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58. Safety Applications

Chapter Editors: Kenneth Gerecke and Charles T. Pope


Table of Contents

Tables and Figures

Systems Analysis
Manh Trung Ho  

Hand and Portable Power Tool Safety
US Department of Labor—Occupational Safety and Health Administration; edited by Kenneth Gerecke

Moving Parts of Machines
Tomas Backström and Marianne Döös

Machine Safeguarding
US Department of Labor— Occupational Safety and Health Administration; edited by Kenneth Gerecke

Presence Detectors
Paul Schreiber

Devices for Controlling, Isolating and Switching Energy
René Troxler

Safety-Related Applications
Dietmar Reinert and Karlheinz Meffert

Software and Computers: Hybrid Automated Systems
Waldemar Karwowski and Jozef Zurada

Principles for the Design of Safe Control Systems
Georg Vondracek

Safety Principles for CNC Machine Tools
Toni Retsch, Guido Schmitter and Albert Marty

Safety Principles for Industrial Robots
Toni Retsch, Guido Schmitter and Albert Marty

Electrical, Electronic and Programmable Electronic Safety-Related Control Systems
Ron Bell

Technical Requirements for Safety-Related Systems Based on Electrical, Electronic and Programmable Electronic Devices
John Brazendale and Ron Bell

Rollover
Bengt Springfeldt

Falls from Elevations
Jean Arteau

Confined Spaces
Neil McManus

Principles of Prevention: Materials Handling and Internal Traffic
Kari Häkkinen

Tables

Click a link below to view table in article context.

1. Possible dysfunctions of a two-button control circuit
2. Machine guards
3. Devices
4. Feeding & ejection methods
5. Circuit structures’ combinations in machine controls
6. Safety integrity levels for protection systems
7. Software design & development
8. Safety integrity level: type B components
9. Integrity requirements: electronic system architectures
10. Falls from elevations: Quebec 1982-1987
11.Typical fall prevention & fall arrest systems
12. Differences between fall prevention & fall arrest
13. Sample form for assessment of hazardous conditions
14. A sample entry permit

Figures

Point to a thumbnail to see figure caption, click to see figure in article context.

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Materials handling and internal traffic are contributing factors in a major portion of accidents in many industries. Depending on the type of industry, the share of work accidents attributed to materials handling varies from 20 to 50%. The control of materials-handling risks is the foremost safety problem in dock work, the construction industry, warehousing, sawmills, shipbuilding and other similar heavy industries. In many process-type industries, such as the chemical products industry, the pulp and paper industry and the steel and foundry industries, many accidents still tend to occur during the handling of final products either manually or by fork-lift trucks and cranes.

This high accident potential in materials-handling activities is due to at least three basic characteristics:

  • High amounts of potential and kinetic energies, which have the propensity for causing injury and damage, are found in transport and handling.
  • The number of people required at transport and handling workplaces is still relatively high, and they are often exposed to the risks associated with such sites.
  • Whenever several dynamic operations have to be carried out simultaneously and require cooperation in varying environments, there is an especially urgent need of clear and timely communication and information. The consequently high liability of many types of human errors and omissions may create hazardous situations.

 

Materials-Handling Accidents

Every time people or machines move loads, an accident risk is present. The magnitude of risk is determined by the technological and organizational characteristics of the system, the environment and the accident prevention measures implemented. For safety purposes, it is useful to depict materials handling as a system in which the various elements are interrelated (figure 1). When changes are introduced in any element of the system—equipment, goods, procedures, environment, people, management and organization—the risk of injuries is likely to change as well.

Figure 1. A materials-handling system

ACC220F1

The most common materials-handling and internal traffic types involved in accidents are associated with manual handling, transport and moving by hand (carts, bicycles, etc.), lorries, fork-lift trucks, cranes and hoists, conveyors and rail transport.

Several types of accidents are commonly found in materials transport and handling at workplaces. The following list outlines the most frequent types:

  • physical strain in manual handling
  • loads falling onto people
  • people trapped between objects
  • collisions between equipment
  • people falling
  • hits, blows and cuts to people from equipment or loads.

 

Elements of Materials-Handling Systems

For each element in a materials-handling system, several design options are available, and the risk of accidents is affected accordingly. Several safety criteria must be considered for each element. It is important that the systems approach is used throughout the lifetime of the system—during the design of the new system, during the normal operation of the system and in following up on past accidents and disturbances in order to introduce improvements into the system.

General Principles of Prevention

Certain practical principles of prevention are generally regarded as applicable to safety in materials handling. These principles can be applied to both manual and mechanical materials-handling systems in a general sense and whenever a factory, warehouse or construction site is under consideration. Many different principles must be applied to the same project to achieve optimum safety results. Usually, no single measure can totally prevent accidents. Conversely, not all of these general principles are needed, and some of them may not work in a specific situation. Safety professionals and materials-handling specialists should consider the most relevant items to guide their work in each specific case. The most important issue is to manage the principles optimally to create safe and practicable materials-handling systems, rather than to settle upon any single technical principle to the exclusion of others.

The following 22 principles can be used for safety purposes in the development and assessment of materials-handling systems in their planned, present or historical stage. All of the principles are applicable in both pro-active and aftermath safety activities. No strict priority order is implied in the list that follows, but a rough division can be made: the first principles are more valid in the initial design of new plant layouts and materials-handling processes, whereas the last principles listed are more directed to the operation of existing materials-handling systems.

Twenty-two Principles of Prevention of Materials-Handling Accidents

  1. Eliminate all unnecessary transport and handling operations. Because many transport and handling processes are inherently dangerous, it is useful to consider whether some materials handling might be eliminated. Many modern manufacturing processes can be arranged in a continuous flow without any separate handling and transport phases. Many assembly and construction operations can be planned and designed to eliminate strenuous and complex movements of loads. Options for more effective and rational transport can also be found by analysing logistics and material flow in the manufacturing and transport processes.
  2. Remove human beings from the transport and handling space. When workers are not physically located under or in the vicinity of loads to be moved, safety conditions are ipso facto improved because of reduced exposure to hazards. People are not allowed to work in the scrap-handling area of steelworks because pieces of scrap may drop from the magnetic grippers that are used to move the scrap, presenting a continuous hazard of falling loads. Materials handling in harsh environments can often be automated by using robots and automatic trucks, an arrangement that reduces the accident risks posed to workers by moving loads. Moreover, by forbidding people to go unnecessarily through loading and unloading yards, exposure to several types of materials-handling hazards is basically eliminated.
  3. Segregate transport operations from each other as much as possible to minimize encounters.The more frequently vehicles encounter one another, other equipment and people, the greater is the probability of collisions. Segregation of transport operations is important when planning for safe in-plant transport. There are many segregations to be considered, such as pedestrians/vehicles; heavy traffic/light traffic; internal traffic/traffic to and from outside; transport between workplaces/materials handling within a workplace; transport/storage; transport/production line; receiving/shipping; hazardous materials transportation/normal transport. When spatial segregation is not practicable, specific times can be allocated when transport and pedestrians respectively are allowed to enter a work area (e.g., in a warehouse open to the public). If separate pathways cannot be arranged for pedestrians, their routes can be designated by markings and signs. When entering a factory building, employees should be able to use separate pedestrian doors. If pedestrian traffic and fork-lift truck traffic are mixed in doorways, they also tend to be mixed beyond the doorways, thus presenting a hazard. During plant modifications, it is often necessary to limit transport and human motion through the areas which are under repair or construction. In overhead crane transport, collisions can be avoided by seeing to it that the tracks of the cranes do not overlap and by installing limit switches and mechanical barriers.
  4. Provide enough space for materials-handling and transport operations. Too narrow a space for materials handling is often a cause of accidents. For example, workers’ hands can be caught between a load and a wall in manual handling, or a person may be pinned between a moving pillar of a transport crane and a stack of materials when the minimum safety distance of 0.5 m is not available. The space needed for transport and handling operations should be carefully considered in plant design and planning of modifications. It is advisable to reserve some “safety margin” of space in order to accommodate future changes in load dimensions and types of equipment. Often, the volume of the products being manufactured tends to grow as time goes on, but the space in which to handle them becomes smaller and smaller. Although the demand for cost-effective space utilization may be a reason for minimizing production space, it should be borne in mind that the manoeuvring space needed for counterbalanced fork-lift trucks to turn and to backtrack is larger than it seems to be at first sight.
  5. Aim at continuous transport processes, avoiding points of discontinuity in materials handling. Continuous material flows reduce the potential for accidents. The basic arrangement of a plant layout is of crucial importance in carrying out this safety principle. Accidents concentrate in places where the material flow is interrupted because the moving and handling equipment is changed, or for production reasons. Human intervention is often required to unload and reload, to fasten, package, lift and drag, and so forth. Depending on the materials handled, conveyors generally give more continuous material flows than cranes or fork-lift trucks. It is good planning to arrange transport operations in such a way that motor vehicles can move in factory premises in a one-way circle, without any zigzag motion or backtracking. Because points of discontinuity tend to develop in boundary lines between departments or between working cells, production and transport should be planned to avoid such “no-man’s lands” with uncontrolled materials movement.
  6. Use standard elements in materials-handling systems. For safety purposes it is generally better to use standard items of loads, equipment and tools in materials handling. The concept of unit load is well-known to most transport professionals. Materials packed in containers and on pallets are easier to attach and move when the other elements in the transport chain (e.g., storage racks, fork-lift trucks, motor vehicles and fastening devices of cranes) are designed for these unit loads. The use of standard types of fork-lift trucks with similar controls decreases the probability of driver error, as accidents have occurred when a driver has changed from one sort of equipment to another with different controls.
  7. Know the materials to be handled. Knowledge of the characteristics of the materials to be transported is a precondition for safe transfer. In order to select appropriate lifting or load restraints, one must take into account the weight, centre of gravity and dimensions of goods that are to be fastened for lifting and transport. When hazardous materials are handled, it is necessary that information be available as to their reactivity, flammability and health hazards. Special hazards are presented in the case of items which are fragile, sharp, dusty, slippery, loose, or when handling explosive materials and living animals, for example. The packages often provide important information for workers as to proper handling methods, but sometimes labels are removed or protective packaging conceals important information. For example, it may not be possible to view the distribution of the contents within a package, with the result that one cannot properly assess the load’s centre of gravity.
  8. Keep the loading below the safe working-load capacity. Overloading is a common cause of damage in materials-handling systems. Loss of balance and material breakage are typical results of overloading handling equipment. The safe working load of slings and other lifting tackle should be clearly marked, and proper configurations of slings must be selected. Overloading can take place when the weight or the centre of gravity of the load is misjudged, leading to improper fastening and manoeuvring of loads. When slings are used to handle loads, the equipment operator should be aware that an inclined pathway may exert forces sufficient to cause the load to drop off or over-balance the equipment. The loading capacity of fork-lift trucks should be marked on the equipment; this varies according to the lifting height and the size of the load. Overloading due to fatigue failure may occur under repeated loadings well below the ultimate breaking load if the component is not correctly designed against this type of failure.
  9. Set the speed limits low enough to maintain safe movement. Speed limits for vehicles moving in workplaces vary from 10 km/h to 40 km/h (about 5 to 25 mph). Lower speeds are required in inside corridors, in doorways, at crossings and in narrow aisles. A competent driver can adapt a vehicle’s speed according to the demands of each situation, but signs notifying drivers of speed limitations are advisable at critical places. The maximum speed of a remote-controlled mobile crane, for example, must be determined first by fixing a vehicle speed comparable to a reasonable walking speed for a human, and then allowing for the time needed for simultaneous observations and control of loads so as not to exceed the response time of the human operator.
  10. Avoid overhead lifting in areas where people are working underneath. Overhead lifting of materials always poses a risk of falling loads. Although people are ordinarily not allowed to work under hanging loads, the routine transportation of loads over people in production can expose them to danger. Fork-lift transport to high storage racks and lifting between floors are further examples of overhead lifting tasks. Overhead conveyors transporting stones, coke or casts may also constitute a risk of falling loads for those walking underneath if protective covers are not installed. In considering a new overhead transport system, the potential greater risks should be compared with the lesser risks associated with a floor-level transport system.
  11. Avoid materials-handling methods that require climbing and working at high levels. When people have to climb up—for example, to unfasten sling hooks, to adjust a vehicle’s canopy or to make markings on loads—they risk falling. This hazard can often be averted by better planning, by changing the sequence of work, by using various lifting accessories and remote-controlled tools, or by mechanization and automation.
  12. Attach guards at danger points. Guards should be installed on danger points in materials-handling equipment such as the chains of fork-lift trucks, the rope drives of cranes and the trapping points of conveyors. Out-of-reach protection is often not enough, because the hazard point may be reached by using ladders and other means. Guards are also used to protect against technical failures that could lead to injuries (e.g., of wire rope retainers on crane sheaves, safety latches in lifting hooks and the protection pads of textile slings that shield against sharp edges). Guardrails and toeboards installed against the edges of loading platforms and overhead storage racks, and around floor openings, can protect both people and things from falling. This sort of protection is often needed when fork-lift trucks and cranes lift materials from one floor to another. People can be protected from falling objects in materials-handling operations by safety nets and permanent guards such as wire mesh or metal plate covers on conveyors.
  13. Transport and lift people only by the equipment designed for the purpose. Cranes, fork-lift trucks, excavators and conveyors are machines for moving materials, not human beings, from one place to another. Special lifting platforms are available to lift persons, for example, to change lamps on ceilings. If a crane or a fork-lift truck is equipped with a special cage which can be securely attached to the equipment and which meets proper safety requirements, persons can be lifted without an excessive risk of severe injury.
  14. Keep equipment and loads stable. Accidents happen when equipment, goods or storage racks lose their stability, especially in the case of fork-lift trucks or mobile cranes. The selection of actively stable equipment is a first step to reduce hazards. Further, it is advisable to use equipment that emits a warning signal before the limit of collapse is reached. Good working practices and qualified operators are the next stops of prevention. Experienced and trained employees are able to estimate centres of gravity and recognize unstable conditions where materials are piled and stacked, and to make the necessary adjustments.
  15. Provide good visibility. Visibility is always limited when handling materials with fork-lift trucks. When new equipment is purchased, it is important to assess how much the driver can see through the mast structures (and, for high-lifting trucks, the visibility through the overhead frame). In any case, the materials handled cause some loss of visibility, and this effect should be considered. Whenever possible, a clear line of sight should be provided—for example, by removing piles of goods or by arranging openings or empty sections at critical points in racks. Mirrors can be applied to the equipment and at suitable locations in factories and warehouses to make blind corners safer. However, mirrors are a secondary means of prevention compared to the actual elimination of blind corners in order to allow direct vision. In crane transport it is often necessary to assign a special signal person to check that the area where the load will be lowered is unoccupied by people. A good safety practice is to paint or otherwise mark danger points and obstructions in the working environment—for example, pillars, edges of doors and of loading docks, protruding machine elements and moving parts of equipment. Appropriate illumination can often improve visibility considerably—for example, on stairs, in corridors and at exit doors.
  16. Eliminate manual lifting and carrying of loads by mechanical and automated handling. About 15% of all work-related injuries involve the manual lifting and carrying of loads. Most of the injuries are due to over-exertion; the rest are slips and falls and hand injuries inflicted by sharp edges. Cumulative trauma disorders and back disorders are typical health problems due to manual-handling work. Although mechanization and automation have eliminated manual-handling tasks to a large extent in industry, there still exist a number of workplaces where people are physically overloaded by lifting and carrying heavy loads. Consideration should be given to providing appropriate handling equipment—for example, hoists, lifting platforms, elevators, fork-lift trucks, cranes, conveyors, palletizers, robots and mechanical manipulators.
  17. Provide and maintain effective communication. A common factor in serious accidents is a failure in communication. A crane driver must communicate with a slinger, who fastens the load, and if the hand signs between the driver and the loader are incorrect or radio phones have a low audibility, critical errors may result. Communication links are important between materials-handling operators, production people, loaders, dock workers, equipment drivers and maintenance people. For instance, a fork-lift truck driver has to pass along information about any safety problems encountered—for example, aisles with blind corners due to stacks of material—when turning over the truck to the next driver during shift change. Drivers of motor vehicles and mobile cranes working as contractors in a workplace are often unfamiliar with the particular risks they may encounter, and should therefore receive special guidance or training. This may include providing a map of the factory premises at the access gate together with the essential safe work and driving instructions. Traffic signs for workplace traffic are not as highly developed as the those for public roads. However, many of the risks encountered in road traffic are common within factory premises, too. It is therefore important to provide appropriate traffic signs for internal traffic in order to facilitate the communication of hazard warnings and to alert drivers to whatever precautions may be required.
  18. Arrange the human interfaces and the manual handling according to ergonomic principles. Materials-handling work should be accommodated to the capacity and skills of people by applying ergonomics so as to obviate errors and improper straining. The controls and displays of cranes and fork-lift trucks should be compatible with the natural expectations and habits of people. In manual handling it is important to make sure that there is enough space for the human motions necessary to carry out the tasks. Furthermore, excessively strenuous working postures should be avoided—for example, manually lifting loads over one’s head, and not exceeding the maximum permissible weights for manual lifting. Individual variations in age, strength, health status, experience and anthropometric considerations may require modification of the workspace and tasks accordingly. Order picking in storage facilities is an example of a task in which ergonomics is of utmost importance for safety and productivity.
  19. Provide adequate training and advice. Materials-handling tasks are often regarded as too low-status to warrant any special training for the workforce. The number of specialized crane operators and fork-lift drivers is decreasing at workplaces; and there is a growing tendency to make crane and fork-lift truck driving a job that almost anybody in a workplace should be prepared to do. Although hazards can be reduced by technical and ergonomic measures, it is the skill of the operator that is ultimately decisive in averting hazardous situations in dynamic work settings. Accident surveys have indicated that many of the victims in materials-handling accidents are people not involved in materials-handling tasks themselves. Therefore, training should also be provided to some extent for bystanders in the materials-handling areas.
  20. Supply the people working in transport and handling with appropriate personal outfits. Several types of injuries can be prevented by using appropriate personal protective equipment. Safety shoes which do not cause slips and falls, heavy gloves, safety glasses or goggles, and hard hats are typical personal protectors worn for materials-handling tasks. When special hazards demand it, fall protection, respirators and special safety garments are used. Appropriate working gear for materials handling should provide good visibility and should not include parts that may easily be caught on equipment or gripped by moving parts.
  21. Carry out proper maintenance and inspection duties. When accidents happen because of failures in equipment, the reasons are often to be found in poor maintenance and inspection procedures. Instructions for maintenance and inspections are given in safety standards and in manufacturers’ manuals. Deviations from the given procedures can lead to dangerous situations. Material-handling equipment users are responsible for daily maintenance and inspection routines involving such tasks as checking batteries, rope and chain drives, lifting tackle, brakes and controls; cleaning windows; and adding oil when needed. More thorough, less frequent, inspections are carried out regularly, such as weekly, monthly, semi-annually or once a year, depending on the conditions of use. Housekeeping, including adequate cleaning of floors and workplaces, is also important for safe materials handling. Oily and wet floors cause people and trucks to slip. Broken pallets and storage racks should be discarded whenever observed. In operations involving the transporting of bulk materials by conveyors it is important to remove accumulations of dust and grain in order to prevent dust explosions and fires.
  22. Plan for changes in the environmental conditions. The capacity to adapt to varying environmental conditions is limited among equipment and people alike. Fork-lift truck operators need several seconds to adapt themselves when driving from a gloomy hall through doorways to a sunlit yard outside, and when moving inside from outdoors. To make these operations safer, special lighting arrangements can be set up at doorways. In the outdoors, cranes are often subjected to high wind loads, which have to be taken into account during lifting operations. In extreme wind conditions, lifting with cranes must be interrupted entirely. Ice and snow may cause considerable extra work for workers who have to clean the surfaces of loads. Sometimes, this also means taking extra risks; for instance, when the work is done upon the load or even under the load during lifting. Planning should cover safe procedures for these tasks, too. An icy load may glide away from a pallet fork during a forklift transport. Corrosive atmospheres, heat, frost conditions and seawater can cause degradation of materials and subsequent failures if the materials are not designed to withstand such conditions.

 

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Monday, 04 April 2011 19:18

Confined Spaces

Written by

Confined spaces are ubiquitous throughout industry as recurring sites of both fatal and nonfatal accidents. The term confined space traditionally has been used to label particular structures, such as tanks, vessels, pits, sewers, hoppers and so on. However, a definition based on description in this manner is overly restrictive and defies ready extrapolation to structures in which accidents have occurred. Potentially any structure in which people work could be or could become a confined space. Confined spaces can be very large or they can be very small. What the term actually describes is an environment in which a broad range of hazardous conditions can occur. These condition include personal confinement, as well as structural, process, mechanical, bulk or liquid material, atmospheric, physical, chemical, biological, safety and ergonomic hazards. Many of the conditions produced by these hazards are not unique to confined spaces but are exacerbated by involvement of the boundary surfaces of the confined space.

Confined spaces are considerably more hazardous than normal workspaces. Seemingly minor alterations in conditions can immediately change the status of these workspaces from innocuous to life-threatening. These conditions may be transient and subtle, and therefore are difficult to recognize and to address. Work involving confined spaces generally occurs during construction, inspection, maintenance, modification and rehabilitation. This work is nonroutine, short in duration, nonrepetitive and unpredictable (often occurring during off-shift hours or when the unit is out of service).

