Reliability Block Diagrams (RBDs)

Abstract

The reliability block diagram (RBD) is perhaps the most popular and widely used approach to deal with safety and dependability modelling. This chapter describes its position within the probabilistic approaches in general and the Boolean family approaches in particular. This is a directed acyclic graph drafted by using specific symbols (blocks) and logic structures (series, parallel, majority vote). It aims to model how a system is in up state (i.e. in good operating conditions) from the logic combination of the up states of its components. The various structures are analysed and simple examples are provided to explain how to effectively build such models. This chapter is the opportunity to introduce, by analogy with an electrical circuit, the concepts of tie sets and cut sets. As explained in further chapters, the minimal cut sets concept is invaluable when dealing with qualitative analyses and for the detection of weak points of safety or production systems. This chapter also raises a warning: the RBD users should not be misled by the name of the approach because the RBDs cannot be used straightforwardly to calculate the reliability of repaired items.