Definition:
Proof Test Interval (TI), also called “Test Interval,” is the maximum allowable time between proof tests for a SIF or portion of a SIF. Its purpose is detecting dangerous undetected (DU) failures. This is determined by the functional safety team when developing the PFDavg calculations for that SIF. It is based on needed PFDavg (target SIL), manufacturer recommendations, and standard facility practices
Key Points:
- Shorter TIs reduce average PFD but cause more operational churn.
- Specified in the Safety Requirements Specification (SRS).
Example:
Proof testing a shutdown valve every 12 months (TI = 1 year).
See also :proof test, proof test coverage (Cpt)
Cited Source:
- IEC 61511-1:2016, Clause 11.9.