Process FMEA (Failure Modes Effects Analysis) Severity Rating and RPN

[Bill Ryan - 2007]

In Reply to Parent Post by bpritts

If you look at true "safety critical'" features of automobiles you'll note that they generally have backups.

So if design is the issue, consider whether there are backup systems
on the product; maybe the initial severity assessment was in error.
(Errors can be corrected, with explanation. As noted above, you can't
just reduce a severity without a change, so you'd have to justify
a reduction.)Brad

I guess I don't follow where the "backyp system" affects the Severity rating. Doesn't it really affect the Occurence?? As Dave said - there is still the Potential for the Failure Mode to happen, even with a backup system, so how has the Severity been impacted?

Just looking for clarification.
Bill

[bpritts]

Bill - responding to your question ...

Many of the PFMEA's I've seen (including some that I've done!) reflect
a lack of deep knowledge of the system that the product is installed in.

What I am suggesting is that if you *think* that you have a high severity
for a component that you make, it becomes your duty to study the
full system and learn about the design safety margins. If the severity # is
real then you need to be thinking about 100% automated on line testing
or some other extremely high confidence control system.

I recognize that this sounds good but may be very difficult - I have rarely
seen a design FMEA from *any* of my customers' customers -- North
American auto companies. I won't
name names but one has a 2 letter name and another a 4 letter name,
and neither are owned by Germans. I am often told that these documents
exist but are company confidential. I wonder sometimes....

It's also an interesting point on how this might really be considered as
an occurrence factor. Maybe the horrible consequence only happens if both
the primary and the backup system fail simultaneously. That could be
handled by decreasing the occurrence index.

But to restate what everyone else has said, the only way to change the
severity figure involves either a change to the design, or, as I suggest,
a change to your knowledge of the design.

Regards,
Brad