UNSAFE ACT AND UNSAFE CONDITIONS BASIC INFORMATION AND TUTORIALS


Organisational defences against accidents can be conceptualised as barriers between hazards and losses. However, these barriers and defences have holes in them, just like slices of Swiss cheese. This is why Reason (1997) called his model, the Swiss cheese model.

The “unsafe acts” slice of cheese represents operations at the “sharp-end”, the front-line employees in an organisation. This slice of cheese represents defences within the realm of direct and immediate human activity at the workface.

The holes in this slice are unsafe acts (or failures to act), meaning human errors. According to Reason (1997), mistakes, slips, lapses and violations which occur here are active failures, and they are the immediate cause(s) of the accident.

Human errors can be immediate causes either singly or in combination: a violation paired with a mistake is a common immediate cause. It should be noted that some immediate causes may be technical; not all are human failures.

However, improvements in engineering reliability mean that simple technical failures are very rarely found to be the primary cause of accidents. Conversely, the proportion of accidents with a human cause has correspondingly increased.


Beyond “unsafe acts”, the holes in the next slice of cheese represent “unsafe conditions”. These unsafe conditions are unsafe practices and conditions within the local workplace and psychological factors, all of which increase the probability of unsafe acts.

These holes are latent factors which contribute to accidents, rather than active failures. In effect, the unsafe conditions are the error promoting conditions and performance shaping factors outlined above. There is a one-to-many mapping between unsafe conditions and unsafe acts, since an unsafe condition can promote many types of human error, and allow them to promulgate.

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