Confined Space Accidents

Accidents involving confined spaces differ from accidents that occur in normal workspaces. A seemingly minor error or oversight in preparation of the space, selection or maintenance of equipment or work activity can precipitate an accident. This is because the tolerance for error in these situations is smaller than for normal workplace activity.

The occupations of victims of confined space accidents span the occupational spectrum. While most are workers, as might be expected, victims also include engineering and technical people, supervisors and managers, and emergency response personnel. Safety and industrial hygiene personnel also have been involved in confined space accidents. The only data on accidents in confined spaces are available from the United States, and these cover only fatal accidents (NIOSH 1994). Worldwide, these accidents claim about 200 victims per year in industry, agriculture and the home (Reese and Mills 1986). This is at best a guess based on incomplete data, but it appears to be applicable today. About two-thirds of the accidents resulted from hazardous atmospheric conditions in the confined space. In about 70% of these the hazardous condition existed prior to entry and the start of work. Sometimes these accidents cause multiple fatalities, some of which are the result of the original incident and a subsequent attempt at rescue. The highly stressful conditions under which the rescue attempt occurs often subject the would-be rescuers to considerably greater risk than the initial victim.

The causes and outcomes of accidents involving work external to structures that confine hazardous atmospheres are similar to those occurring inside confined spaces. Explosion or fire involving a confined atmosphere caused about half of the fatal welding and cutting accidents in the United States. About 16% of these accidents involved “empty” 205 l (45 gal UK, 55 gal US) drums or containers (OSHA 1988).

Identification of Confined Spaces

A review of fatal accidents in confined spaces indicates that the best defences against unnecessary encounters are an informed and trained workforce and a programme for hazard recognition and management. Development of skills to enable supervisors and workers to recognize potentially hazardous conditions is also essential. One contributor to this programme is an accurate, up-to-date inventory of confined spaces. This includes type of space, location, characteristics, contents, hazardous conditions and so on. Confined spaces in many circumstances defy being inventoried because their number and type are constantly changing. On the other hand, confined spaces in process operations are readily identifiable, yet remain closed and inaccessible almost all of the time. Under certain conditions, a space may be considered a confined space one day and would not be considered a confined space the next.

A benefit from identifying confined spaces is the opportunity to label them. A label can enable workers to relate the term confined space to equipment and structures at their work location. The downside to the labelling process includes: (1) the label could disappear into a landscape filled with other warning labels; (2) organizations that have many confined spaces could experience great difficulty in labelling them; (3) labelling would produce little benefit in circumstances where the population of confined spaces is dynamic; and (4) reliance on labels for identification causes dependence. Confined spaces could be overlooked.

Hazard Assessment

The most complex and difficult aspect in the confined space process is hazard assessment. Hazard assessment identifies both hazardous and potentially hazardous conditions and assesses the level and acceptability of risk. The difficulty with hazard assessment occurs because many of the hazardous conditions can produce acute or traumatic injury, are difficult to recognize and assess, and often change with changing conditions. Hazard elimination or mitigation during preparation of the space for entry, therefore, is essential for minimizing the risk during work.

Hazard assessment can provide a qualitative estimate of the level of concern attached to a particular situation at a particular moment (table 1). The breadth of concern within each category ranges from minimal to some maximum. Comparison between categories is not appropriate, since the maximum level of concern can differ considerably.

Table 1. Sample form for assessment of hazardous conditions

Hazardous condition

Real or potential consequence

 

Low

Moderate

High

Hot work

     

Atmospheric hazards

     

oxygen deficiency

     

oxygen enrichment

     

chemical

     

biological

     

fire/explosion

     

Ingestion/skin contact

     

Physical agents

     

noise/vibration

     

heat/cold stress

     

non/ionizing radiation

     

laser

     

Personal confinement

     

Mechanical hazard

     

Process hazard

     

Safety hazards

     

structural

     

engulfment/immersion

     

entanglement

     

electrical

     

fall

     

slip/trip

     

visibility/light level

     

explosive/implosive

     

hot/cold surfaces

     

NA = not applicable. The meanings of certain terms such as toxic substance, oxygen deficiency, oxygen enrichment, mechanical hazard, and so on, require further specification according to standards that exist in a particular jurisdiction.

 

Each entry in table 1 can be expanded to provide detail about hazardous conditions where concern exists. Detail also can be provided to eliminate categories from further consideration where concern is non-existent.

 

Fundamental to the success of hazard recognition and assessment is the Qualified Person. The Qualified Person is deemed capable by experience, education and/or specialized training, of anticipating, recognizing and evaluating exposures to hazardous substances or other unsafe conditions and specifying control measures and/or protective actions. That is, the Qualified Person is expected to know what is required in the context of a particular situation involving work within a confined space.

A hazard assessment should be performed for each of the following segments in the operating cycle of the confined space (as appropriate): the undisturbed space, pre-entry preparation, pre-work inspection work activities (McManus, manuscript) and emergency response. Fatal accidents have occurred during each of these segments. The undisturbed space refers to the status quo established between closure following one entry and the start of preparation for the next. Pre-entry preparations are actions taken to render the space safe for entry and work. Pre-work inspection is the initial entry and examination of the space to ensure that it is safe for the start of work. (This practice is required in some jurisdictions.) Work activities are the individual tasks to be performed by entrants. Emergency response is the activity in the event rescue of workers is required, or other emergency occurs. Hazards that remain at the start of work activity or are generated by it dictate the nature of possible accidents for which emergency preparedness and response are required.

Performing the hazard assessment for each segment is essential because the focus changes continuously. For example, the level of concern about a specific condition could disappear following pre-entry preparation; however, the condition could reappear or a new one could develop as a result of an activity which occurs either inside or outside the confined space. For this reason, assessing a level of concern to a hazardous condition for all time based only on an appraisal of pre-opening or even opening conditions would be inappropriate.

Instrumental and other monitoring methods are used for determining the status of some of the physical, chemical and biological agents present in and around the confined space. Monitoring could be required prior to entry, during entry or during work activity. Lockout/tagout and other procedural techniques are used to deactivate energy sources. Isolation using blanks, plugs and caps, and double block and bleed or other valve configurations prevents entry of substances through piping. Ventilation, using fans and eductors, is often necessary to provide a safe environment for working both with and without approved respiratory protection. Assessment and control of other conditions relies on the judgement of the Qualified Person.

The last part of the process is the critical one. The Qualified Person must decide whether the risks associated with entry and work are acceptable. Safety can best be assured through control. If hazardous and potentially hazardous conditions can be controlled, the decision is not difficult to make. The less the level of perceived control, the greater the need for contingencies. The only other alternative is to prohibit the entry.

Entry Control

The traditional methods for managing on-site confined space activity are the entry permit and the on-site Qualified Person. Clear lines of authority, responsibility and accountability between the Qualified Person and entrants, standby personnel, emergency responders and on-site management are required under either system.

The function of an entry document is to inform and to document. Table 2 (below) provides a formal basis for performing the hazard assessment and documenting the results. When edited to include only information relevant to a particular circumstance, this becomes the basis for the entry permit or entry certificate. The entry permit is most effective as a summary that documents actions performed and indicates by exception, the need for further precautionary measures. The entry permit should be issued by a Qualified Person who also has the authority to cancel the permit should conditions change. The issuer of the permit should be independent of the supervisory hierarchy in order to avoid potential pressure to speed the performance of work. The permit specifies procedures to be followed as well as conditions under which entry and work can proceed, and records test results and other information. The signed permit is posted at the entry or portal to the space or as specified by the company or regulatory authority. It remains posted until it is either cancelled, replaced by a new permit or the work is completed. The entry permit becomes a record upon completion of the work and must be retained for recordkeeping according to requirements of the regulatory authority.

The permit system works best where hazardous conditions are known from previous experience and control measures have been tried and proven effective. The permit system enables expert resources to be apportioned in an efficient manner. The limitations of the permit arise where previously unrecognized hazards are present. If the Qualified Person is not readily available, these can remain unaddressed.

The entry certificate provides an alternative mechanism for entry control. This requires an onsite Qualified Person who provides hands-on expertise in the recognition, assessment and evaluation, and control of hazards. An added advantage is the ability to respond to concerns on short notice and to address unanticipated hazards. Some jurisdictions require the Qualified Person to perform a personal visual inspection of the space prior to the start of work. Following evaluation of the space and implementation of control measures, the Qualified Person issues a certificate describing the status of the space and conditions under which the work can proceed (NFPA 1993). This approach is ideally suited to operations that have numerous confined spaces or where conditions or the configuration of spaces can undergo rapid change.

 


 

Table 2. A sample entry permit

ABC COMPANY

CONFINED SPACE—ENTRY PERMIT

1. DESCRIPTIVE INFORMATION

Department:

Location:

Building/Shop:

Equipment/Space:

Part:

Date:                                                 Assessor:

Duration:                                           Qualification:

2. ADJACENT SPACES

Space:

Description:

Contents:

Process:

3. PRE-WORK CONDITIONS

Atmospheric Hazards

Oxygen Deficiency                       Yes  No  Controlled

Concentration:                              (Acceptable minimum:                             %)

Oxygen Enrichment                     Yes  No  Controlled

Concentration:                              (Acceptable maximum:                            %)

Chemical                                      Yes  No  Controlled

Substance Concentration            (Acceptable standard:                                )

Biological                                      Yes  No  Controlled

Substance Concentration            (Acceptable standard:                                )

Fire/Explosion                              Yes  No  Controlled

Substance Concentration            (Acceptable maximum:                     % LFL)

Ingestion/Skin Contact Hazard   Yes  No  Controlled

Physical Agents

Noise/Vibration                            Yes  No  Controlled

Level:                                          (Acceptable maximum:                        dBA)

Heat/Cold Stress                         Yes  No  Controlled

Temperature:                              (Acceptable range:                                     )

Non/Ionizing Radiation                 Yes  No  Controlled

Type Level                                   (Acceptable maximum:                              )

Laser                                            Yes  No  Controlled

Type Level                                    (Acceptable maximum:                              )

Personal Confinement
(Refer to corrective action.)         Yes  No  Controlled

Mechanical Hazard
(Refer to procedure.)                   Yes  No  Controlled

Process Hazard
(Refer to procedure.)                   Yes  No  Controlled

ABC COMPANY

CONFINED SPACE—ENTRY PERMIT

Safety Hazards

Structural Hazard
(Refer to corrective action.)          Yes  No  Controlled

Engulfment/Immersion
(Refer to corrective action.)          Yes  No  Controlled

Entanglement
(Refer to corrective action.)          Yes  No  Controlled

Electrical
(Refer to procedure.)                    Yes  No  Controlled

Fall
(Refer to corrective action.)          Yes  No  Controlled

Slip/Trip
(Refer to corrective action.)          Yes  No  Controlled

Visibility/light level                          Yes  No  Controlled

Level:                                            (Acceptable range:                                  lux)

Explosive/Implosive
(Refer to corrective action.)           Yes  No  Controlled

Hot/Cold Surfaces
(Refer to corrective action.)           Yes  No  Controlled

For entries in highlighted boxes, Yes or Controlled, provide additional detail and refer to protective measures. For hazards for which tests can be made, refer to testing  requirements. Provide date of most recent calibration. Acceptable maximum, minimum, range or standard depends on the jurisdiction.

4. Work Procedure

Description:

Hot Work
(Refer to protective measure.)            Yes  No  Controlled

Atmospheric Hazard

Oxygen Deficiency 

(Refer to requirement for additional testing. Record results. 
Refer to requirement for protective measures.)

Concentration:                                    Yes  No  Controlled

                                                            (Acceptable minimum:                             %)

Oxygen Enrichment                           

(Refer to requirement for additional testing. Record results.
Refer to requirement for protective measures.)                                    

Concentration:                                   Yes  No  Controlled

                                                           (Acceptable maximum:                             %)

Chemical              

(Refer to requirement for additional testing. Record results. Refer to requirement
for protective measures.)
Substance Concentration                  Yes  No  Controlled

                                                           (Acceptable standard:                                 )

Biological             

(Refer to requirement for additional testing. Record results. Refer to requirement
for protective measures.)
Substance Concentration                 Yes  No  Controlled

                                                          (Acceptable standard:                                 )

Fire/Explosion             

(Refer to requirement for additional testing. Record results. Refer to requirement
for protective measures.)
Substance Concentration                 Yes  No  Controlled

                                                          (Acceptable standard:                                 )

Ingestion/Skin Contact Hazard         Yes  No  Controlled

(Refer to requirement for protective measures.)                      

ABC COMPANY

CONFINED SPACE—ENTRY PERMIT

Physical Agents

Noise/Vibration             

(Refer to requirement for protective measures. Refer to requirement for
additional testing. Record results.)
Level:                                                Yes  No  Controlled

                                                         (Acceptable maximum:                         dBA)

Heat/Cold Stress           

(Refer to requirement for protective measures. Refer to requirement for
additional testing. Record results.)
Temperature:                                    Yes  No  Controlled

                                                          (Acceptable range:                                      )

Non/Ionizing Radiation            

(Refer to requirement for protective measures. Refer to requirement for
additional testing. Record results.)
Type Level                                        Yes  No  Controlled

                                                          (Acceptable maximum:                               )

Laser
(Refer to requirement for protective measures.)            Yes  No  Controlled

Mechanical Hazard
(Refer to requirement for protective measures.)            Yes  No  Controlled

Process Hazard

(Refer to requirement for protective measures.)           Yes  No  Controlled

Safety Hazards

Structural Hazard
(Refer to requirement for protective measures.)            Yes  No  Controlled

Engulfment/Immersion
(Refer to requirement for protective measures.)           Yes  No  Controlled

Entanglement
(Refer to requirement for protective measures.)            Yes  No  Controlled

Electrical
(Refer to requirement for protective measures.)           Yes  No  Controlled

Fall
(Refer to requirement for protective measures.)            Yes  No  Controlled

Slip/Trip
(Refer to requirement for protective measures.)            Yes  No  Controlled

Visibility/light level
(Refer to requirement for protective measures.)            Yes  No  Controlled

Explosive/Implosive
(Refer to requirement for protective measures.)             Yes  No  Controlled

Hot/Cold Surfaces
(Refer to requirement for protective measures.)            Yes  No  Controlled

For entries in highlighted boxes, Yes or Possible, provide additional detail and refer to protective
measures. For hazards for which tests can be made, refer to testing requirements. Provide date of
most recent calibration.

Protective Measures

Personal protective equipment (specify)

Communications equipment and procedure (specify)

Alarm systems (specify)

Rescue Equipment (specify)

Ventilation (specify)

Lighting (specify)

Other (specify)

(Continues on next page)

ABC COMPANY

CONFINED SPACE—ENTRY PERMIT

Testing Requirements

Specify testing requirements and frequency

Personnel

Entry Supervisor

Originating Supervisor

Authorized Entrants

Testing Personnel

Attendants

 

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Monday, 04 April 2011 19:04

Falls from Elevations

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Falls from elevations are severe accidents that occur in many industries and occupations. Falls from elevations result in injuries which are produced by contact between the falling person and the source of injury, under the following circumstances:

  • The motion of the person and the force of impact are generated by gravity.
  • The point of contact with the source of injury is lower than the surface supporting the person at the start of the fall.

 

From this definition, it may be surmised that falls are unavoidable because gravity is always present. Falls are accidents, somehow predictable, occurring in all industrial sectors and occupations and having a high severity. Strategies to reduce the number of falls, or at least reduce the severity of the injuries if falls occur, are discussed in this article.

The Height of the Fall

The severity of injuries caused by falls is intrinsically related to the height of fall. But this is only partly true: the free-fall energy is the product of the falling mass times the height of the fall, and the severity of the injuries is directly proportional to the energy transferred during the impact. Statistics of fall accidents confirm this strong relationship, but show also that falls from a height of less than 3 m can be fatal. A detailed study of fatal falls in construction shows that 10% of the fatalities caused by falls occurred from a height less than 3 m (see figure 1). Two questions are to be discussed: the 3-m legal limit, and where and how a given fall was arrested.

Figure 1. Fatalities caused by falls and the height of fall in the US construction industry, 1985-1993

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In many countries, regulations make fall protection mandatory when the worker is exposed to a fall of more than 3 m. The simplistic interpretation is that falls of less than 3 m are not dangerous. The 3-m limit is in fact the result of a social, political and practical consensus which says it is not mandatory to be protected against falls while working at the height of a single floor. Even if the 3-m legal limit for mandatory fall protection exists, fall protection should always be considered. The height of fall is not the sole factor explaining the severity of fall accidents and the fatalities due to falls; where and how the person falling came to rest must also be considered. This leads to analysis of the industrial sectors with higher incidence of falls from elevations.

Where Falls Occur

Falls from elevations are frequently associated with the construction industry because they account for a high percentage of all fatalities. For example, in the United States, 33% of all fatalities in construction are caused by falls from elevations; in the UK, the figure is 52%. Falls from elevations also occur in other industrial sectors. Mining and the manufacturing of transportation equipment have a high rate of falls from elevations. In Quebec, where many mines are steep, narrow-vein, underground mines, 20% of all accidents are falls from elevations. The manufacture, use and maintenance of transportation equipment such as airplanes, trucks and railroad cars are activities with a high rate of fall accidents (table 1). The ratio will vary from country to country depending on the level of industrialization, the climate, and so on; but falls from elevations do occur in all sectors with similar consequences.


Table 1. Falls from elevations: Quebec 1982-1987

                               Falls from elevations                         Falls from elevations in all accidents
                               per 1,000 workers

Construction                        14.9                                                10.1%

Heavy industry                      7.1                                                  3.6%


Having taken into consideration the height of fall, the next important issue is how the fall is arrested. Falling into hot liquids, electrified rails or into a rock crusher could be fatal even if the height of fall is less than 3 m.

Causes of Falls

So far it has been shown that falls occur in all economic sectors, even if the height is less than 3 m. But why do humans fall? There are many human factors which can be involved in falling. A broad grouping of factors is both conceptually simple and useful in practice:

Opportunities to fall are determined by environmental factors and result in the most common type of fall, namely the tripping or slipping that result in falls from grade level. Other falling opportunities are related to activities above grade.

Liabilities to fall are one or more of the many acute and chronic diseases. The specific diseases associated with falling usually affect the nervous system, the circulatory system, the musculoskeletal system or a combination of these systems.

Tendencies to fall arise from the universal, intrinsic deteriorative changes that characterize normal ageing or senescence. In falling, the ability to maintain upright posture or postural stability is the function that fails as a result of combined tendencies, liabilities and opportunities.

Postural Stability

Falls are caused by the failure of postural stability to maintain a person in an upright position. Postural stability is a system consisting of many rapid adjustments to external, perturbing forces, especially gravity. These adjustments are largely reflex actions, subserved by a large number of reflex arcs, each with its sensory input, internal integrative connections, and motor output. Sensory inputs are: vision, the inner ear mechanisms that detect position in space, the somatosensory apparatus that detects pressure stimuli on the skin, and the position of the weight-bearing joints. It appears that visual perception plays a particularly important role. Very little is known about the normal, integrative structures and functions of the spinal cord or the brain. The motor output component of the reflex arc is muscular reaction.

Vision

The most important sensory input is vision. Two visual functions are related to postural stability and control of gait:

  • the perception of what is vertical and what is horizontal is basic to spatial orientation
  • the ability to detect and discriminate objects in cluttered environments.

 

Two other visual functions are important:

  • the ability to stabilize the direction in which the eyes are pointed so as to stabilize the surrounding world while we are moving and immobilize a visual reference point
  • the ability to fixate and pursue definite objects within the large field (“keep an eye on”); this function requires considerable attention and results in deterioration in the performance of any other simultaneous, attention-demanding tasks.

 

Causes of postural instability

The three sensory inputs are interactive and interrelated. The absence of one input—and/or the existence of false inputs—results in postural instability and even in falls. What could cause instability?

Vision

  • the absence of vertical and horizontal references—for example, the connector at the top of a building
  • the absence of stable visual references—for example, moving water under a bridge and moving clouds are not stable references
  • the fixing a definite object for work purposes, which diminishes other visual functions, such as the ability to detect and discriminate objects that can cause tripping in a cluttered environment
  • a moving object in a moving background or reference—for example, a structural steel component moved by a crane, with moving clouds as background and visual reference.

 

Inner ear

  • having the person’s head upside down while the level equilibrium system is at its optimum performance horizontally
  • travelling in pressurized aircraft
  • very fast movement, as, for example, in a roller-coaster
  • diseases.

 

Somatosensory apparatus (pressure stimuli on the skin and position of weight-bearing joints)

  • standing on one foot
  • numbed limbs from staying in a fixed position for a long period of time—for example, kneeling down
  • stiff boots
  • very cold limbs.

 

Motor output

  • numbed limbs
  • tired muscles
  • diseases, injuries
  • ageing, permanent or temporary disabilities
  • bulky clothing.

 

Postural stability and gait control are very complex reflexes of the human being. Any perturbations of the inputs may cause falls. All perturbations described in this section are common in the workplace. Therefore, falling is somehow natural and prevention must therefore prevail.

