Multiple Safety Systems

safety system comprising several sub safety systems operating one after the other when the prior

ones have failed

Note 1 to entry: Industrial processes often implement multiple safety systems (safety layers). In this case the

failure of an intermediate safety layer provokes a demand on the proximate succeeding safety layer and so on. The

accident occurs only if the demand is transmitted until the ultimate safety layer and it fails to operate.

Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards

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