Fault-tree analysis considering latency of basic events

Severe accidents can occur even in advanced systems such as airplanes and nuclear power plants. In the accident analysis, most of the attention focuses on the active failure which initiates the occurrence of system failure, but the latent condition which allows the active failure to cause system failure must be considered. This paper proposes a simple method to deal with latent conditions in FTA (fault tree analysis) based on the detectability of basic events. In FTA analysis, the system failure occurs if all basic events in a minimal cut set occur, but all the event sequences leading to the occurrence of a minimal cut set do not always occur without their detection. Using the detectability index of a basic event, it is evaluated whether the critical state of a basic event is latent where its occurrence can cause the system failure. Based on the number/probability of critical states that can be detected by the detectability of a basic event, a new measure of its importance in contributing to the reduction of system failure is proposed. A simple illustrative example shows these properties.