Strategy for Fall Protection

As previously noted, the risks of falls are identifiable. Therefore, falls are preventable. Figure 2 shows a very common situation where a gauge must be read. The first illustration shows a traditional situation: a manometer is installed at the top of a tank without means of access In the second, the worker improvises a means of access by climbing on several boxes: a hazardous situation. In the third, the worker uses a ladder; this is an improvement. However, the ladder is not permanently fixed to the tank; it is therefore probable that the ladder may be in use elsewhere in the plant when a reading is required. A situation such as this is possible, with fall arrest equipment added to the ladder or the tank and with the worker wearing a full body harness and using a lanyard attached to an anchor. The fall-from-elevation hazard still exists.

Figure 2. Installations for reading a gauge

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In the fourth illustration, an improved means of access is provided using a stairway, a platform and guardrails; the benefits are a reduction in the risk of falling and an increase in the ease of reading (comfort), thus reducing the duration of each reading and providing a stable work posture allowing for a more precise reading.

The correct solution is illustrated in the last illustration. During the design stage of the facilities, maintenance and operation activities were recognized. The gauge was installed so that it could be read at ground level. No falls from elevations are possible: therefore, the hazard is eliminated.

This strategy puts the emphasis on the prevention of falls by using the proper means of access (e.g., scaffolds, ladders, stairways) (Bouchard 1991). If the fall cannot be prevented, fall arrest systems must be used (figure 3). To be effective, fall arrest systems must be planned. The anchorage point is a key factor and must be pre-engineered. Fall arrest systems must be efficient, reliable and comfortable; two examples are given in Arteau, Lan and Corbeil (to be published) and Lan, Arteau and Corbeil (to be published). Examples of typical fall prevention and fall arrest systems are given in table 2. Fall arrest systems and components are detailed in Sulowski 1991.

Figure 3. Fall prevention strategy

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Table 2. Typical fall prevention and fall arrest systems

 

Fall prevention systems

Fall arrest systems

Collective protection

Guardrails Railings

Safety net

Individual protection

Travel restricting system (TRS)

Harness, lanyard, energy absorber anchorage, etc.

 

The emphasis on prevention is not an ideological choice, but rather a practical choice. Table 3 shows the differences between fall prevention and fall arrest, the traditional PPE solution.

Table 3. Differences between fall prevention and fall arrest

 

Prevention

Arrest

Fall occurrence

No

Yes

Typical equipment

Guardrails

Harness, lanyard, energy absorber and anchorage (fall arrest system)

Design load (force)

1 to 1.5 kN applied horizontally and 0.45 kN applied vertically—both at any point on the upper rail

Minimum breaking strength of the anchorage point

18 to 22 kN

Loading

Static

Dynamic

 

For the employer and the designer, it is easier to build fall prevention systems because their minimum breaking strength requirements are 10 to 20 times less than those of fall arrest systems. For example, the minimum breaking strength requirement of a guard rail is around 1 kN, the weight of a large man, and the minimum breaking strength requirement of the anchorage point of an individual fall arrest system could be 20 kN, the weight of two small cars or 1 cubic metre of concrete. With prevention, the fall does not occur, so the risk of injury does not exist. With fall arrest, the fall does occur and even if arrested, a residual risk of injury exists.

 

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Monday, 04 April 2011 19:01

Rollover

Written by

Tractors and other mobile machinery in agricultural, forestry, construction and mining work, as well as materials handling, can give rise to serious hazards when the vehicles roll over sideways, tip over forwards or rear over backwards. The risks are heightened in the case of wheeled tractors with high centres of gravity. Other vehicles that present a hazard of rollover are crawler tractors, loaders, cranes, fruit-pickers, dozers, dumpers, scrapers and graders. These accidents usually happen too fast for drivers and passengers to get clear of the equipment, and they can become trapped under the vehicle. For example, tractors with high centres of gravity have considerable likelihood of rollover (and narrow tractors have even less stability than wide ones). A mercury engine cut-off switch to shut off power upon sensing lateral movement was introduced on tractors but was proven too slow to cope with the dynamic forces generated in the rollover movement (Springfeldt 1993). Therefore the safety device was abandoned.

The fact that such equipment often is used on sloping or uneven ground or on soft earth, and sometimes in close proximity to ditches, trenches or excavations, is an important contributing cause to rollover. If auxiliary equipment is attached high up on a tractor, the probability of rearing over backwards in climbing a slope (or tipping over forwards when descending) increases. Furthermore, a tractor can roll over because of the loss of control due to the pressure exerted by tractor-drawn equipment (e.g., when the carriage moves downwards on a slope and the attached equipment is not braked and over-runs the tractor). Special hazards arise when tractors are used as tow vehicles, particularly if the tow hook on the tractor is placed on a higher level than the wheel axle.

History

Notice of the rollover problem was taken on the national level in certain countries where many fatal rollovers occurred. In Sweden and New Zealand, development and testing of rollover protective structures (ROPS) on tractors (figure 1) already were in progress in the 1950s, but this work was followed up by regulations only on the part of the Swedish authorities; these regulations were effective from the year 1959 (Springfeldt 1993).

Figure 1. Usual types of ROPS on tractors

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Proposed regulations prescribing ROPS for tractors were met by resistance in the agricultural sector in several countries. Strong opposition was mounted against plans requiring employers to install ROPS on existing tractors, and even against the proposal that only new tractors be equipped by the manufacturers with ROPS. Eventually many countries successfully mandated ROPS for new tractors, and later on some countries were able to require ROPS be retrofitted on old tractors as well. International standards concerning tractors and earth-moving machinery, including testing standards for ROPS, contributed to more reliable designs. Tractors were designed and manufactured with lower centres of gravity and lower-placed tow hooks. Four-wheel drive has reduced the risk of rollover. But the proportion of                                                                                                                     tractors with ROPS in countries with many old tractors and                                                                                                                                 without mandates for retrofitting of ROPS is still rather low.

Investigations

Rollover accidents, particularly those involving tractors, have been studied by researchers in many countries. However, there are no centralized international statistics with respect to the number of accidents caused by the types of mobile machinery reviewed in this article. Available statistics at the national level nevertheless show that the number is high, especially in agriculture. According to a Scottish report of tractor rollover accidents in the period 1968–1976, 85% of the tractors involved had equipment attached at the time of the accident, and of these, half had trailed equipment and half had mounted equipment. Two-thirds of the tractor rollover accidents in the Scottish report occurred on slopes (Springfeldt 1993). It was later proved that the number of accidents would be reduced after the introduction of training for driving on slopes as well as the application of an instrument for measuring slope steepness combined with an indicator of safe slope limits.

In other investigations, New Zealand researchers observed that half of their fatal rollover accidents occurred on flat ground or on slight slopes, and only one-tenth occurred on steep slopes. On flat ground tractor drivers may be less attentive to rollover hazards, and they can misjudge the risk posed by ditches and uneven ground. Of the rollover fatalities in tractors in New Zealand in the period 1949–1980, 80% occurred in wheel tractors, and 20% with crawler tractors (Springfeldt 1993). Studies in Sweden and New Zealand showed that about 80% of the tractor rollover fatalities occurred when tractors rolled over sideways. Half of the tractors involved in the New Zealand fatalities had rolled 180°.

Studies of the correlation between rollover fatalities in West Germany and the model year of farm tractors (Springfeldt 1993) showed that 1 of 10,000 old, unprotected tractors manufactured before 1957 was involved in a rollover fatality. Of tractors with prescribed ROPS, manufactured in 1970 and later, 1 of 25,000 tractors was involved in a rollover fatality. Of fatal tractor rollovers in West Germany in the period 1980–1985, two-thirds of the victims were thrown from their protected area and then run over or hit by the tractor (Springfeldt 1993). Of nonfatal rollovers, one-quarter of the drivers were thrown from the driver’s seat but not run over. It is evident that the fatality risk increases if the driver is thrown out of the protected area (similar to automobile accidents). Most of the tractors involved had a two-pillar bow (figure 1 C) that does not prevent the driver from being thrown out. In a few cases the ROPS had been subject to breakage or strong deformation.

The relative frequencies of injuries per 100,000 tractors in different periods in some countries and the reduction of the fatality rate was calculated by Springfeldt (1993). The effectiveness of ROPS in diminishing injury in tractor rollover accidents has been proven in Sweden, where the number of fatalities per 100,000 tractors was reduced from approximately 17 to 0.3 over the period of three decades (1960–1990) (figure 2). At the end of the period it was estimated that about 98% of the tractors were fitted with ROPS, mainly in the form of a crushproof cab (figure 1 A). In Norway, fatalities were reduced from about 24 to 4 per 100,000 tractors during a similar period. However, worse results were achieved in Finland and New Zealand.

Figure 2. Injuries by rollovers per 100,000 tractors in Sweden between 1957 and 1990

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Prevention of Injuries by Rollovers

The risk of rollover is greatest in the case of tractors; however, in agricultural and forest work there is little that can be done to prevent tractors from rolling over. By mounting ROPS on tractors and those types of earth-moving machinery with potential rollover hazards, the risk of personal injuries can be reduced, provided that the drivers remain on their seats during rollover events (Springfeldt 1993). The frequency of rollover fatalities depends largely on the proportion of protected machines in use and the types of ROPS used. A bow (figure 1 C) gives much less protection than a cab or a frame (Springfeldt 1993). The most effective structure is a crushproof cab, which allows the driver to stay inside, protected, during a rollover. (Another reason for choosing a cab is that it affords weather protection.) The most effective means of keeping the driver within the protection of the ROPS during a rollover is a seat-belt, provided that the driver uses the belt while operating the equipment. In some countries, there are information plates at the driver’s seat advising that the steering wheel be gripped in a rollover event. An additional safety measure is to design the driver’s cab or interior environment and the ROPS so as to prevent exposure to hazards such as sharp edges or protuberances.

In all countries, rollovers of mobile machinery, mainly tractors, are causing serious injures. There are, however, considerable differences among countries concerning technical specifications relating to machinery design, as well as administrative procedures for examinations, testing, inspections and marketing. The international diversity that characterizes safety efforts in this connection may be explained by considerations such as the following:

  • whether there exist mandatory requirements for ROPS (in the form of regulations or legislation), or recommendations only, or no rules at all
  • the need for rules for new machinery and rules applicable to older equipment
  • the availability of inspection carried out by authorities and the existence of social pressure and cultural climate favourable to observance of safety rules; in many countries, the obedience to safety guidelines is not checked by inspection in agricultural work
  • pressure from trade unions; however, it should be borne in mind that workers’ organizations have less influence on working conditions in agriculture than in other sectors, because there are many family farms in agriculture
  • the type of ROPS used in the country
  • information and understanding of the risks to which tractor drivers are exposed; practical problems often stand in the way of reaching farmers and forest workers for the purposes of information and education
  • the geography of the country, especially where agricultural, forestry and road work is carried out.

 

Safety Regulations

The nature of rules governing requirements for ROPS and the degree of implementation of the rules in a country, has a strong influence on rollover accidents, especially fatal ones. With this in mind, the development of safer machinery has been abetted by directives, codes and standards issued by international and national organizations. Additionally, many countries have adopted rigorous prescriptions for ROPS which have resulted in a great reduction of rollover injuries.

European Economic Community

Beginning in 1974 the European Economic Community (EEC) issued directives concerning type-approval of wheeled agricultural and forestry tractors, and in 1977 issued further, special directives concerning ROPS, including their attachment to tractors (Springfeldt 1993; EEC 1974, 1977, 1979, 1982, 1987). The directives prescribe a procedure for type-approval and certification by manufacture of tractors, and ROPS must be reviewed by an EEC Type Approval Examination. The directives have won acceptance by all the member countries.

Some EEC directives concerning ROPS on tractors were repealed as of 31 December 1995 and replaced by the general machinery directive which applies to those sorts of machinery presenting hazards due to their mobility (EEC 1991). Wheeled tractors, as well as some earth-moving machinery with a capacity exceeding 15 kW (namely crawlers and wheel loaders, backhoe loaders, crawler tractors, scrapers, graders and articulated dumpers) must be fitted with a ROPS. In case of a rollover, the ROPS must offer the driver and operators an adequate deflection-limiting volume (i.e., space allowing movement of occupants’ bodies before contacting interior elements during an accident). It is the responsibility of the manufacturers or their authorized representatives to perform appropriate tests.

Organization for Economic Cooperation and Development

In 1973 and 1987 the Organization for Economic Cooperation and Development (OECD) approved standard codes for testing of tractors (Springfeldt 1993; OECD 1987). They give results of tests of tractors and describe the testing equipment and test conditions. The codes require testing of many machinery parts and functions, for instance the strength of ROPS. The OECD Tractor Codes describe a static and a dynamic method of testing ROPS on certain types of tractors. A ROPS may be designed solely to protect the driver in the event of tractor rollover. It must be retested for each model of tractor to which the ROPS is to be fitted. The Codes also require that it be possible to mount a weather protection for the driver onto the structure, of a more or less temporary nature. The Tractor Codes have been accepted by all OECD member bodies from 1988, but in practice the United States and Japan also accept ROPS that do not comply with the code requirements if safety belts are provided (Springfeldt 1993).

International Labour Organization

In 1965, the International Labour Organization (ILO) in its manual, Safety and Health in Agricultural Work, required that a cab or a frame of sufficient strength be adequately fixed to tractors in order to provide satisfactory protection for the driver and passengers inside the cab in case of tractor rollover (Springfeldt 1993; ILO 1965). According to ILO Codes of Practice, agricultural and forestry tractors should be provided with ROPS to protect the operator and any passenger in case of rollover, falling objects or displaced loads (ILO 1976).

The fitting of ROPS should not adversely affect

  • access between the ground and driver’s position
  • access to the tractor’s main controls
  • the manoeuvrability of the tractor in cramped surroundings
  • the attachment or use of any equipment that may be connected to the tractor
  • the control and adjustment of associated equipment.

 

International and national standards

In 1981 the International Organization for Standardization (ISO) issued a standard for tractors and machinery for agriculture and forestry (ISO 1981). The standard describes a static test method for ROPS and sets forth acceptance conditions. The standard has been approved by the member bodies in 22 countries; however, Canada and the United States have expressed disapproval of the document on technical grounds. A Standard and Recommended Practice issued in 1974 by the Society of Automotive Engineers (SAE) in North America contains performance requirements for ROPS on wheeled agricultural tractors and industrial tractors used in construction, rubber-tired scrapers, front-end loaders, dozers, crawler loaders, and motor graders (SAE 1974 and 1975). The contents of the standard have been adopted as regulations in the United States and in the Canadian provinces of Alberta and British Columbia.

Rules and Compliance

OECD Codes and International Standards concern the design and construction of ROPS as well as the control of their strength, but lack the authority to require that this sort of protection be put into practice (OECD 1987; ISO 1981). The European Economic Community also proposed that tractors and earth-moving machinery be equipped with protection (EEC 1974-1987). The aim of the EEC directives is to achieve uniformity among national entities concerning the safety of new machinery at the manufacturing stage. The member countries are obliged to follow the directives and issue corresponding prescriptions. Starting in 1996, the member countries of the EEC intend to issue regulations requiring that new tractors and earth-moving machinery be fitted with ROPS.

In 1959, Sweden became the first country to require ROPS for new tractors (Springfeldt 1993). Corresponding requirements came into effect in Denmark and Finland ten years later. Later on, in the 1970s and 1980s, mandatory requirements for ROPS on new tractors became effective in Great Britain, West Germany, New Zealand, the United States, Spain, Norway, Switzerland and other countries. In all these countries except the United States, the rules were extended to old tractors some years later, but these rules were not always mandatory. In Sweden, all tractors must be equipped with a protective cab, a rule that in Great Britain applies only to all tractors used by agricultural workers (Springfeldt 1993). In Denmark, Norway and Finland, all tractors must be provided with at least a frame, while in the United States and the Australian states, bows are accepted. In the United States tractors must have seat-belts.

In the United States, materials-handling machinery that was manufactured before 1972 and is used in construction work must be equipped with ROPS which meet minimum performance standards (US Bureau of National Affairs 1975). The machines covered by the requirement include some scrapers, front-end loaders, dozers, crawler tractors, loaders, and motor graders. Retrofitting was carried out of ROPS on machines manufactured about three years earlier.

Summary

In countries with mandatory requirements for ROPS for new tractors and retrofitting of ROPS on old tractors, there has been a decrease of rollover injuries, especially fatal ones. It is evident that a crushproof cab is the most effective type of ROPS. A bow gives poor protection in case of rollover. Many countries have prescribed effective ROPS at least on new tractors and as of 1996 on earth-moving machines. In spite of this fact some authorities seem to accept types of ROPS that do not comply with such requirements as have been promulgated by the OECD and the ISO. It is expected that a more general harmonization of the rules governing ROPS will be accomplished gradually all over the world, including the developing countries.

 

Back

Machinery, process plants and other equipment can, if they malfunction, present risks from hazardous events such as fires, explosions, radiation overdoses and moving parts. One of the ways such plants, equipment and machinery can malfunction is from failures of electro-mechanical, electronic and programmable electronic (E/E/PE) devices used in the design of their control or safety systems. These failures can arise either from physical faults in the device (e.g., from wear and tear occurring randomly in time (random hardware failures)); or from systematic faults (e.g., errors made in the specification and design of a system that cause it to fail due to (1) some particular combination of inputs, (2) some environmental condition (3) incorrect or incomplete inputs from sensors, (4) incomplete or erroneous data entry by operators, and (5) potential systematic faults due to poor interface design).

Safety-Related Systems Failures

This article covers the functional safety of safety-related control systems, and considers the hardware and software technical requirements necessary to achieve the required safety integrity. The overall approach is in accordance with the proposed International Electrotechnical Commission Standard IEC 1508, Parts 2 and 3 (IEC 1993). The overall goal of draft international standard IEC 1508, Functional Safety: Safety-Related Systems, is to ensure that plant and equipment can be safety automated. A key objective in the development of the proposed international standard is to prevent or minimize the frequency of:

  • failures of control systems triggering other events which in turn could lead to danger (e.g., control system fails, control is lost, process goes out of control resulting in a fire, release of toxic materials, etc.)
  • failures in alarm and monitoring systems so that operators are not given information in a form that can be quickly identified and understood in order to carry out the necessary emergency actions
  • undetected failures in protection systems, making them unavailable when needed for a safety action (e.g., a failed input card in an emergency shut-down system).

 

The article “Electrical, electronic and programmable electronic safety-related systems” sets out the general safety management approach embodied within Part 1 of IEC 1508 for assuring the safety of control and protection systems that are important to safety. This article describes the overall conceptual engineering design that is needed to reduce the risk of an accident to an acceptable level, including the role of any control or protection systems based on E/E/PE technology.

In figure 1, the risk from the equipment, process plant or machine (generally referred to as equipment under control (EUC) without protective devices) is marked at one end of the EUC Risk Scale, and the target level of risk that is needed to meet the required level of safety is at the other end. In between is shown the combination of safety-related systems and external risk reduction facilities needed to make up the required risk reduction. These can be of various types—mechanical (e.g., pressure relief valves), hydraulic, pneumatic, physical, as well as E/E/PE systems. Figure 2 emphasizes the role of each safety layer in protecting the EUC as the accident progresses.

Figure 1. Risk reduction: General concepts

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Figure 2. Overall model: Protection layers

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Provided that a hazard and risk analysis has been performed on the EUC as required in Part 1 of IEC 1508, the overall conceptual design for safety has been established and therefore the required functions and Safety Integrity Level (SIL) target for any E/E/PE control or protection system have been defined. The Safety Integrity Level target is defined with respect to a Target Failure Measure (see table 1).


Table 1. Safety Integrity Levels for protection systems: Target failure measures

Safety integrity Level                        Demand mode of operation (Probability of failure to perform its design function on demand)

4                                                10-5 ≤ × <10-4

3                                                10-4 ≤ × <10-3

2                                                10-3 ≤ × <10-2

1                                                10-2 ≤ × <10-1 


Protection Systems

This paper outlines the technical requirements that the designer of an E/E/PE safety-related system should consider to satisfy the required Safety Integrity Level target. The focus is on a typical protection system utilizing programmable electronics in order to allow for a more in-depth discussion of the key issues with little loss in generality. A typical protection system is shown in figure 3, which depicts a single channel safety system with a secondary switch-off activated via a diagnostic device. In normal operation the unsafe condition of the EUC (e.g., overspeed in a machine, high temperature in a chemical plant) will be detected by the sensor and transmitted to the programmable electronics, which will command the actuators (via the output relays) to put the system into a safe state (e.g., removing power to electric motor of the machine, opening a valve to relieve pressure).

Figure 3. Typical protection system

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But what if there are failures in the protection system components? This is the function of the secondary switch-off, which is activated by the diagnostic (self-checking) feature of this design. However, the system is not completely fail-safe, as the design has only a certain probability of being available when being asked to carry out its safety function (it has a certain probability of failure on demand or a certain Safety Integrity Level). For example, the above design might be able to detect and tolerate certain types of output card failure, but it would not be able to withstand a failure of the input card. Therefore, its safety integrity will be much lower than that of a design with a higher-reliability input card, or improved diagnostics, or some combination of these.

There are other possible causes of card failures, including “traditional” physical faults in the hardware, systematic faults including errors in the requirements specification, implementation faults in the software and inadequate protection against environmental conditions (e.g., humidity). The diagnostics in this single-channel design may not cover all these types of faults, and therefore this will limit the Safety Integrity Level achieved in practice. (Coverage is a measure of the percentage of faults that a design can detect and handle safely.)

