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Guideline – Demonstrating Prior Use v4
Page 19 of 30
Reference should be made to appendix B for a worked example.
A.3 Assessing the dominant failure mode
Using the data provided by the maintenance management system, or derived from other
sources, the dominant failure mode can be determined by performing a high level
analysis of the component (note that this does not imply analysis of individual entities
that make up the component, for example resistors, capacitors, semiconductors).
It may be sufficient for the dominant failure mode to be determined based on an analysis
of the suitability of the component - if it fails to a safe state on loss of input or motive
power (for example, air supply, electricity supply), and if there are no known duty
vulnerabilities. Where this cannot be established, a more formal review can be
undertaken.
An example of an analysis performed on an interface relay component which provides a
shutdown signal to a Motor Control Centre (MCC) is given in Table 2 below. Assuming
that the failure rate of failure ID #4 is less than that of ID #1 to #3, it can be seen that the
dominant
failure mode is to the Fail To Safe mode (FTS).