The precautionary approach required by the model WHS act forbids, on pain of criminal imprisonment, the exclusive use of target levels of risk (tolerable or acceptable) as a method of establishing whether a situation or circumstance is safe.
Amongst other concerns, this is problematic for those involved in SIL allocation under IEC 61508. The usual comment is; how else can it be done? Actually, it’s quite straight forward.
Disproportionality in safety results from the economist’s law of diminishing returns (the pain is not worth the gain) for precautionary effort based on the significance of the risk vs the effort required to reduce it.
For example, if an initial precaution reduces the risk by 99%, the next precaution can only address the remaining 1% of the risk and so on. This means that in terms of the balance of the significance of the risk vs the effort required to reduce it, the scales thump to the ‘lets not do anymore’ side very quickly for effective precautions. It is always hard to define this point in advance because it does depend on the actual circumstances of the issue of concern. But when considering the cost effectiveness of precautions in the context of all those available (as well as those already in place), it is generally quite obvious where to draw the line.
This has always been R2A’s understanding of the meaning of disproportionality as used in case law and now the model WHS act.