An article entitled Are Your Safety Instrumented Systems Proof Tests Effective? authored by exida Senior Engineer Loren Stewart has been published in Valve Magazine.
Many people assume that a proof test of a safety function is 100% effective. Depending on the test used, this may very well be wrong.
A weak proof test design can impact the effectiveness of a safety function significantly, which can be shown through the average Probability of Failure on Demand (PFDavg) calculations.
This article explains proof test coverage for valves, actuators and solenoids and why this test is an important safety parameter. It also shows that the real objective is to detect failures that are not detected by any automatic diagnostics, and points out that safety instrumented systems (SIS) products will have much higher safety even with a lower proof test coverage if automatic diagnostics are used. In other words, it explains why a lower proof test coverage is not a bad thing.