Glossary · Functional safety
Fault exclusion
In functional safety, a fault exclusion is a justified assumption that a specified fault need not be considered because its occurrence is sufficiently unlikely under defined conditions. It is valid only as long as those conditions hold.
- Functional safety
- Verification
In one sentence
A fault exclusion is a justified decision to disregard a specific fault, and it is only valid under the conditions stated for it.
Example
The designer of a press excludes the breaking of a positively driven contact in a door switch, citing ISO 13849-2 and the manufacturer's data, under the condition that the switch is used within its rated mechanical life.
How it applies
- Functional safety: ISO 13849-2 lists faults to be considered and the conditions under which some may be excluded. Fault exclusions can limit the achievable Performance Level, so standards restrict their use for higher levels.
- Evidence: Every fault exclusion is an assumption with conditions. Document the fault, the justification, the source and the conditions, and link it to the safety function.
- Technical documentation: Conditions such as maximum operating cycles, mounting requirements or environmental limits must appear in the operating and maintenance information. If users do not know them, the exclusion silently becomes invalid.
- Change control: A modification, a component substitution or a new operating mode can remove the basis of an exclusion. Check the list during every change control review.
Fault exclusion vs. ignored fault
A fault exclusion is justified, documented and conditional. A fault that was simply not analyzed is a gap in the evidence.