Technical Requirements

Parts 2 and 3 of draft IEC 1508 provide a framework for identifying the various potential causes of failure in hardware and software and for selecting design features that overcome those potential causes of failure appropriate to the required Safety Integrity Level of the safety-related system. For example, the overall technical approach for the protection system in figure 3 is shown in figure 4. The figure indicates the two basic strategies for overcoming faults and failures: (1) fault avoidance, where care is taken in to prevent faults being created; and (2) fault tolerance, where the design is created specifically to tolerate specified faults. The single-channel system mentioned above is an example of a (limited) fault tolerant design where diagnostics are used to detect certain faults and put the system into a safe state before a dangerous failure can occur.

Figure 4. Design specification: Design solution

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Fault avoidance

Fault avoidance attempts to prevent faults being introduced into a system. The main approach is to use a systematic method of managing the project so that safety is treated as a definable and manageable quality of a system, during design and then subsequently during operation and maintenance. The approach, which is similar to quality assurance, is based on the concept of feedback and involves: (1) planning (defining safety objectives, identifying the ways and means to achieve the objectives); (2) measuring achievement against the plan during implementation and (3) applying feedback to correct for any deviations. Design reviews are a good example of a fault avoidance technique. In IEC 1508 this “quality” approach to fault avoidance is facilitated by the requirements to use a safety lifecycle and employ safety management procedures for both hardware and software. For the latter, these often manifest themselves as software quality assurance procedures such as those described in ISO 9000-3 (1990).

In addition, Parts 2 and 3 of IEC 1508 (concerning hardware and software, respectively) grade certain techniques or measures that are considered useful for fault avoidance during the various safety lifecycle phases. Table 2 gives an example from Part 3 for the design and development phase of software. The designer would use the table to assist in the selection of fault avoidance techniques, depending on the required Safety Integrity Level. With each technique or measure in the tables there is a recommendation for each Safety Integrity Level, 1 to 4. The range of recommendations covers Highly Recommended (HR), Recommended (R), Neutral—neither for or against (—) and Not Recommended (NR).

Table 2. Software design and development

Technique/measure

SIL 1

SIL 2

SIL 3

SIL 4

1. Formal methods including, for example, CCS, CSP, HOL, LOTOS

R

R

HR

2. Semi-formal methods

HR

HR

HR

HR

3. Structured. Methodology including, for example, JSD, MASCOT, SADT, SSADM and YOURDON

HR

HR

HR

HR

4. Modular approach

HR

HR

HR

HR

5. Design and coding standards

R

HR

HR

HR

HR = highly recommended; R = recommended; NR = not recommended;— = neutral: the technique/measure is neither for or against the SIL.
Note: a numbered technique/measure shall be selected according to the safety integrity level.

Fault tolerance

IEC 1508 requires increasing levels of fault tolerance as the safety integrity target increases. The standard recognizes, however, that fault tolerance is more important when systems (and the components that make up those systems) are complex (designated as Type B in IEC 1508). For less complex, “well proven” systems, the degree of fault tolerance can be relaxed.

Tolerance against random hardware faults

Table 3 shows the requirements for fault tolerance against random hardware failures in complex hardware components (e.g., microprocessors) when used in a protection system such as is shown in figure 3. The designer may need to consider an appropriate combination of diagnostics, fault tolerance and manual proof checks to overcome this class of fault, depending on the required Safety Integrity Level.


Table 3. Safety Integrity Level - Fault requirements for Type B components1

1    Safety-related undetected faults shall be detected by the proof check.

2    For components without on-line medium diagnostic coverage, the system shall be able to perform the safety function in the presence of a single fault. Safety-related undetected faults shall be detected by the proof check.

3    For components with on-line high diagnostic coverage, the system shall be able to perform the safety function in the presence of a single fault. For components without on-line high diagnostic coverage, the system shall be able to perform the safety function in the presence of two faults. Safety-related undetected faults shall be detected by the proof check.

4    The components shall be able to perform the safety function in the presence of two faults. Faults shall be detected with on-line high diagnostic coverage. Safety-related undetected faults shall be detected by the proof check. Quantitative hardware analysis shall be based on worst-case assumptions.

1Components whose failure modes are not well defined or testable, or for which there are poor failure data from field experience (e.g., programmable electronic components).


IEC 1508 aids the designer by providing design specification tables (see table 4) with design parameters indexed against the Safety Integrity Level for a number of commonly used protection system architectures.

Table 4. Requirements for Safety Integrity Level 2 - Programmable electronic system architectures for protection systems

PE system configuration

Diagnostic coverage per channel

Off-line proof test Interval (TI)

Mean time to spurious trip

Single PE, Single I/O, Ext. WD

High

6 months

1.6 years

Dual PE, Single I/O

High

6 months

10 years

Dual PE, Dual I/O, 2oo2

High

3 months

1,281 years

Dual PE, Dual I/O, 1oo2

None

2 months

1.4 years

Dual PE, Dual I/O, 1oo2

Low

5 months

1.0 years

Dual PE, Dual I/O, 1oo2

Medium

18 months

0.8 years

Dual PE, Dual I/O, 1oo2

High

36 months

0.8 years

Dual PE, Dual I/O, 1oo2D

None

2 months

1.9 years

Dual PE, Dual I/O, 1oo2D

Low

4 months

4.7 years

Dual PE, Dual I/O, 1oo2D

Medium

18 months

18 years

Dual PE, Dual I/O, 1oo2D

High

48+ months

168 years

Triple PE, Triple I/O, IPC, 2oo3

None

1 month

20 years

Triple PE, Triple I/O, IPC, 2oo3

Low

3 months

25 years

Triple PE, Triple I/O, IPC, 2oo3

Medium

12 months

30 years

Triple PE, Triple I/O, IPC, 2oo3

High

48+ months

168 years

 

The first column of the table represents architectures with varying degrees of fault tolerance. In general, architectures placed near the bottom of the table have a higher degree of fault tolerance than those near the top. A 1oo2 (one out of two) system is able to withstand any one fault, as can 2oo3.

The second column describes the percentage coverage of any internal diagnostics. The higher the level of the diagnostics, the more faults will be trapped. In a protection system this is important because, provided the faulty component (e.g., an input card) is repaired within a reasonable time (often 8 hours), there is little loss in functional safety. (Note: this would not be the case for a continuous control system, because any fault is likely to cause an immediate unsafe condition and the potential for an incident.)

The third column shows the interval between proof tests. These are special tests that are required to be carried out to thoroughly exercise the protection system to ensure that there are no latent faults. Typically these are carried out by the equipment vendor during plant shutdown periods.

The fourth column shows the spurious trip rate. A spurious trip is one that causes the plant or equipment to shut down when there is no process deviation. The price for safety is often a higher spurious trip rate. A simple redundant protection system—1oo2—has, with all other design factors unchanged, a higher Safety Integrity Level but also a higher spurious trip rate than a single-channel (1oo1) system.

If one of the architectures in the table is not being used or if the designer wants to carry out a more fundamental analysis, then IEC 1508 allows this alternative. Reliability engineering techniques such as Markov modelling can then be used to calculate the hardware element of the Safety Integrity Level (Johnson 1989; Goble 1992).

Tolerance against systematic and common cause failures

This class of failure is very important in safety systems and is the limiting factor on the achievement of safety integrity. In a redundant system a component or subsystem, or even the whole system, is duplicated to achieve a high reliability from lower-reliability parts. Reliability improvement occurs because, statistically, the chance of two systems failing simultaneously by random faults will be the product of the reliabilities of the individual systems, and hence much lower. On the other hand, systematic and common cause faults cause redundant systems to fail coincidentally when, for example, a specification error in the software leads the duplicated parts to fail at the same time. Another example would be the failure of a common power supply to a redundant system.

IEC 1508 provides tables of engineering techniques ranked against the Safety Integrity Level considered effective in providing protection against systematic and common cause failures.

Examples of techniques providing defences against systematic failures are diversity and analytical redundancy. The basis of diversity is that if a designer implements a second channel in a redundant system using a different technology or software language, then faults in the redundant channels can be regarded as independent (i.e., a low probability of coincidental failure). However, particularly in the area of software-based systems, there is some suggestion that this technique may not be effective, as most mistakes are in the specification. Analytical redundancy attempts to exploit redundant information in the plant or machine to identify faults. For the other causes of systematic failure—for example, external stresses—the standard provides tables giving advice on good engineering practices (e.g., separation of signal and power cables) indexed against Safety Integrity Level.

Conclusions

Computer-based systems offer many advantages—not only economic, but also the potential for improving safety. However, the attention to detail required to realize this potential is significantly greater than is the case using conventional system components. This article has outlined the main technical requirements that a designer needs to take into account to successfully exploit this technology.

 

Back

This article discusses the design and implementation of safety- related control systems which deal with all types of electrical, electronic and programmable-electronic systems (including computer-based systems). The overall approach is in accordance with proposed International Electrotechnical Commission (IEC) Standard 1508 (Functional Safety: Safety-Related 

Systems) (IEC 1993).

Background

During the 1980s, computer-based systems—generically referred to as programmable electronic systems (PESs)—were increasingly being used to carry out safety functions. The primary driving forces behind this trend were (1) improved functionality and economic benefits (particularly considering the total life cycle of the device or system) and (2) the particular benefit of certain designs, which could be realized only when computer technology was used. During the early introduction of computer-based systems a number of findings were made:

  • The introduction of computer control was poorly thought out and planned.
  • Inadequate safety requirements were specified.
  • Inadequate procedures were developed with respect to the validation of software.
  • Evidence of poor workmanship was disclosed with respect to the standard of plant installation.
  • Inadequate documentation was generated and not adequately validated with respect to what was actually in the plant (as distinct from what was thought to be in the plant).
  • Less than fully effective operation and maintenance procedures had been established.
  • There was evidently justified concern about the competence of persons to perform the duties required of them.

 

In order to solve these problems, several bodies published or began developing guidelines to enable the safe exploitation of PES technology. In the United Kingdom, the Health and Safety Executive (HSE) developed guidelines for programmable electronic systems used for safety-related applications, and in Germany, a draft standard (DIN 1990) was published. Within the European Community, an important element in the work on harmonized European Standards concerned with safety-related control systems (including those employing PESs) was started in connection with the requirements of the Machinery Directive. In the United States, the Instrument Society of America (ISA) has produced a standard on PESs for use in the process industries, and the Center for Chemical Process Safety (CCPS), a directorate of the American Institute of Chemical Engineers, has produced guidelines for the chemical process sector.

A major standards initiative is currently taking place within the IEC to develop a generically based international standard for electrical, electronic and programmable electronic (E/E/PES) safety-related systems that could be used by the many applications sectors, including the process, medical, transport and machinery sectors. The proposed IEC international standard comprises seven Parts under the general title IEC 1508. Functional safety of electrical/electronic/programmable electronic safety-related systems. The various Parts are as follows:

  • Part 1.General requirements
  • Part 2.Requirements for electrical, electronic and programmable electronic systems
  • Part 3.Software requirements
  • Part 4.Definitions
  • Part 5.Examples of methods for the determination of safety integrity levels
  • Part 6.Guidelines on the application of Parts 2 and 3
  • Part 7.Overview of techniques and measures.

 

When finalized, this generically based International Standard will constitute an IEC basic safety publication covering functional safety for electrical, electronic and programmable electronic safety-related systems and will have implications for all IEC standards, covering all application sectors as regards the future design and use of electrical/electronic/programmable electronic safety-related systems. A major objective of the proposed standard is to facilitate the development of standards for the various sectors (see figure 1).

Figure 1. Generic and application sector standards

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PES Benefits and Problems

The adoption of PESs for safety purposes had many potential advantages, but it was recognized that these would be achieved only if appropriate design and assessment methodologies were used, because: (1) many of the features of PESs do not enable the safety integrity (that is, the safety performance of the systems carrying out the required safety functions) to be predicted with the same degree of confidence that has traditionally been available for less complex hardware-based (“hardwired”) systems; (2) it was recognized that while testing was necessary for complex systems, it was not sufficient on its own. This meant that even if the PES was implementing relatively simple safety functions, the level of complexity of the programmable electronics was significantly greater than that of the hardwired systems they were replacing; and (3) this rise in complexity meant that the design and assessment methodologies had to be given much more consideration than previously, and that the level of personal competence required to achieve adequate levels of performance of the safety-related systems was subsequently greater.

The benefits of computer-based PESs include the following:

  • the ability to perform on-line diagnostic proof checks on critical components at a frequency significantly higher than would otherwise be the case
  • the potential to provide sophisticated safety interlocks
  • the ability to provide diagnostic functions and condition monitoring which can be used to analyse and report on the performance of plant and machinery in real time
  • the capability of comparing actual conditions of the plant with “ideal” model conditions
  • the potential to provide better information to operators and hence to improve decision-making affecting safety
  • the use of advanced control strategies to enable human operators to be located remotely from hazardous or hostile environments
  • the ability to diagnose the control system from a remote location.

 

The use of computer-based systems in safety-related applications creates a number of problems which need to be adequately addressed, such as the following:

  • The failure modes are complex and not always predictable.
  • Testing the computer is necessary but is not sufficient in itself to establish that the safety functions will be performed with the degree of certainty required for the application.
  • Microprocessors may have subtle variations between different batches, and therefore different batches may display different behaviour.
  • Unprotected computer-based systems are particularly susceptible to electrical interference (radiated interference; electrical “spikes” in the mains supplies, electrostatic discharges, etc.).
  • It is difficult and often impossible to quantify the probability of failure of complex safety-related systems incorporating software. Because no method of quantification has been widely accepted, software assurance has been based on procedures and standards which describe the methods to be used in the design, implementation and maintenance of the software.

 

Safety Systems under Consideration

The types of safety-related systems under consideration are electrical, electronic and programmable electronic systems (E/E/PESs). The system includes all elements, particularly signals extending from sensors or from other input devices on the equipment under control, and transmitted via data highways or other communication paths to the actuators or other output devices (see figure 2).

Figure 2. Electrical, electronic and programmable electronic system (E/E/PES)

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The term electrical, electronic and programmable electronic device has been used to encompass a wide variety of devices and covers the following three chief classes:

  1. electrical devices such as electro-mechanical relays
  2. electronic devices such as solid state electronic instruments and logic systems
  3. programmable electronic devices, which includes a wide variety of computer-based systems such as the following:
  • microprocessors
  • micro-controllers
  • programmable controllers (PCs)
  • application-specific integrated circuits (ASICs)
  • programmable logic controllers (PLCs)
  • other computer-based devices (e.g., “smart” sensors, transmitters and actuators).

 

By definition, a safety-related system serves two purposes:

  1. It implements the required safety functions necessary to achieve a safe state for the equipment under control or maintains a safe state for the equipment under control. The safety-related system must perform those safety functions that are specified in the safety functions requirements specification for the system. For example, the safety functions requirements specification may state that when the temperature reaches a certain value x, valve y shall open to allow water to enter the vessel.
  2. It achieves, on its own or with other safety-related systems, the necessary level of safety integrity for the implementation of the required safety functions. The safety functions must be performed by the safety-related systems with the degree of confidence appropriate to the application in order to achieve the required level of safety for the equipment under control.

 

This concept is illustrated in figure 3.

Figure 3. Key features of safety-related systems

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System Failures

In order to ensure safe operation of E/E/PES safety-related systems, it is necessary to recognize the various possible causes of safety-related system failure and to ensure that adequate precautions are taken against each. Failures are classified into two categories, as illustrated in figure 4.

Figure 4. Failure categories

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  1. Random hardware failures are those failures which result from a variety of normal degradation mechanisms in the hardware. There are many such mechanisms occurring at different rates in different components, and since manufacturing tolerances cause components to fail on account of these mechanisms after different times in operation, failures of a total item of equipment comprising many components occur at unpredictable (random) times. Measures of system reliability, such as the mean time between failures (MTBF), are valuable but are usually concerned only with random hardware failures and do not include systematic failures.
  2. Systematic failures arise from errors in the design, construction or use of a system which cause it to fail under some particular combination of inputs or under some particular environmental condition. If a system failure occurs when a particular set of circumstances arises, then whenever those circumstances arise in the future there will always be a system failure. Any failure of a safety-related system which does not arise from a random hardware failure is, by definition, a systematic failure. Systematic failures, in the context of E/E/PES safety-related systems, include:
  • systematic failures due to errors or omissions in the safety functions requirements specification
  • systematic failures due to errors in the design, manufacture, installation or operation of the hardware. These would include failures arising from environmental causes and human (e.g., operator) error
  • systematic failures due to faults in the software
  • systematic failures due to maintenance and modification errors.

 

Protection of Safety-Related Systems

The terms that are used to indicate the precautionary measures required by a safety-related system to protect against random hardware failures and systematic failures are hardware safety integrity measures and systematic safety integrity measures respectively. Precautionary measures that a safety-related system can bring to bear against both random hardware failures and systematic failures are termed safety integrity. These concepts are illustrated in figure 5.

Figure 5. Safety performance terms

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Within the proposed international standard IEC 1508 there are four levels of safety integrity, denoted Safety Integrity Levels 1, 2, 3 and 4. Safety Integrity Level 1 is the lowest safety integrity level and Safety Integrity Level 4 is the highest. The Safety Integrity Level (whether 1, 2, 3 or 4) for the safety-related system will depend upon the importance of the role the safety-related system is playing in achieving the required level of safety for the equipment under control. Several safety-related systems may be necessary—some of which may be based on pneumatic or hydraulic technology.

Design of Safety-Related Systems

A recent analysis of 34 incidents involving control systems (HSE) found that 60% of all cases of failure had been “built in” before the safety-related control system had been put into use (figure 7). Consideration of all the safety life cycle phases is necessary if adequate safety-related systems are to be produced.

Figure 7. Primary cause (by phase) of control system failure

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Functional safety of safety-related systems depends not only on ensuring that the technical requirements are properly specified but also in ensuring that the technical requirements are effectively implemented and that the initial design integrity is maintained throughout the life of the equipment. This can be realized only if an effective safety management system is in place and the people involved in any activity are competent with respect to the duties they have to perform. Particularly when complex safety-related systems are involved, it is essential that an adequate safety management system is in place. This leads to a strategy that ensures the following:

  • An effective safety management system is in place.
  • The technical requirements that are specified for the E/E/PES safety-related systems are sufficient to deal with both random hardware and systematic failure causes.
  • The competence of the people involved is adequate for the duties they have to perform.

 

In order to address all the relevant technical requirements of functional safety in a systematic manner, the concept of the Safety Lifecycle has been developed. A simplified version of the Safety Lifecycle in the emerging international standard IEC 1508 is shown in figure 8. The key phases of the Safety Lifecycle are:

Figure 8. Role of the Safety Lifecycle in achieving functional safety

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  • specification
  • design and implementation
  • installation and commissioning
  • operation and maintenance
  • changes after commissioning.

 

Level of Safety

The design strategy for the achievement of adequate levels of safety integrity for the safety-related systems is illustrated in figure 9 and figure 10. A safety integrity level is based on the role the safety-related system is playing in the achievement of the overall level of safety for the equipment under control. The safety integrity level specifies the precautions that need to be taken into account in the design against both random hardware and systematic failures.

Figure 9. Role of safety integrity levels in the design process

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Figure 10. Role of the Safety Lifecycle in the specification and design process

SA059F10

The concept of safety and level of safety applies to the equipment under control. The concept of functional safety applies to the safety-related systems. Functional safety for the safety-related systems has to be achieved if an adequate level of safety is to be achieved for the equipment that is giving rise to the hazard. The specified level of safety for a specific situation is a key factor in the safety integrity requirements specification for the safety-related systems.

The required level of safety will depend upon many factors—for example, the severity of injury, the number of people exposed to danger, the frequency with which people are exposed to danger and the duration of the exposure. Important factors will be the perception and views of those exposed to the hazardous event. In arriving at what constitutes an appropriate level of safety for a specific application, a number of inputs are considered, which include the following:

  • legal requirements relevant to the specific application
  • guidelines from the appropriate safety regulatory authority
  • discussions and agreements with the different parties involved in the application
  • industry standards
  • national and international standards
  • the best independent industrial, expert and scientific advice.

 

Summary

When designing and using safety-related systems, it must be remembered that it is the equipment under control that creates the potential hazard. The safety-related systems are designed to reduce the frequency (or probability) of the hazardous event and/or the consequences of the hazardous event. Once the level of safety has been set for the equipment, the safety integrity level for the safety-related system can be determined, and it is the safety integrity level that allows the designer to specify the precautions that need to be built into the design to be deployed against both random hardware and systematic failures.

 

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Monday, 04 April 2011 18:41

Safety Principles for Industrial Robots

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Industrial robots are found throughout industry wherever high productivity demands must be met. The use of robots, however, requires design, application and implementation of the appropriate safety controls in order to avoid creating hazards to production personnel, programmers, maintenance specialists and system engineers.

Why Are Industrial Robots Dangerous?

One definition of robots is “moving automatic machines that are freely programmable and are able to operate with little or no human interface”. These types of machines are currently used in a wide variety of applications throughout industry and medicine, including training. Industrial robots are being increasingly used for key functions, such as new manufacturing strategies (CIM, JIT, lean production and so on) in complex installations. Their number and breadth of applications and the complexity of the equipment and installations result in hazards such as the following:

  • movements and sequences of movements that are almost impossible to follow, as the robot’s high-speed movements within its radius of action often overlap with those of other machines and equipment
  • release of energy caused by flying parts or beams of energy such as those emitted by lasers or by water jets
  • free programmability in terms of direction and speed
  • susceptibility to influence by external errors (e.g., electromagnetic compatibility)
  • human factors.

