45 Two Types of Priority and Gates and Their Markov Analyses (PSAM-0123)

In general it is difficult to represent the course of time in a fault-tree. Priority AND gates have been used in fault-tree analysis especially in system safety assessment to represent the order relation between several event occurrences. On the other hand, the international standard, IEC 61025, describes only that a priority AND gate can be analyzed by Markov techniques without indication of any concrete techniques/methods. Hence this standard has not presented any information on practical analysis of a priority AND gate.

At first this paper points out that there are (at least) two types of priority AND gates, although such discrimination has not been considered until now. It is important that fault-tree analysis should reflect the types. Hence this paper presents two ways of analyses for each type of priority AND gates. Such discrimination between types of priority AND gates themselves and between ways of analyses is indispensable for actual fault-tree analysis according to such the international standard.