PFMEA - RPN - Severity, Occurrence and Detection - Which one does not change?

R

Rob Nix

#51
Severity of a hazard never changes! The design does not have anything to do with the hazard. It has to do with the probability of occurring and detection to prevent it.
If death is the hazard it does not matter if it was a good design ore not you are still dead.
Design of the process maybe, but design of the product may change the severity. If I change the material of a baseball to "nerf ball" foam, the severity of the hazard, e.g., getting hit in the head with it, goes way down.

I would have liked to have been a fly on the wall when the designers of the electric chair were discussing "severity" in their FMEA. A ten is good, isn't it? :rolleyes:
 
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Jim Wynne

Staff member
Admin
#53
Design of the process maybe, but design of the product may change the severity. If I change the material of a baseball to "nerf ball" foam, the severity of the hazard, e.g., getting hit in the head with it, goes way down.

I would have liked to have been a fly on the wall when the designers of the electric chair were discussing "severity" in their FMEA. A ten is good, isn't it? :rolleyes:
Rob Nix? Where have you been? Maybe you need to go to the "New Folks" thread and introduce yourself again. :tg:
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#54
Design of the process maybe, but design of the product may change the severity. If I change the material of a baseball to "nerf ball" foam, the severity of the hazard, e.g., getting hit in the head with it, goes way down.
Rob - I think that's a good example of reducing severity! The ball can still hit you but there is no way a nerf ball does the damage that a hard ball will do...

Stijloor - Your tire/tyre example is a little more complex and I think that is the crux of the discussion. The difficulty is that you have reduced the severity of a puncture (loss of stabliity) and therefore reduced the probability of a rollover, but not the severity of a rollover. we could reduce the severity of a rollover only by adding protection to the occupants of the vehicle that would make death impossible. Or by adding 'training wheels' or similar design change that makes rollover impossible.

The difference between your two examples is the 'depth' of the faliure in the overall system - how many sequential events are involved?

My example of reducing severity is a chemistry analyzer that has slide jams when ejecting chemistry slides after analysis is complete. The effect is that the user must open the instrument and remove the jam. Workflow is disrupted. fairly low severity. we reduced the severity by having the instrument unjam itself. the jam still occurs but the customer never knows it and since it is very fast (and the tech is doing something else during that time anyway) there is no effect on workflow.

The closer the failure mode is to the end effect, the more probable it is to actually effect the severity and not just the probability of occurence.

But the real crux is that trying to understand whether or not we have effected occurence or severity or detection is a bit like debating how many angels can fit on the head of a pin: how does this debate help the customer? FMEA is intended to make the product better; it is not intended to be an elegant explanation of severity, occurence and detection.

Yes, we must have basic even strong understanding of these terms so we do the right thing - but in the end we must do the right thing...
 
G

gholland

#55
I completely disagree with the nerf ball reducing severity. The severity of you getting plunked in the head with something hard doesn't go down as it is independant of your design. Severity of a failure never changes.

The occurence of the event does. If I dip a nerfball in water and freeze it, voila a harmful projectile. How often does that happen? Probably never without malicious intent so your occurence changes to improbable (lowest occurence).

:2cents:
 
#56
I completely disagree with the nerf ball reducing severity. The severity of you getting plunked in the head with something hard doesn't go down as it is independant of your design. Severity of a failure never changes.

The occurence of the event does. If I dip a nerfball in water and freeze it, voila a harmful projectile. How often does that happen? Probably never without malicious intent so your occurence changes to improbable (lowest occurence).
By freezing the waterlogged nerfball, you have added a new failure mode (in my opinion). Now, it is not the nerfball that does the damage, but the frozen water.
 
#57
HEY!!!! That was my 2000th post! A milestone.....

Great......

I spent my 2000th post talking about getting clunked in the head by a frozen, water-logged nerfball! :bonk:
 
G

gholland

#58
Glad to be of service... :tg:

I disagree that it is a new failure mode, you're still getting whacked in the head by a ball that causes injury. The change to a nerf ball means you have a very low occurence that you would get whacked in the head and get injured but the failure is still the same.
 

Jim Wynne

Staff member
Admin
#59
Glad to be of service... :tg:

I disagree that it is a new failure mode, you're still getting whacked in the head by a ball that causes injury. The change to a nerf ball means you have a very low occurence that you would get whacked in the head and get injured but the failure is still the same.
"Occurrence" applies to the likelihood of getting hit in the head, not to the severity of being hit. The question is, once hit in the head, what's the severity, and how can it be mitigated? The Nerf ball example is appropriate. =
 
Last edited:
M

MedQE

#60
And to expand on how we use Occurrence (at least where I have worked) it is the probability not only that the hazard occurs but that it also results in the severity you are considering. In the ball example the severity of death would result in a very low probability. If you evaluated this hazard relative to a lesser severity (say injury like eye damage) then the probability may be higher.
In any case we would only evaluate credible hazards so death due to a nerf ball impact would not be considered.
 
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