Abstract: In fault tree analysis, the system failure probability and the component importance measures cannot totally include the contribution of all the component existing states to system reliability. Hence, an equivalent failure probability concept and calculation method is proposed. First, the system existing states are analyzed by probability decomposition method. Markov chain method and the expectation theory are used to calculate the expected number of working circle. And the equivalent failure probability is finally attained. Analysis shows that, equivalent failure probability not only includes the contribution of the critical states of component to system reliability, but also considers the non critical states of component, the equivalent failure probability may compare the relative reliability when different systems have the same failure probability, and the concept of equivalent failure probability is an important exten-sion of current component importance measures and is useful for reliability design.

Key words: Equivalent failure probability, Expectation, Fault tree, Importance measure, Markov chain