 

Investigations in Japan indicate that more than 50% of working accidents with robots can be attributed to faults in the electronic circuits of the control system. In the same investigations, “human error” was responsible for less than 20%. The logical conclusion of this finding is that hazards which are caused by system faults cannot be avoided by behavioural measures taken by human beings. Designers and operators therefore need to provide and implement technical safety measures (see figure 1).

Figure 1. Special operating control system for the setting up of a mobile welding robot

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Accidents and Operating Modes

Fatal accidents involving industrial robots began to occur in the early 1980s. Statistics and investigations indicate that the majority of incidents and accidents do not take place in normal operation (automatic fulfilment of the assignment concerned). When working with industrial robot machines and installations, there is an emphasis on special operation modes such as commissioning, setting up, programming, test runs, checks, troubleshooting or maintenance. In these operating modes, persons are usually in a danger zone. The safety concept must protect personnel from negative events in these types of situations.

International Safety Requirements

The 1989 EEC Machinery Directive (89/392/EEC (see the article “Safety principles for CNC machine tools” in this chapter and elsewhere in this Encyclopaedia)) establishes the principal safety and health requirements for machines. A machine is considered to be the sum total of interlinked parts or devices, of which at least one part or device can move and correspondingly has a function. Where industrial robots are concerned, it must be noted that the entire system, not just one single piece of equipment on the machine, must meet the safety requirements and be fitted with the appropriate safety devices. Hazard analysis and risk assessment are suitable methods of determining whether these requirements have been satisfied (see figure 2).

Figure 2. Block diagram for a personnel security system

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Requirements and Safety Measures in Normal Operation

The use of robot technology places maximum demands on hazard analysis, risk assessment and safety concepts. For this reason, the following examples and suggestions can serve only as guidelines:

1. Given the safety goal that manual or physical access to hazardous areas involving automatic movements must be prevented, suggested solutions include the following:

  • Prevent manual or physical access into danger zones by means of mechanical barriers.
  • Use safety devices of the sort which respond when approached (light barriers, safety mats), and take care to switch off machinery safely when accessed or entered.
  • Permit manual or physical access only when the entire system is in a safe state. For example, this can be achieved by the use of interlocking devices with closure mechanisms on the access doors.

 

2. Given the safety goal that no person may be injured as a result of the release of energy (flying parts or beams of energy), suggested solutions include:

  • Design should prevent any release of energy (e.g., correspondingly dimensioned connections, passive gripper interlocking devices for gripper change mechanisms, etc.).
  • Prevent the release of energy from the danger zone, for example, by a correspondingly dimensioned safety hood.

 

3. The interfaces between normal operation and special operation (e.g., door interlocking devices, light barriers, safety mats) are necessary to enable the safety control system to automatically recognize the presence of personnel.

Demands and Safety Measures in Special Operation Modes

Certain special operation modes (e.g., setting up, programming) on an industrial robot require movements which must be assessed directly at the site of operation. The relevant safety goal is that no movements may endanger the persons involved. The movements should be

  • only of the scheduled style and speed
  • prolonged only as long as instructed
  • those which may be performed only if it can be guaranteed that no parts of the human body are in the danger zone.

 

A suggested solution to this goal could involve the use of special operating control systems which permit only controllable and manageable movements using acknowledgeable controls. The speed of movements is thus safely reduced (energy reduction by the connection of an isolation transformer or the use of fail-safe state monitoring equipment) and the safe condition is acknowledged before the control is allowed to activate (see figure 3).

Figure 3. Six-axis industrial robot in a safety cage with material gates

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Demands on Safety Control Systems

One of the features of a safety control system must be that the required safety function is guaranteed to work whenever any faults arise. Industrial robot machines should be almost instantaneously directed from a hazardous state to a safe state. Safety control measures needed to achieve this include the following safety goals:

  • A fault in the safety control system may not trigger off a hazardous state.
  • A fault in the safety control system must be identified (immediately or at intervals).

Suggested solutions to providing reliable safety control systems would be:

  • redundant and diverse layout of electro-mechanical control systems including test circuits
  • redundant and diverse set-up of microprocessor control systems developed by different teams. This modern approach is considered to be state-of-the-art; for example, those complete with safety light barriers.

 

Safety Goals for the Construction and Use of Industrial Robots.

When industrial robots are built and used, both manufacturers as well as users are required to install state-of-the-art safety controls. Apart from the aspect of legal responsibility, there may also be a moral obligation to ensure that robot technology is also a safe technology.

Normal operation mode

The following safety conditions should be provided when robot machines are operating in the normal mode:

  • The field of movement of the robot and the processing areas used by peripheral equipment must be secured in such a way as to prevent manual or physical access by persons to areas which are hazardous as a result of automatic movements.
  • Protection should be provided so that flying workpieces or tools are not allowed to cause damage.
  • No persons must be injured by parts, tools or workpieces ejected by the robot or by the release of energy, due to faulty gripper(s), gripper power failure, inadmissible speed, collision(s) or faulty workpiece(s).
  • No persons may be injured by the release of energy or by parts ejected by peripheral equipment.
  • Feed and removal apertures must be designed to prevent manual or physical access to areas which are hazardous as a result of automatic movements. This condition must also be fulfilled when production material is removed. If production material is fed to the robot automatically, no hazardous areas may be created by feed and removal apertures and the moving production material.

 

Special operation modes

The following safety conditions should be provided when robot machines are operating in special modes:

The following must be prevented during rectification of a breakdown in the production process:

  • manual or physical access to areas which are hazardous due to automatic movements by the robot or by peripheral equipment
  • hazards which arise from faulty behaviour on the part of the system or from inadmissible command input if persons or parts of the body are in the area exposed to hazardous movements
  • hazardous movements or conditions initiated by the movement or removal of production material or waste products
  • injuries caused by peripheral equipment
  • movements that have to be carried out with the safety guard(s) for normal operation removed, to be carried out only within the operational scope and speed, and only as long as instructed. Additionally, no person(s) or parts of the body may be present in the area at risk.

 

The following safe conditions should be assured during set up:

No hazardous movements may be initiated as a result of a faulty command or incorrect command input.

  • The replacement of robot machine or peripheral parts must not initiate any hazardous movements or conditions.
  • If movements have to be carried out with the safety guard(s) for normal operation removed when conducting setting-up operations, such movements may be carried out only within the directed scope and speed and only as long as instructed. Additionally, no person(s) or parts of the body may be present in the area at risk.
  • During setting-up operations, the peripheral equipment must not make any hazardous movements or initiate any hazardous conditions.

 

During programming, the following safety conditions are applicable:

  • Manual or physical access to areas which are hazardous due to automatic movements must be prevented.
  • If movements are carried out with the safety guard(s) for normal operation removed, the following conditions must be fulfilled:
  • (a)Only the command to move may be carried out, and only for as long as it is issued.
  • (b)Only controllable movements may be carried out (i.e., they must be clearly visible, low-speed movements).
  • (c)Movements may be initiated only if they do not constitute a hazard to the programmer or other persons.
  • Peripheral equipment must not represent a hazard to the programmer or other persons.

 

Safe test operations require the following precautions:

Prevent manual or physical access to areas which are hazardous due to automatic movements.

  • Peripheral equipment must not be a source of danger.

 

When inspecting robot machines, safe procedures include the following:

  • If it is necessary to enter the robot’s field of movement for inspection purposes, this is permissible only if the system is in a safe state.
  • Hazards caused by faulty behaviour on the part of the system or by inadmissible command input must be prevented.
  • Peripheral equipment must not be a source of danger to inspection personnel.

 

Troubleshooting often requires starting the robot machine while it is in a potentially hazardous condition, and special safe work procedures such as the following should be implemented:

  • Access to areas which are hazardous as a result of automatic movements must be prevented.
  • The starting up of a drive unit as a result of a faulty command or false command input must be prevented.
  • In handling a defective part, all movements on the part of the robot must be prevented.
  • Injuries caused by machine parts which are ejected or fall off must be prevented.
  • If, during troubleshooting, movements have to be carried out with the safety guard(s) for normal operation removed, such movements may be carried out only within the scope and speed laid down and only as long as instructed. Additionally, no person(s) or parts of the body may be present in the area at risk.
  • Injuries caused by peripheral equipment must be prevented.

 

Remedying a fault and maintenance work also may require start-up while the machine is in an unsafe condition, and therefore require the following precautions:

  • The robot must not be able to start up.
  • The handling of various machine parts, either manually or with ancillary equipment, must be possible without risk of exposure to hazards.
  • It must not be possible to touch parts that are “live”.
  • Injuries caused by the escape of liquid or gaseous media must be prevented.
  • Injuries caused by peripheral equipment must be prevented.

 

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Monday, 04 April 2011 18:33

Safety Principles for CNC Machine Tools

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Whenever simple and conventional production equipment, such as machine tools, is automated, the result is complex technical systems as well as new hazards. This automation is achieved through the use of computer numeric control (CNC) systems on machine tools, called CNC machine tools (e.g., milling machines, machining centres, drills and grinders). In order to be able to identify the potential hazards inherent in automatic tools, the various operating modes of each system should be analysed. Previously conducted analyses indicate that a differentiation should be made between two types of operation: normal operation and special operation.

It is often impossible to prescribe the safety requirements for CNC machine tools in the shape of specific measures. This may be because there are too few regulations and standards specific to the equipment which provide concrete solutions. Safety requirements can be determined only if the possible hazards are identified systematically by conducting a hazard analysis, particularly if these complex technical systems are fitted with freely programmable control systems (as with CNC machine tools).

In the case of newly developed CNC machine tools, the manufacturer is obliged to carry out a hazard analysis on the equipment in order to identify whatever dangers may be present and to show by means of constructive solutions that all dangers to persons, in all of the different operating modes, are eliminated. All the hazards identified must be subjected to a risk assessment wherein each risk of an event is dependent on the scope of damage and the frequency with which it may occur. The hazard to be assessed is also given a risk category (minimized, normal, increased). Wherever the risk cannot be accepted on the basis of the risk assessment, solutions (safety measures) must be found. The purpose of these solutions is to reduce the frequency of occurrence and the scope of damage of an unplanned and potentially hazardous incident (an “event”).

The approaches to solutions for normal and increased risks are to be found in indirect and direct safety technology; for minimized risks, they are to be found in referral safety technology:

  • Direct safety technology. Care is taken at the design stage to eliminate any hazards (e.g., the elimination of shearing and trapping points).
  • Indirect safety technology. The hazard remains. However, the addition of technical arrangements prevents the hazard from turning into an event (e.g., such arrangements may include the prevention of access to dangerous moving parts by means of physical safety hoods, the provision of safety devices which turn power off, shielding from flying parts using safety guards, etc.).
  • Referral safety technology. This applies only to residual hazards and minimized risks—that is, hazards which can lead to an event as a result of human factors. The occurrence of such an event can be prevented by appropriate behaviour on the part of the person concerned (e.g., instructions on behaviour in the operating and maintenance manuals, personnel training, etc.).

 

International Safety Requirements

The EC Machinery Directive (89/392/EEC) of 1989 lays down the principal safety and health requirements for machines. (According to the Machinery Directive, a machine is considered to be the sum total of interlinked parts or devices, of which at least one can move and correspondingly has a function.) In addition, individual standards are created by international standardization bodies to illustrate possible solutions (e.g., by attending to fundamental safety aspects, or by examining electrical equipment fitted to industrial machinery). The aim of these standards is to specify protection goals. These international safety requirements give manufacturers the necessary legal basis to specify these requirements in the above-mentioned hazard analyses and risk assessments.

Operating Modes

When using machine tools, a differentiation is made between normal operation and special operation. Statistics and investigations indicate that the majority of incidents and accidents do not take place in normal operation (i.e., during the automatic fulfilment of the assignment concerned). With these types of machines and installations, there is an emphasis on special modes of operations such as commissioning, setting up, programming, test runs, checks, troubleshooting or maintenance. In these operating modes, persons are usually in a danger zone. The safety concept must protect personnel from harmful events in these types of situations.

Normal operation

The following applies to automatic machines when carrying out normal operation: (1) the machine fulfils the assignment for which it was designed and constructed without any further intervention by the operator, and (2) applied to a simple turning machine, this means that a workpiece is turned to the correct shape and chips are produced. If the workpiece is changed manually, changing the workpiece is a special mode of operation.

Special modes of operation

Special modes of operation are working processes which allow normal operation. Under this heading, for example, one would include workpiece or tool changes, rectifying a fault in a production process, rectifying a machine fault, setting up, programming, test runs, cleaning and maintenance. In normal operation, automatic systems fulfil their assignments independently. From the viewpoint of working safety, however, automatic normal operation becomes critical when the operator has to intervene working processes. Under no circumstances may the persons intervening in such processes be exposed to hazards.

Personnel

Consideration must be given to the persons working in the various modes of operation as well as to third parties when safeguarding machine tools. Third parties also include those indirectly concerned with the machine, such as supervisors, inspectors, assistants for transporting material and dismantling work, visitors and others.

Demands and Safety Measures for Machine Accessories

Interventions for jobs in special operation modes mean that special accessories have to be used to assure work can be conducted safely. The first type of accessories include equipment and items used to intervene in the automatic process without the operator’s having to access a hazardous zone. This type of accessory includes (1) chip hooks and tongs which have been so designed that chips in the machining area can be removed or pulled away through the apertures provided in the safety guards, and (2) workpiece clamping devices with which the production material can be manually inserted into or removed from an automatic cycle

Various special modes of operation—for example, remedial work or maintenance work—make it necessary for personnel to intervene in a system. In these cases, too, there is a whole range of machine accessories designed to increase working safety—for example, devices to handle heavy grinding wheels when the latter are changed on grinders, as well as special crane slings for dismantling or erecting heavy components when machines are overhauled. These devices are the second type of machine accessory for increasing safety during work in special operations. Special operation control systems can also be considered to represent a second type of machine accessory. Particular activities can be carried out safely with such accessories—for example, a device can be set up in the machine axes when feed movements are necessary with the safety guards open.

These special operation control systems must satisfy particular safety requirements. For example, they must ensure that only the movement requested is carried out in the way requested and only for as long as requested. The special operation control system must therefore be designed in such a way as to prevent any faulty action from turning into hazardous movements or states.

Equipment which increases the degree of automation of an installation can be considered to be a third type of machine accessory for increasing working safety. Actions which were previously carried out manually are done automatically by the machine in normal operation, such as equipment including portal loaders, which change the workpieces on machine tools automatically. The safeguarding of automatic normal operation causes few problems because the intervention of an operator in the course of events is unnecessary and because possible interventions can be prevented by safety devices.

Requirements and Safety Measures for the Automation of Machine Tools

Unfortunately, automation has not led to the elimination of accidents in production plants. Investigations simply show a shift in the occurrence of accidents from normal to special operation, primarily due to the automation of normal operation so that interventions in the course of production are no longer necessary and personnel are thus no longer exposed to danger. On the other hand, highly automatic machines are complex systems which are difficult to assess when faults occur. Even the specialists employed to rectify faults are not always able to do so without incurring accidents. The amount of software needed to operate increasingly complex machines is growing in volume and complexity, with the result that an increasing number of electrical and commissioning engineers suffer accidents. There is no such thing as flawless software, and changes in software often lead to changes elsewhere which were neither expected nor wanted. In order to prevent safety from being affected, hazardous faulty behaviour caused by external influence and component failures must not be possible. This condition can be fulfilled only if the safety circuit is designed as simply as possible and is separate from the rest of the controls. The elements or sub-assemblies used in the safety circuit must also be fail-safe.

It is the task of the designer to develop designs that satisfy safety requirements. The designer cannot avoid having to consider the necessary working procedures, including the special modes of operation, with great care. Analyses must be made to determine which safe work procedures are necessary, and the operating personnel must become familiar with them. In the majority of cases, a control system for special operation will be necessary. The control system usually observes or regulates a movement, while at the same time, no other movement must be initiated (as no other movement is needed for this work, and thus none is expected by the operator). The control system does not necessarily have to carry out the same assignments in the various modes of special operation.

Requirements and Safety Measures in Normal and Special Modes of Operation

Normal operation

The specification of safety goals should not impede technical progress because adapted solutions can be selected. The use of CNC machine tools makes maximum demands on hazard analysis, risk assessment and safety concepts. The following describes several safety goals and possible solutions in greater detail.

Safety goal

  • Manual or physical access to hazardous areas during automatic movements must be prevented.

 

Possible solutions

  • Prevent manual or physical access into danger zones by means of mechanical barriers.
  • Provide safety devices that respond when approached (light barriers, safety mats) and switch off machinery safely during interventions or entry.
  • Allow manual or physical access to machinery (or its vicinity) only when the entire system is in a safe state (e.g., by using interlocking devices with closure mechanisms on the access doors).

 

Safety goal

  • The possibility of any persons being injured as a result of the release of energy (flying parts or beams of energy) should be eliminated.

 

Possible solution

  • Prevent the release of energy from the danger zone—for example, by a correspondingly dimensioned safety hood.

 

Special operation

The interfaces between normal operation and special operation (e.g., door interlocking devices, light barriers, safety mats) are necessary to enable the safety control system to recognize automatically the presence of personnel. The following describes certain special operation modes (e.g., setting up, programming) on CNC machine tools which require movements that must be assessed directly at the site of operation.

Safety goals

  • Movements must take place only in such a way that they cannot be a hazard for the persons concerned. Such movements must be executed only in the scheduled style and speed and continued only as long as instructed.
  • They are to be attempted only if it can be guaranteed that no parts of the human body are in the danger zone.

 

Possible solution

  • Install special operating control systems which permit only controllable and manageable movements using finger-tip control via “acknowledge-type” push buttons. The speed of movements is thus safely reduced (provided that energy has been reduced by means of an isolation transformer or similar monitoring equipment).

 

Demands on Safety Control Systems

One of the features of a safety control system must be that the safety function is guaranteed to work whenever any faults arise so as to direct processes from a hazardous state to a safe state.

Safety goals

  • A fault in the safety control system must not trigger off a dangerous state.
  • A fault in the safety control system must be identified (immediately or at intervals).

 

Possible solutions

  • Put in place a redundant and diverse layout of electro-mechanical control systems, including test circuits.
  • Put in place a redundant and diverse set-up of microprocessor control systems developed by different teams. This approach is considered to be state of the art, for example, in the case of safety light barriers.

 

Conclusion

It is apparent that the increasing trend in accidents in normal and special modes of operation cannot be halted without a clear and unmistakable safety concept. This fact must be taken into account in the preparation of safety regulations and guidelines. New guidelines in the shape of safety goals are necessary in order to allow advanced solutions. This objective enables designers to choose the optimum solution for a specific case while at the same time demonstrating the safety features of their machines in a fairly simple way by describing a solution to each safety goal. This solution can then be compared with other existing and accepted solutions, and if it is better or at least of equal value, a new solution can then be chosen. In this way, progress is not hampered by narrowly formulated regulations.


Main Features of the EEC Machinery Directive

The Council Directive of 14 June 1989 on the approximation of the laws of the Member States relating machinery (89/392/EEC) applies to each individual state.

  • Each individual state must integrate the directive in its legislation.
  • Valid from 1 January 1993.
  • Requires that all manufacturers adhere to the state of the art.
  • The manufacturer must produce a technical construction file which contains full information on all fundamental aspects of safety and health care.
  • The manufacturer must issue the declaration of conformity and the CE marking of the machines.
  • Failure to place a complete technical documentation at the disposal of a state supervisory centre is considered to represent the non-fulfilment of the machine guidelines. A pan-EEC sales prohibition may be the consequence.

 

Safety Goals for the Construction and Use of CNC Machine Tools

1. Lathes

1.1            Normal mode of operation

1.1.1            The work area is to be safeguarded so that it is impossible to reach or step into the danger zones of automatic movements, either intentionally or unintentionally.

1.1.2             The tool magazine is to be safeguarded so that it is impossible to reach or step into the danger zones of automatic movements, either intentionally or unintentionally.

1.1.3             The workpiece magazine is to be safeguarded so that it is impossible to reach or step into the danger zones of automatic movements, either intentionally or unintentionally.

1.1.4             Chip removal must not result in personal injury due to the chips or moving parts of the machine.

1.1.5             Personal injuries resulting from reaching into drive systems must be prevented.

1.1.6             The possibility of reaching into the danger zones of moving chip conveyors must be prevented.

1.1.7             No personal injury to operators or third persons must result from flying workpieces or parts thereof.

For example, this can occur

  • due to insufficient clamping
  • due to inadmissible cutting force
  • due to inadmissible rotation speed
  • due to collision with the tool or machine parts
  • due to workpiece breakage
  • due to defective clamping fixtures
  • due to power failure

 

1.1.8            No personal injury must result from flying workpiece clamping fixtures.

1.1.9             No personal injury must result from flying chips.

1.1.10             No personal injury must result from flying tools or parts thereof.

For example, this can occur

  • due to material defects
  • due to inadmissible cutting force
  • due to a collision with the workpiece or a machine part
  • due to inadequate clamping or tightening

 

1.2            Special modes of operation

1.2.1             Workpiece changing.

1.2.1.1             Workpiece clamping must be done in such a way that no parts of the body can become trapped between closing clamping fixtures and workpiece or between the advancing sleeve tip and workpiece.

