System Definition
Reliability models are central to the entire design process. Therefore, they must be based on a thorough understanding of the proposed system design and the requirements that the design must satisfy. System definition involves making studies of documents ranging from staff requirements to circuit diagrams, working closely with design staff and carrying out detailed engineering analyses to determine functional dependencies within the design. Necessary data may not always be clearly specified, and assumptions may have to be made. However, such assumptions must always be recorded and agreed to by both customer and supplier.
Compared to only a few years ago, the analysis of reliability models, even using powerful analytical techniques, now presents little difficulty in the light of techniques that have been developed and, in many cases, this analysis is supported by commercially available software packages. Ensuring that the models produced results that actually represent reality is where there still is, or can be, a considerable amount of difficulty.
The objective of system definition is to bring together all available information relating to the system and its components and to record in an ordered manner:
• The specified operational requirements and any constraints, for the system.
• The proposed system configuration, including the functional relationships between items comprising the system and failure criteria.
• The operating and maintenance conditions that apply to the system.
These aspects are considered more fully in the following paragraphs. It must be emphasised that, in practice, the various factors are all closely interrelated and must be considered as a whole.
During the detailed design stage of a project, data should be available to define a system down to the component/part level, and this must be done because most tables of base failure rates (e.g., see
Data Tables) provide data only at this level. Clearly, during earlier project stages, system definition will be restricted to higher levels of assembly but the same general principles for gathering data and analysing the system will apply. System definition involves much detailed and time-consuming work, which may be done by many different people. Therefore, a reference or coding method that readily identifies any particular item within the system hierarchy must be adopted so that data can be cross-referenced easily and accurately.