The possibility of a fault exclusion is linked to the compromise between the requirement to consider all dangerous faults and the theoretical possibility that certain type of dangerous fault could not occur.
Fault exclusion is based on:
- The very low probability of occurrence of some faults
- The accepted technical robusteness of a certain type of component, regardless of the application considered
- The technical requirements concerning the application and the type of risk
Since a fault exclusion can lead to a very high PL, a detailed justification of the exclusion must be provided in the technical documentation.
For new components or components that are not in the lists of ISO 13849-2, an FMEA analysis (see IEC 60812) must be carried out to establish the faults that must be considered for these components and those that can be excluded.
If dangerous faults can be excluded for a component, the contribution of the component to the MTTFD is zero.
For electromechanical components, the analysis on the fault exclusion must be conducted separately for the mechanical part and for the electrical part, considering the environmental conditions and possible external influences.