1.2.1.2             The starting of a drive (spindles, axes, sleeves, turret heads or chip conveyors) as a consequence of a defective command or invalid command must be prevented.

1.2.1.3             It must be possible to manipulate the workpiece manually or with tools without danger.

1.2.2             Tool changing in tool holder or tool turret head.

1.2.2.1             Danger resulting from the defective behaviour of the system or due to entering an invalid command must be prevented.

1.2.3             Tool changing in the tool magazine.

1.2.3.1             Movements in the tool magazine resulting from a defective or invalid command must be prevented during tool changing.

1.2.3.2             It must not be possible to reach into other moving machine parts from the tool loading station.

1.2.3.3             It must not be possible to reach into danger zones on the further movement of the tool magazine or during the search. If taking place with the guards for normal operation mode removed, these movements may only be of the designated kind and only be carried out during the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

1.2.4             Measurement check.

1.2.4.1             Reaching into the work area must only be possible after all movements have been brought to a standstill.

1.2.4.2             The starting of a drive resulting from a defective command or invalid command input must be prevented.

1.2.5             Set-up.

1.2.5.1             If movements are executed during set-up with the guards for normal mode of operation removed, then the operator must be safeguarded by another means.

1.2.5.2             No dangerous movements or changes of movements must be initiated as a result of a defective command or invalid command input.

1.2.6             Programming.

1.2.6.1             No movements may be initiated during programming which endanger a person in the work area.

1.2.7             Production fault.

1.2.7.1             The starting of a drive resulting from a defective command on invalid command input setpoint must be prevented.

1.2.7.2             No dangerous movements or situations are to be initiated by the movement or removal of the workpiece or waste.

1.2.7.3             Where movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

1.2.8             Troubleshooting.

1.2.8.1             Reaching into the danger zones of automatic movements must be prevented.

1.2.8.2             The starting of a drive as a result of a defective command or invalid command input must be prevented.

1.2.8.3             A movement of the machine on manipulation of the defective part must be prevented.

1.2.8.4             Personal injury resulting from a machine part splintering off or dropping must be prevented.

1.2.8.5             If, during troubleshooting, movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

1.2.9             Machine malfunction and repair.

1.2.9.1             The machine must be prevented from starting.

1.2.9.2             Manipulation of the different parts of the machine must be possible either manually or with tools without any danger.

1.2.9.3             It must not be possible to touch live parts of the machine.

1.2.9.4             Personal injury must not result from the issue of fluid or gaseous media.

 

2. Milling machines

2.1            Normal mode of operation

2.1.1             The work area is to be safeguarded so that it is impossible to reach or step into the danger zones of automatic movements, either intentionally or unintentionally.

2.1.2             Chip removal must not result in personal injury due to the chips or moving parts of the machine.

2.1.3             Personal injuries resulting from reaching into drive systems must be prevented.

No personal injury to operators or third persons must result from flying workpieces or parts thereof.

For example, this can occur

  • due to insufficient clamping
  • due to inadmissible cutting force
  • due to collision with the tool or machine parts
  • due to workpiece breakage
  • due to defective clamping fixtures
  • due to power failure

 

2.1.4             No personal injury must result from flying workpiece clamping fixtures.

2.1.5             No personal injury must result from flying chips.

2.1.6             No personal injury must result from flying tools or parts thereof.

For example, this can occur

  • due to material defects
  • due to inadmissible speed of rotation
  • due to inadmissible cutting force
  • due to collision with workpiece or machine part
  • due to inadequate clamping or tightening
  • due to power failure

 

Special modes of operation

2.2.1             Workpiece changing.

2.2.1.1             Where power-operated clamping fixtures are used, it must not be possible for parts of the body to become trapped between the closing parts of the clamping fixture and the workpiece.

2.2.1.2             The starting of a drive (spindle, axis) resulting from a defective command or invalid command input must be prevented.

2.2.1.3             The manipulation of the workpiece must be possible manually or with tools without any danger.

2.2.2             Tool changing.

2.2.2.1             The starting of a drive resulting from a defective command or invalid command input must be prevented.

2.2.2.2             It must not be possible for fingers to become trapped when putting in tools.

2.2.3             Measurement check.

2.2.3.1             Reaching into the work area must only be possible after all movements have been brought to a standstill.

2.2.3.2             The starting of a drive resulting from a defective command or invalid command input must be prevented.

2.2.4             Set-up.

2.2.4.1             If movements are executed during set-up with guards for normal mode of operation removed, the operator must be safeguarded by another means.

2.2.4.2             No dangerous movements or changes of movements must be initiated as a result of a defective command or invalid command input.

2.2.5             Programming.

2.2.5.1             No movements must be initiated during programming which endanger a person in the work area.

2.2.6             Production fault.

2.2.6.1             The starting of drive resulting from a defective command or invalid command input must be prevented.

2.2.6.2             No dangerous movements or situations must be initiated by the movement or removal of the workpiece or waste.

2.2.6.3             Where movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

2.2.7             Troubleshooting.

2.2.7.1             Reaching into the danger zones of automatic movements must be prevented.

2.2.7.2             The starting of a drive as a result of a defective command or invalid command input must be prevented.

2.2.7.3             Any movement of the machine on manipulation of the defective part must be prevented.

2.2.7.4             Personal injury resulting from a machine part splintering off or dropping must be prevented.

2.2.7.5             If, during troubleshooting, movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

2.2.8             Machine malfunction and repair.

2.2.8.1             Starting the machine must be prevented.

2.2.8.2             Manipulation of the different parts of the machine must be possible manually or with tools without any danger.

2.2.8.3             It must not be possible to touch live parts of the machine.

2.2.8.4             Personal injury must not result from the issue of fluid or gaseous media.

 

3. Machining centres

3.1            Normal mode of operation

3.1.1             The work area must be safeguarded so that is impossible to reach or step into the danger zones of automatic movements, either intentionally or unintentionally.

3.1.2             The tool magazine must be safeguarded so that it is impossible to reach or step into the danger zones of automatic movements.

3.1.3             The workpiece magazine must be safeguarded so that it is impossible to reach or step into the danger zones of automatic movements.

3.1.4             Chip removal must not result in personal injury due to the chips or moving parts of the machine.

3.1.5             Personal injuries resulting from reaching into drive systems must be prevented.

3.1.6             The possibility of reaching into danger zones of moving chip conveyors (screw conveyors, etc.) must be prevented.

3.1.7             No personal injury to operators or third persons must result from flying workpieces or parts thereof.

For example, this can occur

  • due to insufficient clamping
  • due to inadmissible cutting force
  • due to collision with the tool or machine parts
  • due to workpiece breakage
  • due to defective clamping fixtures
  • due to changing to the wrong workpiece
  • due to power failure

 

3.1.8             No personal injury must result from flying workpiece clamping fixtures.

3.1.9             No personal injury must result from flying chips.

3.1.10             No personal injury must result from flying tools or parts thereof.

For example, this can occur

  • due to material defects
  • due to inadmissible speed of rotation
  • due to inadmissible cutting force
  • due to collision with workpiece or machine part
  • due to inadequate clamping or tightening
  • due to the tool flying out of the tool changer
  • due to selecting the wrong tool
  • due to power failure

 

3.2            Special modes of operation

3.2.1             Workpiece changing.

3.2.1.1             Where power-operated clamping fixtures are used, it must not be possible for parts of the body to become trapped between the closing parts of the clamping fixture and the workpiece.

3.2.1.2             The starting of a drive resulting from a defective command or invalid command input must be prevented.

3.2.1.3             It must be possible to manipulate the workpiece manually or with tools without any danger.

3.2.1.4             Where workpieces are changed in a clamping station, it must not be possible from this location to reach or step into automatic movement sequences of the machine or workpiece magazine. No movements must be initiated by the control while a person is present in the clamping zone. The automatic insertion of the clamped workpiece into the machine or workpiece magazine is only to take place when the clamping station is also safeguarded with a protective system corresponding to that for normal mode of operation.

3.2.2             Tool changing in the spindle.

3.2.2.1             The starting of a drive resulting from a defective command or invalid command input must be prevented.

3.2.2.2             It must not be possible for fingers to become trapped when putting in tools.

3.2.3             Tool changing in tool magazine.

3.2.3.1             Movements in the tool magazine resulting from defective commands or invalid command input must be prevented during tool changing.

3.2.3.2             It must not be possible to reach into other moving machine parts from the tool loading station.

3.2.3.3             It must not be possible to reach into danger zones on the further movement of the tool magazine or during the search. If taking place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

3.2.4             Measurement check.

3.2.4.1             Reaching into the work area must only be possible after all movements have been brought to a standstill.

3.2.4.2             The starting of a drive resulting from a defective command or invalid command input must be prevented.

3.2.5             Set-up.

3.2.5.1             If movements are executed during set-up with the guards for normal mode of operation removed, then the operator must be safeguarded by another means.

3.2.5.2             No dangerous movements or changes of movement must be initiated as a result of a defective command or invalid command input.

3.2.6             Programming.

3.2.6.1             No movements must be initiated during programming which endanger a person in the work area.

3.2.7             Production fault.

3.2.7.1             The starting of a drive resulting from a defective command or invalid command input must be prevented.

3.2.7.2             No dangerous movements or situations must be initiated by the movement or removal of the workpiece or waste.

3.2.7.3             Where movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

3.2.8             Troubleshooting.

3.2.8.1             Reaching into the danger zones of automatic movements must be prevented.

3.2.8.2             The starting of a drive as a result of a defective command or invalid command input must be prevented.

3.2.8.3             Any movement of the machine on manipulation of the defective part must be prevented.

3.2.8.4             Personal injury resulting from a machine part splintering off or dropping must be prevented.

3.2.8.5             If, during troubleshooting, movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

3.2.9             Machine malfunction and repair.

3.2.9.1             Starting the machine must be prevented.

3.2.9.2             Manipulation of the different parts of the machine must be possible manually or with tools without any danger.

3.2.9.3             It must not be possible to touch live parts of the machine.

3.2.9.4             Personal injury must not result from the issue of fluid or gaseous media.

 

4. Grinding machines

4.1            Normal mode of operation

4.1.1             The work area is to be safeguarded so that it is impossible to reach or step into the danger zones of automatic movements, either intentionally or unintentionally.

4.1.2             Personal injuries resulting from reaching into drive systems must be prevented.

4.1.3             No personal injury to operators or third persons must result from flying workpieces or parts thereof.

For example, this can occur

  • due to insufficient clamping
  • due to inadmissible cutting force
  • due to inadmissible rotation speed
  • due to collision with the tool or machine parts
  • due to workpiece breakage
  • due to defective clamping fixtures
  • due to power failure

 

4.1.4             No personal injury must result from flying workpiece clamping fixtures.

4.1.5             No personal injury or fires must result from sparking.

4.1.6             No personal injury must result from flying parts of grinding wheels.

For example, this can occur

  • due to inadmissible rotation speed
  • due to inadmissible cutting force
  • due to material defects
  • due to collision with workpiece or machine part
  • due to inadequate clamping (flanges)
  • due to using incorrect grinding wheel

 

Special modes of operation

4.2.1             Workpiece changing.

4.2.1.1             Where power-operated clamping fixtures are used, it must not be possible for parts of the body to become trapped between the closing parts of the clamping fixture and the workpiece.

4.2.1.2             The starting of a feed drive resulting from a defective command or invalid command input must be prevented.

4.2.1.3             Personal injury caused by the rotating grinding wheel must be prevented when manipulating the workpiece.

4.2.1.4             Personal injury resulting from a bursting grinding wheel must not be possible.

4.2.1.5             The manipulation of the workpiece must be possible manually or with tools without any danger.

4.2.2             Tool changing (grinding wheel changing)

4.2.2.1             The starting of a feed drive resulting from .a defective command or invalid command input must be prevented.

4.2.2.2             Personal injury caused by the rotating grinding wheel must not be possible during measuring procedures.

4.2.2.3             Personal injury resulting from a bursting grinding wheel must not be possible.

4.2.3             Measurement check.

4.2.3.1             The starting of a feed drive resulting from a defective command or invalid command input must be prevented.

4.2.3.2             Personal injury caused by the rotating grinding wheel must not be possible during measuring procedures.

4.2.3.3             Personal injury resulting from a bursting grinding wheel must not be possible.

4.2.4.             Set-up.

4.2.4.1             If movements are executed during set-up with the guards for normal mode of operation removed, then the operator must be safeguarded by another means.

4.2.4.2             No dangerous movements or changes of movement must be initiated as a result of a defective command or invalid command input.

4.2.5             Programming.

4.2.5.1             No movements must be initiated during programming which endanger a person in the work area.

4.2.6             Production fault.

4.2.6.1             The starting of a feed drive resulting from a defective command or invalid command input must be prevented.

4.2.6.2             No dangerous movements or situations must be initiated by the movement or removal of the workpiece or waste.

4.2.6.3             Where movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

4.2.6.4             Personal injury caused by the rotating grinding wheel must be prevented.

4.2.6.5             Personal injury resulting from a bursting grinding wheel must not be possible.

4.2.7             Troubleshooting.

4.2.7.1             Reaching into the danger zones of automatic movements must be prevented.

4.2.7.2             The starting of a drive as a result of a defective command or invalid command input must be prevented.

4.2.7.3             Any movement of the machine on manipulation of the defective part must be prevented.

4.2.7.4             Personal injury resulting from a machine part splintering off or dropping must be prevented.

4.2.7.5             Personal injury caused the operator’s contacting or by the bursting of the rotating grinding wheel must be prevented.

4.2.7.6             If, during troubleshooting, movements have to take place with the guards for the normal mode of operation removed, these movements may only be of the kind designated and only executed for the period of time ordered and only when it can be ensured that no parts of the body are in these danger zones.

4.2.8             Machine malfunction and repair.

4.2.8.1             Starting the machine must be prevented.

4.2.8.2             Manipulation of the different parts of the machine must be possible manually or with tools without any danger.

4.2.8.3             It must not be possible to touch live parts of the machine.

4.2.8.4             Personal injury must not result from the issue of fluid or gaseous media.

 

Back

It is generally agreed that control systems must be safe during use. With this in mind, most modern control systems are designed as shown in figure 1.

Figure 1. General design of control systems

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The simplest way to make a control system safe is to construct an impenetrable wall around it so as to prevent human access or interference into the danger zone. Such a system would be very safe, albeit impractical, since it would be impossible to gain access in order to perform most testing, repair and adjustment work. Because access to danger zones must be permitted under certain conditions, protective measures other than just walls, fences and the like are required to facilitate production, installation, servicing and maintenance.

 

Some of these protective measures can be partly or fully integrated into control systems, as follows:

  • Movement can be stopped immediately should anybody enter the danger zone, by means of emergency stop (ES) buttons.
  • Push-button controls permit movement only when the push-button is activated.
  • Double-hand controls (DHC) permit movement only when both hands are engaged in depressing the two control elements (thus ensuring that hands are kept away from the danger zones).

 

These types of protective measures are activated by operators. However, because human beings often represent a weak point in applications, many functions, such as the following, are performed automatically:

  • Movements of robot arms during the servicing or “teach-in” are very slow. Nonetheless, speed is continuously monitored. If, because of a control system failure, the speed of automatic robot arms were to increase unexpectedly during either the servicing or teach-in period, the monitoring system would activate and immediately terminate movement.
  • A light barrier is provided to prevent access into a danger zone. If the light beam is interrupted, the machine will stop automatically.

 

Normal function of control systems is the most important precondition for production. If a production function is interrupted due to a control failure, it is at most inconvenient but not hazardous. If a safety-relevant function is not performed, it could result in lost production, equipment damage, injury or even death. Therefore, safety-relevant control system functions must be more reliable and safer than normal control system functions. According to European Council Directive 89/392/EEC (Machine Guidelines), control systems must be designed and constructed so that they are safe and reliable.

Controls consist of a number of components connected together so as to perform one or more functions. Controls are subdivided into channels. A channel is the part of a control that performs a specific function (e.g., start, stop, emergency stop). Physically, the channel is created by a string of components (transistors, diodes, relays, gates, etc.) through which, from one component to the next, (mostly electrical) information representing that function is transferred from input to output.

In designing control channels for safety-relevant functions (those functions which involve humans), the following requirements must be fulfilled:

  • Components used in control channels with safety-relevant functions must be able to withstand the rigours of normal use. Generally, they must be sufficiently reliable.
  • Errors in the logic must not cause dangerous situations. Generally, the safety-relevant channel is to be sufficiently failure proof.
  • External influences (factors) should not lead to temporary or permanent failures in safety-relevant channels.

 

Reliability

Reliability is the ability of a control channel or component to perform a required function under specified conditions for a given period of time without failing. (Probability for specific components or control channels can be calculated using suitable methods.) Reliability must always be specified for a specific time value. Generally, reliability can be expressed by the formula in figure 2.

Figure 2. Reliability formula

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Reliability of complex systems

Systems are built from components. If the reliabilities of the components are known, the reliability of the system as a whole can be calculated. In such cases, the following apply:

Serial systems

The total reliability Rtot of a serial system consisting of N components of the same reliability RC is calculated as in figure 3.

Figure 3. Reliability graph of serially connected components

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The total reliability is lower than the reliability of the least reliable component. As the number of serially connected components increases, the total reliability of the chain falls significantly.

Parallel systems

The total reliability Rtot of a parallel system consisting of N components of the same reliability RC is calculated as in figure 4.

Figure 4. Reliability graph of parallel connected components

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Total reliability can be improved significantly through the parallel connection of two or more components.

Figure 5 illustrates a practical example. Note that the circuitry will switch off the motor more reliably. Even if relay A or B fails to open its contact, the motor will still be switched off.

Figure 5. Practical example of figure 4

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To calculate the total reliability of a channel is simple if all necessary component reliabilities are known and available. In the case of complex components (integrated circuits, microprocessors, etc.) the calculation of the total reliability is difficult or impossible if the necessary information is not published by the manufacturer.

Safety

When professionals speak about safety and call for safe machines, they mean the safety of the entire machine or system. This safety is, however, too general, and not precisely enough defined for the designer of controls. The following definition of safety may be practical and usable to designers of control circuitry: Safety is the ability of a control system to perform the required function within prescribed limits, for a given duration, even when anticipated fault(s) occur. Consequently, it must be clarified during the design how “safe” the safety-related channel must be. (The designer can develop a channel that is safe against first failure, against any one failure, against two failures, etc.) Furthermore, a channel that performs a function which is used to prevent accidents may be essentially reliable, but it does not have to be inevitably safe against failures. This may be best explained by the following examples:

Example 1

The example illustrated in figure 6 is a safety-relevant control channel performing the required safety function. The first component may be a switch that monitors, for example, the position of an access door to a dangerous area. The last component is a motor which drives moving mechanical parts within the danger area.

Figure 6. A safety-relevant control channel performing the required safety function

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The required safety function in this case is a dual one: If the door is closed, the motor may run. If the door is open, the motor must be switched off. Knowing reliabilities R1 to R6, it is possible to calculate reliability Rtot. Designers should use reliable components in order to maintain sufficiently high reliability of the whole control system (i.e., the probability that this function may still be performed in, say, even 20 years should be accounted for in the design). As a result, designers must fulfil two tasks: (1) the circuitry must perform the required function, and (2) the reliability of the components and of the whole control channel must be adequate.

The following question should now be asked: Will the aforementioned channel perform the required safety functions even if a failure occurs in the system (e.g., if a relay contact sticks or a component malfunctions)? The answer is “No”. The reason is that a single control channel consisting only of serially connected components and working with static signals is not safe against one failure. The channel can have only a certain reliability, which guarantees the probability that the function will be carried out. In such situations, safety is always meant as failure related.

Example 2

If a control channel is to be both reliable and safe, the design must be modified as in figure 7. The example illustrated is a safety-relevant control channel consisting of two fully separated subchannels.

Figure 7. A safety-relevant control channel with two fully separate subchannels

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This design is safe against the first failure (and possible further failures in the same subchannel), but is not safe against two failures which may occur in two different subchannels (simultaneously or at different times) because there is no failure detection circuit. Consequently, initially both subchannels work with a high reliability (see parallel system), but after the first failure only one subchannel will work, and reliability decreases. Should a second failure occur in the subchannel still working, both will have then failed, and the safety function will no longer be performed.

Example 3

The example illustrated in figure 8 is a safety-relevant control channel consisting of two fully separate subchannels which monitor each other.

Figure 8. A safety-relevant control channel with two fully separate subchannels which monitor each other

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Such a design is failure safe because after any failure, only one subchannel will be non-functional, while the other subchannel remains available and will perform the safety function. Moreover, the design has a failure detection circuit. If, due to a failure, both subchannels fail to work in the same way, this condition will be detected by “exclusive or” circuitry, with the result that the machine will be automatically switched off. This is one of the best ways of designing machine controls—designing safety-relevant subchannels. They are safe against one failure and at the same time provide enough reliability so that the chances that two failures will occur simultaneously is minuscule.

Redundancy

It is apparent that there are various methods by which a designer may improve reliability and/or safety (against failure). The previous examples illustrate how a function (i.e., door closed, motor may run; door opened, motor must be stopped) can be realized by various solutions. Some methods are very simple (one subchannel) and others more complicated (two subchannels with mutual supervising). (See figure 9.)

