(215d) Strategic Proof Testing

A safety protection system depends on periodic proof testing to assure the integrity of the system. A new safety system design standard, ANSI ISA 84.00.01-2004 (IEC 61511 Mod.), allows proof test intervals to be calculated and optimized according to specified risk criteria. Many designers do these calculations assuming that the proof test is perfect. A closer examination shows that proof test effectiveness is a critical input to the calculation.

As safety system designers consider proof test effectiveness it has become clear that many conventional methods are weak and surprisingly ineffective. It is also becoming clear that new more effective methods can be devised that are not only more effective but easier to implement.

This paper describes the criteria for planning of proof test methods that are more effective and easier to perform. A means to measure proof test effectiveness is described. This FMEDA based approach not only measures proof test effectiveness but identifies specific tests to detect specific failures. Several examples are shown with cost savings and safety improvement calculated.