From the reliability computation viewpoint, classification of the system depends primarily on how system performance is affected by its components or modes of operation. A multiple-component system called a series system (see Fig. 7.1) requires that all its components perform satisfactorily to allow satisfactory performance of the entire system. Similarly, for a single-component system involving several modes of operation, it is also viewed as a series system if satisfactory performance of the system requires satisfactory performance of all its different modes of operation.
A second basic type of system is called a parallel system (see Fig. 7.2). A parallel system is characterized by the property that the system would serve its intended purpose satisfactorily as long as at least one of its components or modes of operation performs satisfactorily.
For most real-life problems, system configurations are complex, in which the components are arranged as a mixture of series and parallel subsystems or in the form of a loop. In dealing with the reliability analysis of a complex system, the general approach is to reduce the system configuration, based on the arrangement of its components or modes of operation, to a simpler situation for which the reliability analysis can be performed easily. However, this goal may not always be achievable, in which case a special procedure would have to be devised.