Figure 9. Reliability of redundant systems with or without failure detection

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There is a certain redundancy in the complex circuitry and/or components in comparison with the simple ones. Redundancy can be defined as follows: (1) Redundancy is the presence of more means (components, channels, higher safety factors, additional tests and so on) than are really necessary for the simple fulfilling of the desired function; (2) redundancy obviously does not “improve” the function, which is performed anyway. Redundancy only improves reliability and/or safety.

Some safety professionals believe that redundancy is only the doubling or tripling, and so on, of the system. This is a very limited interpretation, as redundancy may be interpreted much more broadly and flexibly. Redundancy may be not only included in the hardware; it may be included in the software too. Improving the safety factor (e.g., a stronger rope instead of a weaker rope) may also be considered as a form of redundancy.

Entropy

Entropy, a term found mostly in thermodynamics and astronomy, may be defined as follows: Everything tends towards decay. Therefore, it is absolutely certain that all components, subsystems or systems, independently of the technology in use, will fail sometime. This means that there are no 100% reliable and/or safe systems, subsystems or components. All of them are merely more or less reliable and safe, depending on the structure’s complexity. The failures which inevitably occur earlier or later demonstrate the action of entropy.

The only means available to designers to counter entropy is redundancy, which is achieved by (a) introducing more reliability into the components and (b) providing more safety throughout the circuit architecture. Only by sufficiently raising the probability that the required function will be performed for the required period of time, can designers in some degree defend against entropy.

Risk Assessment

The greater the potential risk, the higher the reliability and/or safety (against failures) that is required (and vice versa). This is illustrated by the following two cases:

Case 1

Access to the mould tool fixed in an injection moulding machine is safeguarded by a door. If the door is closed, the machine may work, and if the door is opened, all dangerous movements have to be stopped. Under no circumstances (even in case of failure in the safety-related channel) may any movements, especially those which operate the tool, occur.

Case 2

Access to an automatically controlled assembly line that assembles small plastic components under pneumatic pressure is guarded by a door. If this door is opened, the line will have to be stopped.

In Case 1, if the door-supervising control system should fail, a serious injury may occur if the tool is closed unexpectedly. In Case 2, only slight injury or insignificant harm may result if the door-supervising control system fails.

It is obvious that in the first case much more redundancy must be introduced to attain the reliability and/or safety (against failure) required to protect against extreme high risk. In fact, according to European Standard EN 201, the supervising control system of the injection moulding machine door has to have three channels; two of which are electrical and mutually supervised and one of which is mostly equipped with hydraulics and testing circuits. All these three supervising functions relate to the same door.

Conversely, in applications like that described in Case 2, a single channel activated by a switch with positive action is appropriate to the risk.

Control Categories

Because all of the above considerations are generally based on information theory and consequently are valid for all technologies, it does not matter whether the control system is based on electronic, electro-mechanical, mechanical, hydraulic or pneumatic components (or a mixture of them), or on some other technology. The inventiveness of the designer on the one hand and economic questions on the other hand are the primary factors affecting a nearly endless number of solutions as to how to realize safety-relevant channels.

To prevent confusion, it is practical to set certain sorting criteria. The most typical channel structures used in machine controls for performing safety-related functions are categorized according to:

  • reliability
  • behaviour in case of failure
  • failure-disclosing time.

 

Their combinations (not all possible combinations are shown) are illustrated in table 1.

Table 1. Some possible combinations of circuit structures in machine controls for safety-related functions

Criteria (Questions)

Basic strategy

 

By raising the reliability (is the occurrence of failure shifted to the possibly far future?)

By suitable circuit structure (architecture) failure will be at least detected (Cat. 2) or failure effect on the channel will be eliminated (Cat. 3) or failure will be disclosed immediately (Cat. 4)

 

Categories

 

This solution is basically wrong

B

1

2

3

4

Can the circuit components with stand the expected influences; are they constructed according to state of the art?

No

Yes

Yes

Yes

Yes

Yes

Have well tried components and/or methods been used?

No

No

Yes

Yes

Yes

Yes

Can a failure be detected automatically?

No

No

No

Yes

Yes

Yes

Does a failure prevent the performing of the safety-related function?

Yes

Yes

Yes

Yes

No

No

When will the failure be detected?

Never

Never

Never

Early (latest at the end of interval that is not longer than one machine cycle)

Immediately (when the signal loses dynamical
character)

   

In consumer products

To be used in machines

 

The category applicable for a specific machine and its safety-related control system is mostly specified in the new European standards (EN), unless the national authority, the user and the manufacturer mutually agree that another category should be applied. The designer then develops a control system which fulfils the requirements. For example, considerations governing the design of a control channel may include the following:

  • The components have to withstand the expected influences. (YES/NO)
  • Their construction should be according to state-of-the-art standards. (YES/NO)
  • Well-tried components and methods are used. (YES/NO)
  • Failure must be detected. (YES/NO)
  • Will the safety function be performed even in case of failure? (YES/NO)
  • When will the failure be detected? (NEVER, EARLY, IMMEDIATELY)

 

This process is reversible. Using the same questions, one can decided which category an existing, previously developed control channel belongs to.

Category examples

Category B

The control channel components primarily used in consumer wares have to withstand the expected influences and be designed according to state of the art. A well-designed switch may serve as an example.

Category 1

The use of well-tried components and methods is typical for Category 1. A Category 1 example is a switch with positive action (i.e., requires forced opening of contacts). This switch is designed with robust parts and is activated by relatively high forces, thus reaching extremely high reliability only in contact opening. In spite of sticking or even welded contacts, these switches will open. (Note: Components such as transistors and diodes are not considered as being well-tried components.) Figure 10 will serve as an illustration of a Category 1 control.

Figure 10. A switch with a positive action

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This channel uses switch S with positive action. The contactor K is supervised by the light L. The operator is advised that the normally open (NO) contacts stick by means of indication light L. The contactor K has forced guided contacts. (Note: Relays or contactors with forced guidance of contacts have, in comparison with usual relays or contactors, a special cage made from insulating material so that if normally closed (NC) contacts are closed, all NO contacts have to be opened, and vice versa. This means that by use of NC contacts a check may be made to determine that the working contacts are not sticking or welded together.)

Category 2

Category 2 provides for automatic detection of failures. Automatic failure detection has to be generated before each dangerous movement. Only if the test is positive may the movement be performed; otherwise the machine will be stopped. Automatic failure detection systems are used for light barriers to prove that they are still working. The principle is illustrated in figure 1.

Figure 11. Circuit including a failure detector

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This control system is tested regularly (or occasionally) by injecting an impulse to the input. In a properly working system this impulse will then be transferred to the output and compared to an impulse from a test generator. When both impulses are present, the system obviously works. Otherwise, if there is no output impulse, the system has failed.

Category 3

Circuitry has been previously described under Example 3 in the Safety section of this article, figure 8.

The requirement—that is, automatic failure detection and the ability to perform the safety function even if one failure has occurred anywhere—can be fulfilled by two-channel control structures and by mutual supervising of the two channels.

For machine controls only, the dangerous failures have to be investigated. It should be noted that there are two kinds of failure:

  • Non-dangerous failures are those that, after their occurrence, cause a “safe state” of the machine by providing for switching off the motor.
  • Dangerous failures are those that, after their occurrence, cause an “unsafe state” of the machine, as the motor cannot be switched off or the motor starts to move unexpectedly.

Category 4

Category 4 typically provides for the application of a dynamic, continuously changing signal on the input. The presence of a dynamic signal on the output means running (“1”), and the absence of a dynamic signal means stop (“0”).

For such circuitry it is typical that after failure of any component the dynamic signal will no longer be available on the output. (Note: The static potential on the output is irrelevant.) Such circuits may be called “fail-safe”. All failures will be disclosed immediately, not after the first change (as in Category 3 circuits).

Further comments on control categories

Table 1 has been developed for usual machine controls and shows the basic circuit structures only; according to the machine directive it should be calculated on the assumption that only one failure will occur in one machine cycle. This is why the safety function does not have to be performed in the case of two coincident failures. It is assumed that a failure will be detected within one machine cycle. The machine will be stopped and then repaired. The control system then starts again, fully operable, without failures.

The first intent of the designer should be not to permit “standing” failures, which would not be detected during one cycle as they might later be combined with newly occurring failure(s) (failure cumulation). Such combinations (a standing failure and a new failure) can cause a malfunction of even Category 3 circuitry.

In spite of these tactics, it is possible that two independent failures will occur at the same time within the same machine cycle. It is only very improbable, especially if highly reliable components have been used. For very high-risk applications, three or more subchannels should be used. This philosophy is based on the fact that the mean time between failures is much longer than the machine cycle.

This does not mean, however, that the table cannot be further expanded. Table 1 is basically and structurally very similar to the Table 2 used in EN 954-1. However, it does not try to include too many sorting criteria. The requirements are defined according to the rigorous laws of logic, so that only clear answers (YES or NO) can be expected. This allows a more exact assessment, sorting and classification of submitted circuitry (safety-related channels) and, last but not least, significant improvement of assessment reproducibility.

It would be ideal if risks could be classified in various risk levels and then a definite link established between risk levels and categories, with this all independent of the technology in use. However, this is not fully possible. Early after creating categories it became clear that even given the same technology, various questions were not sufficiently answered. Which is better: a very reliable and well-designed component of Category 1, or a system fulfilling the requirements of Category 3 with poor reliability?

To explain this dilemma one must differentiate between two qualities: reliability and safety (against failures). They are not comparable, as both these qualities have different features:

  • The component with highest reliability has the unpleasant feature that in the event of failure (even if highly improbable) the function will cease to perform.
  • Category 3 systems, where even in case of one failure the function will be performed, are not safe against two failures at the same time (what may be important is whether sufficiently reliable components have been used).

Considering the above, it may be that the best solution (from the high-risk point of view) is to use highly reliable components and configure them so that the circuitry is safe against at least one failure (preferably more). It is clear that such a solution is not the most economical. In practice, the optimization process is mostly the consequence of all these influences and considerations.

Experience with practical use of the categories shows that it is rarely possible to design a control system that can utilize only one category throughout. Combination of two or even three parts, each of a different category, is typical, as illustrated in the following example:

Many safety light barriers are designed in Category 4, wherein one channel works with a dynamic signal. At the end of this system there usually are two mutually supervised subchannels which work with static signals. (This fulfils the requirements for Category 3.)

According to EN 50100, such light barriers are classified as Type 4 electro-sensitive protective devices, although they are composed of two parts. Unfortunately, there is no agreement how to denominate control systems consisting of two or more parts, each part of another category.

Programmable Electronic Systems (PESs)

The principles used to create table 1 can, with certain restrictions of course, be generally appled to PESs too.

PES-only system

In using PESs for control, the information is transferred from the sensor to the activator through a large number of components. Beyond that, it even passes “through” software. (See figure 12).

Figure 12. A PES system circuit

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Although modern PESs are very reliable, the reliability is not as high as may be required for processing safety functions. Beyond that, the usual PES systems are not safe enough, since they will not perform the safety-related function in case of a failure. Therefore, using PESs for processing of safety functions without any additional measures is not permitted.

Very low-risk applications: Systems with one PES and additional measures

When using a single PES for control, the system consists of the following primary parts:

Input part

The reliability of a sensor and input of a PES can be improved by doubling them. Such a double-system input configuration can be further supervised by software to check if both subsystems are delivering the same information. Thus the failures in the input part can be detected. This is nearly the same philosophy as required for Category 3. However, because the supervising is done by software and only once, this may be denominated as 3- (or not as reliable as 3).

Middle part

Although this part cannot be well doubled, it can be tested. Upon switching on (or during operation), a check of the entire instruction set can be performed. At the same intervals, the memory can also be checked by suitable bit patterns. If such checks are conducted without failure, both parts, CPU and memory, are obviously working properly. The middle part has certain features typical of Category 4 (dynamic signal) and others typical of Category 2 (testing performed regularly at suitable intervals). The problem is that these tests, in spite of their extensiveness, cannot be really complete, as the one-PES system inherently does not allow them.

Output part

Similar to an input, the output (including activators) can also be doubled. Both subsystems can be supervised with respect to the same result. Failures will be detected and the safety function will be performed. However, there are the same weak points as in the input part. Consequently, Category 3 is chosen in this case.

In figure 13 the same function is brought to relays A and B. The control contacts a and b, then informs two input systems whether both relays are doing the same work (unless a failure in one of the channels has occurred). Supervising is done again by software.

Figure 13. A PES circuit with a failure-detection system

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The whole system can be described as Category 3-/4/2/3- if properly and extensively done. Nevertheless, the weak points of such systems as above described cannot be fully eliminated. In fact, improved one PESs are actually used for safety-related functions only where the risks are rather low (Hölscher and Rader 1984).

Low- and medium-risk applications with one PES

Today almost every machine is equipped with a PES control unit. To solve the problem of insufficient reliability and usually insufficient safety against failure, the following design methods are commonly used:

  • In relatively simple machines such as lifts, the functions are divided into two groups: (1) the functions that are not related to safety are processed by the PES; (2) the safety-related functions are combined in one chain (safety circuit) and processed outside of the PES (see figure 14).

 

Figure 14. State of the art for stop category 0

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  • The method given above is not suitable for more complex machines. One reason is that such solutions usually are not safe enough. For medium-risk applications, solutions should fulfil the requirements for category 3. General ideas of how such designs may look are presented in figure 15 and figure 16.

 

Figure 15. State of the art for stop category 1

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Figure 16. State of the art for stop category 2

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High-risk applications: systems with two (or more) PESs

Aside from complexity and expense, there are no other factors that would prevent designers from using fully doubled PES systems such as Siemens Simatic S5-115F, 3B6 Typ CAR-MIL and so on. These typically include two identical PESs with homogenous software, and assume the use of “well-tried” PESs and “well-tried” compilers (a well-tried PES or compiler can be considered one that in many practical applications over 3 or more years has shown that systematic failures have been obviously eliminated). Although these doubled PES systems do not have the weak points of single-PES systems, it does not mean that doubled PES systems solve all problems. (See figure 17).

Figure 17. Sophisticated system with two PESs

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Systematic Failures

Systematic failures may result from errors in specifications, design and other causes, and may be present in hardware as well as in software. Double-PES systems are suitable for use in safety-related applications. Such configurations allow the detection of random hardware failures. By means of hardware diversity such as the use of two different types, or products of two different manufacturers, systematic hardware failures could be disclosed (it is highly unlikely that an identical hardware systematic failure would occur in both PES).

Software

Software is a new element in safety considerations. Software is either correct or incorrect (with respect to failures). Once correct, software cannot become instantly incorrect (as compared to hardware). The aims are to eradicate all errors in the software or to at least identify them.

There are various ways of achieving this goal. One is the verification of the program (a second person attempts to discover the errors in a subsequent test). Another possibility is diversity of the software, wherein two different programs, written by two programmers, address the same problem. If the results are identical (within certain limits), it can be assumed that both program sections are correct. If the results are different, it is presumed that errors are present. (N.B., The architecture of the hardware naturally must also be considered.)

Summary

When using PESs, generally the same following basic considerations are to be taken in account (as described in the previous sections).

  • One control system without any redundancy may be allocated to Category B. One control system with additional measures may be Category 1 or even higher, but not higher than 2.
  • A two-part control system with mutual comparison of results may be allocated to Category 3. A two-part control system with mutual comparison of results and more or less diversity may be allocated to Category 3 and is suitable for higher-risk applications.

A new factor is that for the system with a PES, even software should be evaluated from the correctness point of view. Software, if correct, is 100% reliable. At this stage of technological development, the best possible and known technical solutions will probably not be used, since the limiting factors are still economic. Furthermore, various groups of experts are continuing to develop the standards for safety applications of PESs (e.g., EC, EWICS). Although there are various standards already available (VDE0801, IEC65A and so on), this matter is so broad and complex that none of them may be considered as final.

 

Back

A hybrid automated system (HAS) aims to integrate the capabilities of artificially intelligent machines (based on computer technology) with the capacities of the people who interact with these machines in the course of their work activities. The principal concerns of HAS utilization relate to how the human and machine subsystems should be designed in order to make the best use of the knowledge and skills of both parts of the hybrid system, and how the human operators and machine components should interact with each other to assure their functions complement one another. Many hybrid automated systems have evolved as the products of applications of modern information- and control-based methodologies to automate and integrate different functions of often complex technological systems. HAS was originally identified with the introduction of computer-based systems used in the design and operation of real-time control systems for nuclear power reactors, for chemical processing plants and for discrete parts-manufacturing technology. HAS can now also be found in many service industries, such as air traffic control and aircraft navigation procedures in the civil aviation area, and in the design and use of intelligent vehicle and highway navigation systems in road transportation.

With continuing progress in computer-based automation, the nature of human tasks in modern technological systems shifts from those that require perceptual-motor skills to those calling for cognitive activities, which are needed for problem solving, for decision making in system monitoring, and for supervisory control tasks. For example, the human operators in computer-integrated manufacturing systems primarily act as system monitors, problem solvers and decision makers. The cognitive activities of the human supervisor in any HAS environment are (1) planning what should be done for a given period of time, (2) devising procedures (or steps) to achieve the set of planned goals, (3) monitoring the progress of (technological) processes, (4) “teaching” the system through a human-interactive computer, (5) intervening if the system behaves abnormally or if the control priorities change and (6) learning through feedback from the system about the impact of supervisory actions (Sheridan 1987).

Hybrid System Design

The human-machine interactions in a HAS involve utilization of dynamic communication loops between the human operators and intelligent machines—a process that includes information sensing and processing and the initiation and execution of control tasks and decision making—within a given structure of function allocation between humans and machines. At a minimum, the interactions between people and automation should reflect the high complexity of hybrid automated systems, as well as relevant characteristics of the human operators and task requirements. Therefore, the hybrid automated system can be formally defined as a quintuple in the following formula:

HAS = (T, U, C, E, I)

where T = task requirements (physical and cognitive); U = user characteristics (physical and cognitive); C = the automation characteristics (hardware and software, including computer interfaces); E = the system’s environment; I = a set of interactions among the above elements.

The set of interactions I embodies all possible interactions between T, U and C in E regardless of their nature or strength of association. For example, one of the possible interactions might involve the relation of the data stored in the computer memory to the corresponding knowledge, if any, of the human operator. The interactions I can be elemental (i.e., limited to a one-to-one association), or complex, such as would involve interactions between the human operator, the particular software used to achieve the desired task, and the available physical interface with the computer.

Designers of many hybrid automated systems focus primarily on the computer-aided integration of sophisticated machines and other equipment as parts of computer-based technology, rarely paying much attention to the paramount need for effective human integration within such systems. Therefore, at present, many of the computer-integrated (technological) systems are not fully compatible with the inherent capabilities of the human operators as expressed by the skills and knowledge necessary for the effective control and monitoring of these systems. Such incompatibility arises at all levels of human, machine and human-machine functioning, and can be defined within a framework of the individual and the entire organization or facility. For example, the problems of integrating people and technology in advanced manufacturing enterprises occur early in the HAS design stage. These problems can be conceptualized using the following system integration model of the complexity of interactions, I, between the system designers, D, human operators, H, or potential system users and technology, T:

I (H, T) = F [ I (H, D), I (D, T)]

where I stands for relevant interactions taking place in a given HAS’s structure, while F indicates functional relationships between designers, human operators and technology.

The above system integration model highlights the fact that the interactions between the users and technology are determined by the outcome of the integration of the two earlier interactions—namely, (1) those between HAS designers and potential users and (2) those between the designers and the HAS technology (at the level of machines and their integration). It should be noted that even though strong interactions typically exist between the designers and technology, only very few examples of equally strong interrelationships between designers and human operators can be found.

It can be argued that even in the most automated systems, the human role remains critical to successful system performance at the operational level. Bainbridge (1983) identified a set of problems relevant to the operation of the HAS which are due to the nature of automation itself, as follows:

  1. Operators “out of the control loop”. The human operators are present in the system to exercise control when needed, but by being “out of the control loop” they fail to maintain the manual skills and long-term system knowledge that are often required in case of an emergency.
  2. Outdated “mental picture”. The human operators may not be able to respond quickly to changes in the system behaviour if they have not been following the events of its operation very closely. Furthermore, the operators’ knowledge or mental picture of the system functioning may be inadequate to initiate or exercise required responses.
  3. Disappearing generations of skills. New operators may not be able to acquire sufficient knowledge about the computerized system achieved through experience and, therefore, will be unable to exercise effective control when needed.
  4. Authority of automatics. If the computerized system has been implemented because it can perform the required tasks better than the human operator, the question arises, “On what basis should the operator decide that correct or incorrect decisions are being made by the automated systems?”
  5. Emergence of the new types of “human errors” due to automation. Automated systems lead to new types of errors and, consequently, accidents which cannot be analysed within the framework of traditional techniques of analysis.

 

Task Allocation

One of the important issues for HAS design is to determine how many and which functions or responsibilities should be allocated to the human operators, and which and how many to the computers. Generally, there are three basic classes of task allocation problems that should be considered: (1) the human supervisor–computer task allocation, (2) the human–human task allocation and (3) the supervisory computer–computer task allocation. Ideally, the allocation decisions should be made through some structured allocation procedure before the basic system design is begun. Unfortunately such a systematic process is seldom possible, as the functions to be allocated may either need further examination or must be carried out interactively between the human and machine system components—that is, through application of the supervisory control paradigm. Task allocation in hybrid automated systems should focus on the extent of the human and computer supervisory responsibilities, and should consider the nature of interactions between the human operator and computerized decision support systems. The means of information transfer between machines and the human input-output interfaces and the compatibility of software with human cognitive problem-solving abilities should also be considered.

