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DEFENCE IN DEPTH
Defence in depth: facilities

WHAT MAKES A SAFETY LAYER?

 

A safety provision that cannot be broken down into redundant parts

You have read this in the Manual, but it is worth stressing: if two separate indications are routed through a single interlock, the indications and the interlock together provide a single safety layer.

The number of layers that should be counted for independent systems depends somewhat on the reliability of the systems and whether there are any common support systems.

The only quantitative guidance given in the Manual is that a safety layer would be expected to have a failure rate of about 1 in a 100. Most of the layers are not susceptible to sophisticated reliability analysis, but this figure was provided to give some indication of what is expected of a safety layer.