Glossary · Verification and validation
Proof test
Also known as: proof testing
German: Wiederholungsprüfung
In functional safety, a proof test is a periodic test intended to reveal hidden dangerous failures and confirm continued safety function capability. After a proof test, the tested function is assumed to be restored to an as-new condition, as far as the test covers it.
- Verification
- Functional safety
In one sentence
A proof test is a periodic test that reveals hidden dangerous failures, so a safety function can be restored to its claimed integrity.
Example
Once a year, the operator of a hydraulic press tests the redundant safety valves by forcing each one separately and checking that the ram still stops, which reveals a stuck valve that the diagnostics could not detect.
How it applies
- Functional safety: The proof test interval is an input to the reliability calculation, especially for low-demand functions in IEC 61508. A longer interval than assumed lowers the achieved integrity.
- Technical documentation: The manufacturer must describe the proof test: steps, required tools, expected results and interval. Vague instructions like "check function regularly" do not support a SIL claim.
- Evidence: Record every proof test and its result. A failed test is also a data point for field data and reliability review.
- Maintenance: Proof tests are safety-critical maintenance. They need competent staff and, often, lockout/tagout or other hazard control during the test.
Proof test vs. diagnostics
Diagnostics detect some faults automatically during operation. The proof test targets the rest: dangerous failures that stay hidden until a demand occurs.