Common causes
Probabilité de défaillance à la sollicitation d'une fonction instrumentée de sécurité

KooN-type redundancy reduces the probability of RIS failure. However, it is necessary to take into account the possibility that certain causes may lead to the simultaneous failure of redundant equipment.

ISO 12489 introduces the notion of common cause of failure by pointing out that, while mathematically it is possible to consider that the probability of failure of "N" similar pieces of equipment with 1ooN redundancy is "p ^N " (p being the probability of failure of a piece of equipment), in practice it is not possible to "indefinitely" reduce the probability of failure of a system simply by increasing the number of redundant pieces of equipment. To illustrate this, the standard takes the example of...