Functional Safety
What is Proof Testing in Functional Safety?
Vinit Pandey · Published 4 August 2026
In short: Proof testing is a periodic test performed on a safety instrumented function to reveal dangerous failures not detected by automatic diagnostics, restoring the function's assumed failure rate and keeping its calculated probability of failure on demand valid over its operating life.
Key takeaways
- Real-world reliability drifts from the SIL verification's assumed value without actual proof testing at interval
- Proof testing targets dangerous undetected failures that automatic diagnostics can't catch alone
- A thorough proof test exercises sensor, logic solver, and final element, not just the easiest component
- Extending a proof test interval without re-verifying SIL compliance can silently lower actual integrity
A safety instrumented function's calculated PFDavg during SIL verification assumes a specific proof test interval; without actually performing proof tests at that interval in practice, the real-world reliability of the function silently drifts from its assumed value over time.
Proof testing specifically targets dangerous undetected failures — faults that would prevent the safety function from acting when needed, but that automatic diagnostics built into the instrumentation cannot detect on their own.
A thorough proof test exercises the complete safety instrumented function end to end where practical — sensor response, logic solver action, and final element movement — rather than testing only the component most convenient to access, since a partial test can miss failures elsewhere in the loop.
Proof test intervals are not arbitrary maintenance scheduling choices; they are a direct input to the SIL verification calculation, meaning a facility that extends a proof test interval without re-verifying SIL compliance may be operating with a lower actual integrity level than documented.