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Systems Model of Construction Accident Causation

System model proposes the failure in coordination of three systems i.e. man, machine and environment as shown in Fig 6. This model has been used for many years by people at all levels in organisations from supervisors through to safety managers to investigate incidents with good success due to its ease of use and thoroughness of approach.

This perspective has a limited view of accident causality, as it ignores the work system factors and their interactions that generate the hazardous situations and shape the work behaviors. Understanding and addressing these causal factors that lead to accidents is necessary to develop effective accident prevention strategies. The model takes a systems view of accidents it focuses on how the characteristics of the production system generate hazardous situations and shape the work behaviors, and analyzes the conditions that trigger the release of the hazards. The model is based on descriptive rather than prescriptive models of work behaviors it takes into account the actual production behaviors, as opposed to the normative behaviors and procedures that workers "should" follow. The model identifies the critical role of task unpredictability in generating unexpected hazardous situations, and acknowledges the inevitability of exposures and errors. The model identifies the need for two accident prevention strategies:

  1. reliable production planning to reduce task unpredictability, and
  2. error management to increase the workers' ability to avoid, trap, and mitigate errors. This model contributes to safety research by increasing understanding of the production system factors that affect the frequency of accident. The practical benefit of The model is that it provides practitioners with strategies to reduce the likelihood of accidents.

System Theory Model

The model advocates the proper assessment of cumulative hazards during the interaction of three systems and guides the risk assessment before to decision of doing work.

The "pure chance" theory

According to the pure chance theory, every one of any given set of workers has an equal chance of being involved in an accident. It further implies that there is no single discernible pattern of events that leads to an accident. In this theory, all accidents are treated as corresponding to Heinrich's acts of God, and it is held that there exist no interventions to prevent them.

Biased liability theory

Biased liability theory is based on the view that once a worker is involved in an accident, the chances of the same worker becoming involved in future accidents are either increased or decreased as compared to the rest of workers. This theory contributes very little, if anything at all, towards developing preventive actions for avoiding accidents.

Accident proneness theory

Accident proneness theory maintains that within a given set of workers, there exists a subset of workers who are more liable to be involved in accidents. Researchers have not been able to prove this theory conclusively because most of the research work has been poorly conducted and most of the findings are contradictory and inconclusive. This theory is not generally accepted. It is felt that if indeed this theory is supported by any empirical evidence at all, it probably accounts for only a very low proportion of accidents without any statistical significance.

The energy transfer theory

Those who accept the energy transfer theory put forward the claim that a worker incurs injury or equipment suffers damage through a change of energy, and that for every change of energy there is a source, a path and a receiver. This theory is useful for determining injury causation and evaluating energy hazards and control methodology. Strategies can be developed which are either preventive, limiting or ameliorating with respect to the energy transfer. Control of energy transfer at the source can be achieved by the following means:

  • elimination of the source
  • changes made to the design or specification of elements of the work station
  • preventive maintenance.

The path of energy transfer can be modified by:

  • enclosure of the path
  • installation of barriers
  • installation of absorbers
  • positioning of isolators.

The receiver of energy transfer can be assisted by adopting the following measures:

  • limitation of exposure
  • use of personal protective equipment.

Modern theory of accident

According to modern thinking, accidents require the coming together of a number of enabling-factors each one necessary but in itself not sufficient to breach system defences. Major equipment failures or operational personnel errors are seldom the sole cause of breaches in safety defences. Often these breakdowns are the consequence of human failures in decision-making. The breakdowns may involve active failures at the operational level, or latent conditions conducive to facilitating a breach of the system's inherent safety defences. Most accidents include both active and latent conditions.

The figure portrays an accident causation model that assists in understanding the interplay of organizational and management factors (i.e. system factors) in accident causation.
Various "defences" are built into the plant system to protect against inappropriate performance or poor decisions at all levels of the system (i.e. the front-line workplace, the supervisory levels and senior management). This model shows that while organizational factors, including management decisions, can create latent conditions that could lead to an accident, they also contribute to the system's defences.

Errors and violations having an immediate adverse effect can be viewed as unsafe acts; these unsafe acts may penetrate the various defences put in place to protect the system by company management, the regulatory authorities, etc., resulting in an accident. These unsafe acts may be the result of normal errors, or they may result from deliberate violations of prescribed procedures and practices. The model recognizes that there are many error- or violation-producing conditions in the work environment that may affect individual or team behaviour.
These unsafe acts are committed in an operational context which includes latent unsafe conditions. A latent condition is the result of an action or decision made well before an accident. Its consequences may remain dormant for a long time. Individually, these latent conditions are usually not harmful since they are not perceived as being failures in the first place.

Latent unsafe conditions may only become evident once the system's defences have been breached. They may have been present in the system well before an accident and are generally created by decision-makers, regulators and other people far removed in time and space from the accident. Front-line operational personnel can inherit defects in the system, such as those created by poor equipment or task design; conflicting goals (e.g. service that is on time versus safety); defective organizations (e.g. poor internal communications); or bad management decisions (e.g. deferral of a maintenance item). Effective safety management efforts aim to identify and mitigate these latent unsafe conditions on a system-wide basis, rather than by localized efforts to minimize unsafe acts by individuals. Such unsafe acts may only be symptoms of safety problems, not causes.

Even in the best-run organizations, most latent unsafe conditions start with the decision makers. These decision-makers are subject to normal human biases and limitations, as well as to very real constraints of time, budget, politics, etc. Since some of the unsafe decisions cannot be prevented, steps must be taken to detect them and to reduce their adverse consequences.

Fallible decisions by line management may take the form of inadequate procedures, poor scheduling or neglect of recognizable hazards. They may lead to inadequate knowledge and skills or inappropriate operating procedures. How well line management and the organization as a whole perform their functions sets the scene for error- or violation-producing conditions. For example, how effective is management with respect to setting attainable work goals, organizing tasks and resources, managing day to-day affairs, and communicating internally and externally? The fallible decisions made by company management and regulatory authorities are too often the consequence of inadequate resources. However, avoiding the costs of strengthening the safety of the system can facilitate accidents that are so expensive as to bankrupt the operator.

The domino theory found that 88 per cent of accidents are caused by unsafe acts of people, 10 per cent by unsafe actions and 2 per cent by 'acts of God'. Interestingly, the 'acts of God' concept alludes that there may be a level of risk that is not controllable and therefore, non controllable.


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