Application of a computerized methodology for performing common cause failure analysis: The Mocus-Bacfire beta factor (MOBB) code

A fault tree analysis and qualitative common cause failure analysis of a power distribution box system was performed utilizing the Mocus-Bacfire Beta Factor (MOBB) code developed previously. This paper illustrates the advantages of this method over other available common cause procedures. This study indicates that the accepted use of the rare event approximation in performing fault tree analysis may lead to underestimates of risk by neglecting common cause dependencies. These dependencies have been fully modelled here, where it is shown that the error factor for the total system failure rate is high when common cause failures dominate random failures.