4.1 Introduction

A system is a collection of components, subsystems or assemblies arranged in a specific pattern in order to achieve desired function(s) with acceptable performance and reliability. The types of components, their quantities, their quality and the manner in which they are arranged within the system have a direct bearing on the reliability of the system. In order to calculate the reliability of a system the reliabilities of the subsystems are calculated and combined using reliability models that are based on certain probability laws. The level to which the decomposition is done must be such that the reliabilities of the resulting components are known with reasonable and acceptable precision. It may not therefore be necessary to decompose the system into individual real components but into a set of devices or subsystems, the reliabilities of which are known from experience. This approach requires knowledge of the physical structure of the system and the nature of its functions sufficiently to determine the behaviour of the system in the event of failure of a subsystem. The system reliability models discussed in this chapter are based on the assumption that the components fail independent of one another; i.e. the failure of one component does not affect the functioning of other components.

In order to understand the relationship between the system and its components for calculating the reliability, the reliability block diagram is constructed. In practice a system is frequently represented...