Common cause failure probabilities in standby safety system fault tree analysis with testing—scheme and timing dependencies

Modelling and quantification of common cause failures (CCFs) in redundant standby safety systems can be implemented by implicit or explicit fault tree techniques. Common cause event probabilities are derived for both methods for systems with time-related CCFs modelled by general multiple failure rates. The probabilities are determined so that the correct time-average risk can be obtained by a single computation. The impacts of test intervals and test staggering are included. Staggered testing is best with a certain extra-testing rule, although extra testing is not important for 1-out-of-n:G systems. An economic model provides insights into the impacts of various parameters: the optimal test interval increases with increasing redundancy and testing cost, and it decreases with increasing accident cost and initiating event rate. Staggered testing with extra tests allows for the longest optimal test intervals. A practical technique is outlined for incorporating assessment uncertainties in the estimation of multiple failure rates based on data from many plants or systems.