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