Inspection, Testing, and Maintenance
Test the function, not just the device
Inspection, testing, and maintenance (ITM) preserve the integrity claimed in design. Distinguish inspection (visual or condition check), proof test (demonstrates the safety function operates when demanded), functional test (confirms specified behavior), and calibration (adjusts to reference). Proof-test interval is a risk decision: longer intervals generally increase average probability of failure on demand (PFDavg). Test coverage matters; a partial proof test that exercises only the sensor leaves hidden failures in the logic or final element. Record as-found and as-left conditions, test method, date, tester, and any bypass used.
Traps that appear in questions
- Calling a sensor calibration a complete proof test of the loop.
- Forcing a signal at the logic solver and declaring the final element tested.
- Leaving a bypass active after testing.
- Choosing intervals by calendar convenience rather than risk and failure data.
- Failing to record as-found condition, so degradation trends are lost.
Worked rule of thumb
Design the proof test to drive the loop from sensor input through logic to final-element movement, and confirm the safe state. If full end-to-end testing is impractical, document the partial coverage and compensate with inspection, diagnostics, or a shorter interval for the untested portion. A test that cannot detect a dangerous failure provides little risk reduction.