The probability of failure on demand, or the failure rate of a protective system is critically dependent upon the frequency of proof testing and its ability to detect previously unrevealed failures of the system. The proof test interval should therefore be established accordingly, and as a rule of thumb for low demand systems, should be an order of magnitude less than the mean time between failure of the system and the demand rate.
Proof test procedures should be available which specify the success/failure criteria and detail how the test will be performed safely, including any management arrangements, operating restrictions and competence of personnel.
The tests should be arranged to reveal all dangerous failures which have been unrevealed in normal operation including the following measures:
Tests performed at the conditions which would be expected at trip. (Where test under trip conditions cannot be performed, for example for safety reasons, then measures to ensure that potential failures at trip conditions will be revealed should be clarified);
End to end tests at appropriate intervals, including proving sample/impulse lines. (Different elements of the protective system may require proof testing at different intervals).