The severity of a failure mode remains high even though the occurrence may be low. To a nation of consumers, a failure rate of 1 ppm has low criticality. To the one consumer out of a million who is injured by a malfunction, the criticality is high. Process controls and error-proofing measures may reduce the risk of occurrence, but over time, changes occur which were never anticipated or countermeasures have a way of becoming removed or defeated. Someone once said, it is tough to make any design foolproof because they keep making better fools. Marking a KPC on a design document ensures that the potential risk is communicated and not overlooked, now and long into the future.
But that is why the PFMEA is a living document right? To mark a KPC solely on severity alone with the assumption that the occurrence will change over time may lead to way too much over-analysis. As of the current revision of the PFMEA, the supporting KCC to the KPC is well engrained in the process with such a low occurrence rating that to call it out as a KPC would be ineffective.
This is my position, the design engineer I'm working with shares your position. What are your thoughts on the overuse of KPC? Wouldn't that lead to uneconomical processing and increased customer scrutiny?