Proof test coverage is an important variable in Safety Instrumented System design that has the potential to impact risk reduction by an entire Safety Integrity Level (SIL). This is shown in the probability of failure on demand calculations. Most engineers who design and verify safety instrumented functions (SIFs) understand how hard it is to design a manual proof test with high effectiveness.
This paper will explore the concept that the real objective of a proof test is to detect failures NOT detected by automatic diagnostics, explain proof test coverage, why this is an important safety parameter, and will explain how proof test coverage is calculated, showing several examples of proof test procedures along with the test coverage. We will explore different levels of proof test coverage and the associated effects on their failure rates.