FMEA Severity Rating - Will a Process Design Change also Change the Severity Rating?

[sowmya]

When the severity rating will change?. In my understanding, it will change only there is a change in design. Will the process design change can also change the rating? Example, implementation of error proofing gauge will change the severity? Please help

[Miner]

The Severity Rating can only be changed by a change in the product design.

A Mistake Proof can improve the Detection Rating if it is the type that alarms when a defect occurrs. It may also improve the Occurrence Rating if it is the type that prevents defects from being built.

[Bill Ryan - 2007]

In Reply to Parent Post by sowmya

When the severity rating will change?. In my understanding, it will change only there is a change in design. Will the process design change can also change the rating? Example, implementation of error proofing gauge will change the severity? Please help

An "error-proofing gage" does nothing to change the Severity rating. It may/will affect the Detection rating only. If you have error-proofing designed into the process so that the Failure Mode can't be produced, you will affect only the Occurence rating.
Severity rating can only be changed by a design change that mitigates the Effect of the Failure Mode (such as a redundancy setup for the final system).

Hope that helps.

[Michael Walmsley]

In general , the severity rating for an item in the PFMEA must minimally be at the same level as that in the design FMEA.

The caveat is that the PFMEA rating CAN be higher for severity. Especially when an operator can be harmed by manufacturing tooling or practices.

This increased level in the PFMEA (process) does not find its way back to the design FMEA (product). It should be addressed thru a process design change,in which case it will revert back to the same level as found in the design FMEA.

The only way to reduce severity in a design FMEA IS thru a design (product) change.

I agree with Bills comments on Error proofing.

[tymer5]

I agree with Bill's comments on error proofing, and I also agree with Michael's comments on severity rating. The severity rating should not be any lower than the severity rating coming from the Design FMEA. The severity rating can be higher if the Process endangers the operator, or has a worse effect on a down stream process. In this specific case it appears that the Sowmya is attempting to reduce the severity by error proofing a gage. My take on this is that the severity should not be reduced in this case. Just because I have a better gage doesn't mean that the effect of a failure has changed. The word gage leads me to believe the cause or failure mode has all ready occurred. So a change in the detection rating is more appropriate.

Bill, I am interested in hearing you thoughts on why you think redundancy only affects the occurrence rating. It is my experience that redundancy would reduce the severity because we only consider one failure mode at a time. If we consider both the primary and redundant system/component failures occurring then we are considering multiple failure modes. When people start considering multiple failure modes then even trivial failures can have severity rankings driven to 10.

[Bill Ryan - 2007]

In Reply to Parent Post by tymer5

Bill, I am interested in hearing you thoughts on why you think redundancy only affects the occurrence rating. It is my experience that redundancy would reduce the severity because we only consider one failure mode at a time. If we consider both the primary and redundant system/component failures occurring then we are considering multiple failure modes. When people start considering multiple failure modes then even trivial failures can have severity rankings driven to 10.

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 )

[Michael Walmsley]

"FMEA Severity Rating - Will a Process Design Change also Change the Severity Rating?"

If you add redundancy here yes.

[tymer5]

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.

Regards.