Component importance analysis in systems with common cause basic events

Consulta en la Biblioteca ETSI Industriales (Riunet)

[EN] A common cause basic event (CCBE) is an event involving failure of a specific set of components due to a common cause. For instance, in a system with a common cause component group (CCCG) made of components A, B, and C, the CCBEs are CAB, CAC, CBC and CABC.The CCBEs (e.g. CAB, CAC, and CABC) involving the same component (i.e. A) and the independent failure event of this component (AI) are all exclusive events.This means that a minimal cut set (MCS) with combination of these exclusive events (e.g. CAB˙CAC) is not reasonable: the system failure MCS equation should be reduced. In practice, these unreasonable MCSs appear in the system failure MCS equations used, leading to risk estimation errors. Furthermore, the importance indexes (e.g. RAW and RRW) provided by commercial codes are computed by directly substituting the basic event probability (i.e. set equal to 1 or 0) into the system failure MCS equation, without reminimizing. This may lead to conclude a wrong importance ranking because some of the MCSs are not minimal anymore. In this paper, we show this by calculating the importance index for some simple systems with CCBEs, highlighting the impact of the traditional approximation i.e. using single MCS equation without reminimizing, ignoring the exclusive events, and rare event assumption. We conclude that the calculation of the importance index of CCBEs cannot ignore the exclusive events effect.