Total System Reliability
Based on the RBD principles described in Reliability Block Diagrams, the functional relationships within each level of assembly, and also between each level of assembly, can be set down for a total system. A simple example is shown in Figure 2-7.
Note that the levels to which relationships can be developed will vary according to the complexity of the system and the stage of the project. For example, for a complex system:
• During the conceptual and feasibility stages, data will probably be limited to system and sub-system levels.
• In the early design stage, data should be available at the unit level.
• As the detailed design is developed, data will become available at the module and component/part levels.
Having defined the functional relationships for a total system, reliability expressions such as those described in Combining Reliabilities (No Repair) can be used to compute the reliabilities of individual elements within the system, and, progressively, to combine these reliabilities at the higher levels of assembly up to the total system level.
However, it must be noted that standard sources of failure rate data normally provide only component or part failure rates. Detailed prediction cannot therefore be carried out until the detailed design stage, when component and part population data become available. Before this stage, prediction depends on the use of broader and more comparative methods; for example, by comparison with some similar system or by the broad assessment of the numbers and types of components and parts that may be expected in the future design. (For additional information, see , Reliability Prediction Methods.)
