Definition:
Proof Test Coverage (Cpt) is the percentage of dangerous undetected failures that a proof test is capable of detecting in a Safety Instrumented Function (SIF) or safety device. Higher Cpt values mean that a proof test is more effective in uncovering hidden failures that would otherwise contribute to the Probability of Failure on Demand (PFDavg).
This has nothing to do with diagnostics or any automated checking the SIF does on itself. Only the manual proof test done by an operator.
The value of Cpt used in the calculations often comes from the manufacturer via the safety manual for devices that are certified to IEC 61508. If not in the documentation there are other sources that can be referenced.
Key Points:
- Expressed as a percentage (e.g., 90% Cpt).
- Not all dangerous undetected failures can be found during a typical proof test — Cpt accounts for this limitation.
- Directly impacts the calculated PFDavg for a SIF. It is often a balance of deciding how a proof test will be done, which dictates the Cpt, which impacts the PFDavg/RRF, and could even impact the architecture chosen.
- Must be factored into SIL verification and documented in the Safety Requirements Specification (SRS).
Example:
A partial stroke test of a shutdown valve might only detect 70% of possible dangerous failures, meaning its Proof Test Coverage (Cpt) is 70%. A full functional test would have a higher Cpt, closer to 90–95%.
See also: Proof Test, Proof Test Interval (TI)
Cited Source:
- IEC 61508-6:2010, Annex B.
- IEC 61511-1:2016, Clause 16.2.