8 – Methods of modelling

To establish a reliability block for a system, it is necessary to describe a system as a number of functional blocks which are interconnected according to the effect of each block failure on the overall system reliability. There may well be more than one type of system failure, in which case a number of predictions giving different reliabilities is required. This step is absolutely essential if the predictions are to have any significance. Failure Mode and Effect Analysis (FMEA) provides block failure rates by examining individual component failure modes and failure rates. Given a constant failure rate and no internal redundancy, each block will have a failure rate predicted from the sum of the failure rates on the FMEA worksheet.