In Reply to Parent Post by Bill Ryan
I've thought about this and, coupled with Michael's comments, I think I've decided that I need to think about it some more
Let's see if I understand somewhat. On the shuttle (with its back up/redundant computers), the Severity of the main computer would not be as high as the Severity of the 4th, correct? I do understand Michael's "Mission Statement" concept but don't I think I see why the Severity isn't the same for each computer at the PFMEA level. I do see at the SFMEA level where the failure of the system would be addressed with redundancy to address a high severity, but does that mean the DFMEA would have a reduced severity (which might be OK) and then that ranking cascades into the PFMEA? If that's how it works, why wouldn't each computer failing have an equal severity ranking? (Am I making any sense
Bill & Michael,
When I am working on an FMEA and a confusing scenario like this is presented, I keep two rules in mind. 1) We only consider one failure mode at a time, and 2) We stay at the same level (system vs sub-system vs component.)
In Bill's example above the level we are considering is the computer systems level, where multiple computers exists. We are considering the functions the system needs to meet. If only one computer existed and there was a failure then the Severity may be 10. If there is a redundant computer then the Severity may be 8. The reason we are allowed to reduce the severity is because we are assuming only one failure mode at a time. That is only one computer fails in a computer system. The redundancy means the computers fulfill the same function and are independent of one another.
When we do the sub-system FMEA (the computer in this case) then we are considering the functions the computer is required to meet. If it fails to meet these functions then it is evaluated by the failure to meet this function. In this case it is a severity of 10 because it is the same function as the system but we are evaluating only one computer.
Now when you do the process FMEA on a single computer you will cascade the single computer severity into the process FMEA as was discussed above.
For evaluating multiple Failure Modes at once you should turn to another tool such as Fault Tree Analysis.
I hope this helps.