It’s a common problem that some end users assume a perfect proof test is possible, whereby all dangerous failures can be detected, removed and the SIF restored, thereby negating the effects of “mission time”. Another misconception surrounds Proof Testing and what this means. Some consider Proof Testing to be a means of ensuring the SIF is operational and works, however, in reality, the purpose of the Proof Test is to be able to find dangerous faults that the automatic diagnostics are not able to find.
Since there is no such thing as a “perfect Proof Test”, we have to consider the effectiveness in terms of the percentage of dangerous faults we can find during a Proof Test (i.e. 80% coverage means we can find 80% of potential dangerous faults, leaving 20% we can’t find). The webinar will explain and demonstrate the impact of imperfect Proof Testing on the PFDavg for Low Demand SIFs and how assuming 100% coverage can lead to a significant overestimation of the SIL capability of the SIF.
This is important for all SIS designers and End Users to understand since they could be fooling themselves into thinking they have more protection than they actually have.