In traditional approaches to the design and management of hybrid automated systems, workers were considered as deterministic input-output systems, and there was a tendency to disregard the teleological nature of human behaviour—that is, the goal-oriented behaviour relying on the acquisition of relevant information and the selection of goals (Goodstein et al. 1988). To be successful, the design and management of advanced hybrid automated systems must be based on a description of the human mental functions needed for a specific task. The “cognitive engineering” approach (described further below) proposes that human-machine (hybrid) systems need to be conceived, designed, analysed and evaluated in terms of human mental processes (i.e., the operator’s mental model of the adaptive systems is taken into account). The following are the requirements of the human-centred approach to HAS design and operation as formulated by Corbett (1988):

  1. Compatibility. System operation should not require skills unrelated to existing skills, but should allow existing skills to evolve. The human operator should input and receive information which is compatible with conventional practice in order that the interface conform to the user’s prior knowledge and skill.
  2. Transparency. One cannot control a system without understanding it. Therefore, the human operator must be able to “see” the internal processes of the system’s control software if learning is to be facilitated. A transparent system makes it easy for users to build up an internal model of the decision-making and control functions that the system can perform.
  3. Minimum shock. The system should not do anything which operators find unexpected in the light of the information available to them, detailing the present state of the system.
  4. Disturbance control. Uncertain tasks (as defined by the choice structure analysis) should be under human operator control with computer decision-making support.
  5. Fallibility. The implicit skills and knowledge of the human operators should not be designed out of the system. The operators should never be put in a position where they helplessly watch the software direct an incorrect operation.
  6. Error reversibility. Software should supply sufficient feedforward of information to inform the human operator of the likely consequences of a particular operation or strategy.
  7. Operating flexibility. The system should offer human operators the freedom to trade off requirements and resource limits by shifting operating strategies without losing the control software support.

 

Cognitive Human Factors Engineering

Cognitive human factors engineering focuses on how human operators make decisions at the workplace, solve problems, formulate plans and learn new skills (Hollnagel and Woods 1983). The roles of the human operators functioning in any HAS can be classified using Rasmussen’s scheme (1983) into three major categories:

  1. Skill-based behaviour is the sensory-motor performance executed during acts or activities which take place without conscious control as smooth, automated and highly integrated patterns of behaviour. Human activities that fall under this category are considered to be a sequence of skilled acts composed for a given situation. Skill-based behaviour is thus the expression of more or less stored patterns of behaviours or pre-programmed instructions in a space-time domain.
  2. Rule-based behaviour is a goal-oriented category of performance structured by feedforward control through a stored rule or procedure—that is, an ordered performance allowing a sequence of subroutines in a familiar work situation to be composed. The rule is typically selected from previous experiences and reflects the functional properties which constrain the behaviour of the environment. Rule-based performance is based on explicit know-how as regards employing the relevant rules. The decision data set consists of references for recognition and identification of states, events or situations.
  3. Knowledge-based behaviour is a category of goal-controlled performance, in which the goal is explicitly formulated based on knowledge of the environment and the aims of the person. The internal structure of the system is represented by a “mental model”. This kind of behaviour allows the development and testing of different plans under unfamiliar and, therefore, uncertain control conditions, and is needed when skills or rules are either unavailable or inadequate so that problem solving and planning must be called upon instead.

 

In the design and management of a HAS, one should consider the cognitive characteristics of the workers in order to assure the compatibility of system operation with the worker’s internal model that describes its functions. Consequently, the system’s description level should be shifted from the skill-based to the rule-based and knowledge-based aspects of human functioning, and appropriate methods of cognitive task analysis should be used to identify the operator’s model of a system. A related issue in the development of a HAS is the design of means of information transmission between the human operator and automated system components, at both the physical and the cognitive levels. Such information transfer should be compatible with the modes of information utilized at different levels of system operation—that is, visual, verbal, tactile or hybrid. This informational compatibility ensures that different forms of information transfer will require minimal incompatibility between the medium and the nature of the information. For example, a visual display is best for transmission of spatial information, while auditory input may be used to convey textual information.

Quite often the human operator develops an internal model that describes the operation and function of the system according to his or her experience, training and instructions in connection with the given type of human-machine interface. In light of this reality, the designers of a HAS should attempt to build into the machines (or other artificial systems) a model of the human operator’s physical and cognitive characteristics—that is, the system’s image of the operator (Hollnagel and Woods 1983). The designers of a HAS must also take into consideration the level of abstraction in the system description as well as various relevant categories of the human operator’s behaviour. These levels of abstraction for modelling human functioning in the working environment are as follows (Rasmussen 1983): (1) physical form (anatomical structure), (2) physical functions (physiological functions), (3) generalized functions (psychological mechanisms and cognitive and affective processes), (4) abstract functions (information processing) and (5) functional purpose (value structures, myths, religions, human interactions). These five levels must be considered simultaneously by the designers in order to ensure effective HAS performance.

System Software Design

Since the computer software is a primary component of any HAS environment, software development, including design, testing, operation and modification, and software reliability issues must also be considered at the early stages of HAS development. By this means, one should be able to lower the cost of software error detection and elimination. It is difficult, however, to estimate the reliability of the human components of a HAS, on account of limitations in our ability to model human task performance, the related workload and potential errors. Excessive or insufficient mental workload may lead to information overload and boredom, respectively, and may result in degraded human performance, leading to errors and the increasing probability of accidents. The designers of a HAS should employ adaptive interfaces, which utilize artificial intelligence techniques, to solve these problems. In addition to human-machine compatibility, the issue of human-machine adaptability to each other must be considered in order to reduce the stress levels that come about when human capabilities may be exceeded.

Due to the high level of complexity of many hybrid automated systems, identification of any potential hazards related to the hardware, software, operational procedures and human-machine interactions of these systems becomes critical to the success of efforts aimed at reduction of injuries and equipment damage. Safety and health hazards associated with complex hybrid automated systems, such as computer-integrated manufacturing technology (CIM), is clearly one of the most critical aspects of system design and operation.

System Safety Issues

Hybrid automated environments, with their significant potential for erratic behaviour of the control software under system disturbance conditions, create a new generation of accident risks. As hybrid automated systems become more versatile and complex, system disturbances, including start-up and shut-down problems and deviations in system control, can significantly increase the possibility of serious danger to the human operators. Ironically, in many abnormal situations, operators usually rely on the proper functioning of the automated safety subsystems, a practice which may increase the risk of severe injury. For example, a study of accidents related to malfunctions of technical control systems showed that about one-third of the accident sequences included human intervention in the control loop of the disturbed system.

Since traditional safety measures cannot be easily adapted to the needs of HAS environments, injury control and accident prevention strategies need to be reconsidered in view of the inherent characteristics of these systems. For example, in the area of advanced manufacturing technology, many processes are characterized by the existence of substantial amounts of energy flows which cannot be easily anticipated by the human operators. Furthermore, safety problems typically emerge at the interfaces between subsystems, or when system disturbances progress from one subsystem to another. According to the International Organization for Standardization (ISO 1991), the risks associated with hazards due to industrial automation vary with the types of industrial machines incorporated into the specific manufacturing system and with the ways in which the system is installed, programmed, operated, maintained and repaired. For example, a comparison of robot-related accidents in Sweden to other types of accidents showed that robots may be the most hazardous industrial machines used in advanced manufacturing industry. The estimated accident rate for industrial robots was one serious accident per 45 robot-years, a higher rate than that for industrial presses, which was reported to be one accident per 50 machine-years. It should be noted here that industrial presses in the United States accounted for about 23% of all metalworking machine-related fatalities for the 1980–1985 period, with power presses ranked first with respect to the severity-frequency product for non-fatal injuries.

In the domain of advanced manufacturing technology, there are many moving parts which are hazardous to workers as they change their position in a complex manner outside the visual field of the human operators. Rapid technological developments in computer-integrated manufacturing created a critical need to study the effects of advanced manufacturing technology on the workers. In order to identify the hazards caused by various components of such a HAS environment, past accidents need to be carefully analysed. Unfortunately, accidents involving robot use are difficult to isolate from reports of human operated machine-related accidents, and, therefore, there may be a high percentage of unrecorded accidents. The occupational health and safety rules of Japan state that “industrial robots do not at present have reliable means of safety and workers cannot be protected from them unless their use is regulated”. For example, the results of the survey conducted by the Labour Ministry of Japan (Sugimoto 1987) of accidents related to industrial robots across the 190 factories surveyed (with 4,341 working robots) showed that there were 300 robot-related disturbances, of which 37 cases of unsafe acts resulted in some near accidents, 9 were injury-producing accidents, and 2 were fatal accidents. The results of other studies indicate that computer-based automation does not necessarily increase the overall level of safety, as the system hardware cannot be made fail-safe by safety functions in the computer software alone, and system controllers are not always highly reliable. Furthermore, in a complex HAS, one cannot depend exclusively on safety-sensing devices to detect hazardous conditions and undertake appropriate hazard-avoidance strategies.

Effects of Automation on Human Health

As discussed above, worker activities in many HAS environments are basically those of supervisory control, monitoring, system support and maintenance. These activities may also be classified into four basic groups as follows: (1) programming tasks i.e., encoding the information that guides and directs machinery operation, (2) monitoring of HAS production and control components, (3) maintenance of HAS components to prevent or alleviate machinery malfunctions, and (4) performing a variety of support tasks, etc. Many recent reviews of the impact of the HAS on worker well-being concluded that although the utilization of a HAS in the manufacturing area may eliminate heavy and dangerous tasks, working in a HAS environment may be dissatisfying and stressful for the workers. Sources of stress included the constant monitoring required in many HAS applications, the limited scope of the allocated activities, the low level of worker interaction permitted by the system design, and safety hazards associated with the unpredictable and uncontrollable nature of the equipment. Even though some workers who are involved in programming and maintenance activities feel the elements of challenge, which may have positive effects on their well-being, these effects are often offset by the complex and demanding nature of these activities, as well as by the pressure exerted by management to complete these activities quickly.

Although in some HAS environments the human operators are removed from traditional energy sources (the flow of work and movement of the machine) during normal operating conditions, many tasks in automated systems still need to be carried out in direct contact with other energy sources. Since the number of different HAS components is continually increasing, special emphasis must be placed on workers’ comfort and safety and on the development of effective injury control provisions, especially in view of the fact that the workers are no longer able to keep up with the sophistication and complexity of such systems.

In order to meet the current needs for injury control and worker safety in computer integrated manufacturing systems, the ISO Committee on Industrial Automation Systems has proposed a new safety standard entitled “Safety of Integrated Manufacturing Systems” (1991). This new international standard, which was developed in recognition of the particular hazards which exist in integrated manufacturing systems incorporating industrial machines and associated equipment, aims to minimize the possibilities of injuries to personnel while working on or adjacent to an integrated manufacturing system. The main sources of potential hazards to the human operators in CIM identified by this standard are shown in figure 1.

Figure 1. Main source of hazards in computer-intergrated manufacturing (CIM) (after ISO 1991)

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Human and System Errors

In general, hazards in a HAS can arise from the system itself, from its association with other equipment present in the physical environment, or from interactions of human personnel with the system. An accident is only one of the several outcomes of human-machine interactions that may emerge under hazardous conditions; near accidents and damage incidents are much more common (Zimolong and Duda 1992). The occurrence of an error can lead to one of these consequences: (1) the error remains unnoticed, (2) the system can compensate for the error, (3) the error leads to a machine breakdown and/or system stoppage or (4) the error leads to an accident.

Since not every human error that results in a critical incident will cause an actual accident, it is appropriate to distinguish further among outcome categories as follows: (1) an unsafe incident (i.e., any unintentional occurrence regardless whether it results in injury, damage or loss), (2) an accident (i.e., an unsafe event resulting in injury, damage or loss), (3) a damage incident (i.e., an unsafe event which results only in some kind of material damage), (4) a near accident or “near miss” (i.e., an unsafe event in which injury, damage or loss was fortuitously avoided by a narrow margin) and (5) the existence of accident potential (i.e., unsafe events which could have resulted in injury, damage, or loss, but, owing to circumstances, did not result in even a near accident).

One can distinguish three basic types of human error in a HAS:

  1. skill-based slips and lapses
  2. rule-based mistakes
  3. knowledge-based mistakes.

 

This taxonomy, devised by Reason (1990), is based on a modification of Rasmussen’s skill-rule-knowledge classification of human performance as described above. At the skill-based level, human performance is governed by stored patterns of pre-programmed instructions represented as analogue structures in a space-time domain. The rule-based level is applicable to tackling familiar problems in which solutions are governed by stored rules (called “productions”, since they are accessed, or produced, at need). These rules require certain diagnoses (or judgements) to be made, or certain remedial actions to be taken, given that certain conditions have arisen that demand an appropriate response. At this level, human errors are typically associated with the misclassification of situations, leading either to the application of the wrong rule or to the incorrect recall of consequent judgements or procedures. Knowledge-based errors occur in novel situations for which actions must be planned “on-line” (at a given moment), using conscious analytical processes and stored knowledge. Errors at this level arise from resource limitations and incomplete or incorrect knowledge.

The generic error-modelling systems (GEMS) proposed by Reason (1990), which attempts to locate the origins of the basic human error types, can be used to derive the overall taxonomy of human behaviour in a HAS. GEMS seeks to integrate two distinct areas of error research: (1) slips and lapses, in which actions deviate from current intention due to execution failures and/or storage failures and (2) mistakes, in which the actions may run according to plan, but the plan is inadequate to achieve its desired outcome.

Risk Assessment and Prevention in CIM

According to the ISO (1991), risk assessment in CIM should be performed so as to minimize all risks and to serve as a basis for determining safety objectives and measures in the development of programmes or plans both to create a safe working environment and to ensure the safety and health of personnel as well. For example, work hazards in manufacturing-based HAS environments can be characterized as follows: (1) the human operator may need to enter the danger zone during disturbance recovery, service and maintenance tasks, (2) the danger zone is difficult to determine, to perceive and to control, (3) the work may be monotonous and (4) the accidents occurring within computer-integrated manufacturing systems are often serious. Each identified hazard should be assessed for its risk, and appropriate safety measures should be determined and implemented to minimize that risk. Hazards should also be ascertained with respect to all of the following aspects of any given process: the single unit itself; the interaction between single units; the operating sections of the system; and the operation of the complete system for all intended operating modes and conditions, including conditions under which normal safeguarding means are suspended for such operations as programming, verification, troubleshooting, maintenance or repair.

The design phase of the ISO (1991) safety strategy for CIM includes:

  • specification of the limits of system parameters
  • application of a safety strategy
  • identification of hazards
  • assessment of the associated risks
  • removal of the hazards or diminution of the risks as much as practicable.

 

The system safety specification should include:

  • a description of system functions
  • a system layout and/or model
  • the results of a survey undertaken to investigate the interaction of different working processes and manual activities
  • an analysis of process sequences, including manual interaction
  • a description of the interfaces with conveyor or transport lines
  • process flow charts
  • foundation plans
  • plans for supply and disposal devices
  • determination of the space required for supply and disposal of material
  • available accident records.

 

In accordance with the ISO (1991), all necessary requirements for ensuring a safe CIM system operation need to be considered in the design of systematic safety-planning procedures. This includes all protective measures to effectively reduce hazards and requires:

  • integration of the human-machine interface
  • early definition of the position of those working on the system (in time and space)
  • early consideration of ways of cutting down on isolated work
  • consideration of environmental aspects.

 

The safety planning procedure should address, among others, the following safety issues of CIM:

  • Selection of the operating modes of the system. The control equipment should have provisions for at least the following operating modes:(1) normal or production mode (i.e., with all normal safeguards connected and operating), (2) operation with some of the normal safeguards suspended and (3) operation in which system or remote manual initiation of hazardous situations is prevented (e.g., in the case of local operation or of isolation of power to or mechanical blockage of hazardous conditions).
  • Training, installation, commissioning and functional testing. When personnel are required to be in the hazard zone, the following safety measures should be provided in the control system: (1) hold to run, (2) enabling device, (3) reduced speed, (4) reduced power and (5) moveable emergency stop.
  • Safety in system programming, maintenance and repair. During programming, only the programmer should be allowed in the safeguarded space. The system should have inspection and maintenance procedures in place to ensure continued intended operation of the system. The inspection and maintenance programme should take into account the recommendations of the system supplier and those of suppliers of various elements of the systems. It scarcely needs mentioning that personnel who perform maintenance or repairs on the system should be trained in the procedures necessary to perform the required tasks.
  • Fault elimination. Where fault elimination is necessary from inside the safeguarded space, it should be performed after safe disconnection (or, if possible, after a lockout mechanism has been actuated). Additional measures against erroneous initiation of hazardous situations should be taken. Where hazards can occur during fault elimination at sections of the system or at the machines of adjoining systems or machines, these should also be taken out of operation and protected against unexpected starting. By means of instruction and warning signs, attention should be drawn to fault elimination in system components which cannot be observed completely.

 

System Disturbance Control

In many HAS installations utilized in the computer-integrated manufacturing area, human operators are typically needed for the purpose of controlling, programming, maintaining, pre-setting, servicing or troubleshooting tasks. Disturbances in the system lead to situations that make it necessary for workers to enter the hazardous areas. In this respect, it can be assumed that disturbances remain the most important reason for human interference in CIM, because the systems will more often than not be programmed from outside the restricted areas. One of the most important issues for CIM safety is to prevent disturbances, since most risks occur in the troubleshooting phase of the system. The avoidance of disturbances is the common aim as regards both safety and cost-effectiveness.

A disturbance in a CIM system is a state or function of a system that deviates from the planned or desired state. In addition to productivity, disturbances during the operation of a CIM have a direct effect on the safety of the people involved in operating the system. A Finnish study (Kuivanen 1990) showed that about one-half of the disturbances in automated manufacturing decrease the safety of the workers. The main causes for disturbances were errors in system design (34%), system component failures (31%), human error (20%) and external factors (15%). Most machine failures were caused by the control system, and, in the control system, most failures occurred in sensors. An effective way to increase the level of safety of CIM installations is to reduce the number of disturbances. Although human actions in disturbed systems prevent the occurrence of accidents in the HAS environment, they also contribute to them. For example, a study of accidents related to malfunctions of technical control systems showed that about one-third of the accident sequences included human intervention in the control loop of the disturbed system.

The main research issues in CIM disturbance prevention concern (1) major causes of disturbances, (2) unreliable components and functions, (3) the impact of disturbances on safety, (4) the impact of disturbances on the function of the system, (5) material damage and (6) repairs. The safety of HAS should be planned early at the system design stage, with due consideration of technology, people and organization, and be an integral part of the overall HAS technical planning process.

HAS Design: Future Challenges

To assure the fullest benefit of hybrid automated systems as discussed above, a much broader vision of system development, one which is based on integration of people, organization and technology, is needed. Three main types of system integration should be applied here:

  1. integration of people, by assuring effective communication between them
  2. human-computer integration, by designing suitable interfaces and interaction between people and computers
  3. technological integration, by assuring effective interfacing and interactions between machines.

 

The minimum design requirements for hybrid automated systems should include the following: (1) flexibility, (2) dynamic adaptation, (3) improved responsiveness, and (4) the need to motivate people and make better use of their skills, judgement and experience. The above also requires that HAS organizational structures, work practices and technologies be developed to allow people at all levels of the system to adapt their work strategies to the variety of systems control situations. Therefore, the organizations, work practices and technologies of HAS will have to be designed and developed as open systems (Kidd 1994).

An open hybrid automated system (OHAS) is a system that receives inputs from and sends outputs to its environment. The idea of an open system can be applied not only to system architectures and organizational structures, but also to work practices, human-computer interfaces, and the relationship between people and technologies: one may mention, for example, scheduling systems, control systems and decision support systems. An open system is also an adaptive one when it allows people a large degree of freedom to define the mode of operating the system. For example, in the area of advanced manufacturing, the requirements of an open hybrid automated system can be realized through the concept of human and computer-integrated manufacturing (HCIM). In this view, the design of technology should address the overall HCIM system architecture, including the following: (1) considerations of the network of groups, (2) the structure of each group, (3) the interaction between groups, (4) the nature of the supporting software and (5) technical communication and integration needs between supporting software modules.

The adaptive hybrid automated system, as opposed to the closed system, does not restrict what the human operators can do. The role of the designer of a HAS is to create a system that will satisfy the user’s personal preferences and allow its users to work in a way that they find most appropriate. A prerequisite for permitting user input is the development of an adaptive design methodology—that is, an OHAS that allows enabling, computer-supported technology for its implementation in the design process. The need to develop a methodology for adaptive design is one of the immediate requirements to realize the OHAS concept in practice. A new level of adaptive human supervisory control technology needs also to be developed. Such technology should allow the human operator to “see through” the otherwise invisible control system of HAS functioning—for example, by application of an interactive, high-speed video system at each point of system control and operation. Finally, a methodology for development of an intelligent and highly adaptive, computer-based support of human roles and human functioning in the hybrid automated systems is also very much needed.

 

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Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides