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

sowmya

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#1
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
 
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Miner

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#2
Re: Will a Process Design Change also Change the FMEA Severity Rating?

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.
 
B

Bill Ryan - 2007

#3
Re: Will a Process Design Change also Change the FMEA Severity Rating?

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.
 
M

Michael Walmsley

#4
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.
 
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Bill Ryan - 2007

#5
Re: Will a Process Design Change also Change the FMEA Severity 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).
I thought some about this last night. Even a redundancy in the system would not affect the Severity. It would certainly (hopefully:rolleyes: ) help reduce the Occurence.
 
B

Baldrick

#6
Most of the changes we make to processes as a result of FMEA will result in lower detection and/or occurrence scores. It's rare that the changes we make reduce the severity.

I'm sure someone will post examples - I'm not saying you CAN'T reduce severity, just that (by its very nature) it's the most difficult of the S/O/D values to reduce.

A lot of people make the mistake of thinking that because they have reduced the chances of something happening that they have reduced the severity - this is not the case. Occurrence is "how often will this thing happen". Severity is "when this thing happens, how serious is it?".

For an FMEA to be effective in prioritising risk you have to consider the three values independently. The overall combined effect of the three is your RPN number.
 

sowmya

Involved - Posts
#7
All,
Thanks for all your replies. In the manual it is given that "A reduction in severity ranking index can be effected through a design change to system, sub system or component, or a redesign of the process". Am I interpreting wrongly?
 

Manix

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#8
All,
Thanks for all your replies. In the manual it is given that "A reduction in severity ranking index can be effected through a design change to system, sub system or component, or a redesign of the process". Am I interpreting wrongly?
That is a very misleading statement, as I tend to agree with everyone above. I read the severity rating in a Process FMEA as:

"How Serious is the Effect". Looking at a standard FMEA format, the effects column comes before any actions you take, so how can the sevrity rating be affected by a change in the process. The effect will always be the same no matter what you do to the process. The only way you can change the severity of an effect is to change the design, so that the effect is mitigated To use the example given in the FMEA manual,

If an effect of a failure is a tyre blow out, in order to reduce the serverity of this effect, you can design in features that allow the tyre to run flat and therefore this reduces the severity of the blow out!

I think you will find that the Severity rating for failures related to operator safety can be reduced using a redesign of the process. So for example, if you have a drill and the failure mode is that the operator could put their hand into the dangerous area, the severity would be high for operator injury. However, if you introduce a device that cuts the drill off if their hand enters the the dangerous area, you are reducing the severity of the effect of putting the hand in the area!!!!! :confused: You follow?

I think this is might be what it means, relating to operator safety and comfort issues rather than customer satisfaction/end product issues!
 

Manix

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#9
Also as an addtion to the above, you could also relate this to environmental issues. If you have a process that impacts heavily on the environment, then the severity of the failure will be high. However, take steps to reduce the environmental impact (design of the process), then you can reduce the severity.

This is more in line with EFMEA's (Environmental FMEA's) but the widespread adoption of ISO 14001 would also benefit from Environmental issues being considered in all FMEA's!

Hope this helps give you an idea!
 

Helmut Jilling

Auditor / Consultant
#10
As with all general "rules of thumb," it should not be taken as a absolute. Perhaps there are cases where a process design change would reduce Severity, but not likely. The Severity rating is generally looking at the product which results from the process. If anything, a change in the process is likely to reduce the Occurance, or perhaps Detection.
 
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