Glossary · Verification and validation
Proof-test interval
Also known as: proof test interval, T1
German: Wiederholungsprüfintervall
In functional safety, the proof-test interval is the time between two proof tests of a safety function or its components. It is a parameter of the failure probability calculation, because dangerous undetected failures can remain hidden until the next proof test.
- Verification
- Functional safety
In one sentence
The proof-test interval is the time between proof tests; longer intervals let hidden dangerous failures accumulate and raise PFDavg.
Example
The SIL verification for a high-level trip assumes a proof-test interval of 12 months, so the maintenance plan schedules a full functional test of the level transmitter, logic and valve every year.
How it applies
- Calculation: In low-demand mode, the average probability of failure on demand grows with the proof-test interval. Changing the interval changes the result of SIL verification.
- Maintenance: The interval assumed in the calculation must be the interval actually applied in the plant. Postponing a proof test, for example to align with a turnaround, has to be assessed.
- Coverage: A proof test rarely detects every dangerous failure. The proof-test coverage and the interval together determine how well hidden failures are revealed.
- Documentation: State the interval next to each safety function in the Safety requirements specification (SRS), the maintenance manual and the test procedure. The documentation team should flag it as a safety-relevant value so that edits go through change control.
Proof-test interval vs. maintenance interval
A Maintenance interval keeps equipment in working condition. A proof-test interval is a safety parameter taken from the reliability calculation. Both can be scheduled together, but the proof-test interval may only be changed with a new safety assessment.