ASTRA 3.x: Theoretical Manual

Abstract

This report describes the main algorithms implemented in ASTRA 3.x to analyse coherent and non-coherent fault trees. ASTRA 3.x is fully based on the state-of-the-art of Binary Decision Diagrams (BDD) approach. In case of non-coherent fault trees ASTRA 3.x dynamically assigns to each node of the graph a label that identifies the type of the associated variable in order to drive the application of the most suitable analysis algorithms. The resulting BDD is referred to as Labelled BDD (LBDD). Exact values of the unavailability, expected number of failure and repair are calculated; the unreliability upper bound is automatically determined under given conditions. Several importance measures of basic events are also provided. From the LBDD a ZBDD embedding all MCS is obtained from which a subset of Significant Minimal Cut Sets (SMCS) is determined through the application of the cut-off techniques. An important issue is related to the analysis of safety related systems according to the IEC 61508 international standard. In order to simplify the fault tree modelling and analysis a new component type has been defined allowing determining, for any configuration, the PFDavg and PFHavg values. The Staggered testing policy is also applicable besides the Sequential testing implicitly considered by the IEC standard