Deriving Failure Rates in a Hierarchy of FMEAs

When analyzing the failure rate (or occurrence) of a system failure cause, the typical approach is to obtain an occurrence rating from the results of testing. However, in many cases, the occurrence of a system failure cause can be derived from a combination of occurrences of failure causes of the element (sub-system) failure mode coinciding with the system failure cause being assessed. This paper explores a few approaches for deriving occurrences from element FMEAs over a majority of cases before settling on a probabilistic approach that converts occurrences to worst-case failure rates to achieve the most fine-tuned combined occurrence rating. Finally, a “complex analysis” worksheet, where the logical combination of occurrences and failure rates is custom defined by the engineer, is introduced for handling special cases.