For the third blog of this series, the focus is on proof testing. This article will look at what proof testing is, why it's needed, and give some outline examples of proof test procedure requirements. It will also introduce some issues I've personally experienced during audits and assessments of proof test procedures in the process industry sector.
The term "proof testing", in the context of safety instrumented systems, was first introduced in the IEC 61511  standard when it was first published back in 2003. The idea of a proof test comes from the assumed fact that a safety function which is rarely needed (low demand) must be exercised at some frequency to check for "undetected" faults.