"I'm assuming here that you are discussing the source of the contamination as a part beign dropped on the floor, then the operator must inspect the part for contamination? "
sounds like teh PFMEA
process is working...remember its not about assigning ratings and calculating RPNs. its about determining weakness in your processes and taking action to prevent failures. so you're half way there.
that said, the use of contaminated parts is a faliure mode. I'm a bit unclear about the cause of contamination - I'm assuming here that you are discussing the source of the contamination as a part beign dropped on the floor, then the operator must inspect the part for contamination? so the proximate cause may be that the operator misses the contamination (not intentionally) or they intentionally do not inspect or assemble the part even with observed contamination. either way you are describing a very low detectability situation. so just record it as very low...THEN (and this is the important part) take actions to correct the causes. including why the part is dropped, why contamination occurs when its dropped, why an operator may not not follow the process, etc.
---I mean the components dropped on the floor will be contaminated by dusty, or the dropping may cause some damage that not easy be inspected by eye, so it's not allowed to use the components dropped on the floor, the operator doesn't need to inspect the components, just leave the dropped components there, they will be treated as scrap components.
Why I say the operator maybe intentionally? For example, maybe they dropped too much components and are worry about been criticized(seems funny, but it's only a example).
I know we should consider "why the operator may drop the component", because the dimension is not very big, like a spring in the automatic ball pen, and you know, spring is very easy to jump away. And I know automatic feeding of the component can reduce even make it disappear for the cause, but now because of cost saving, we have to keep the machine as manual operation by operators.
So I think adding some detection to avoid the preventive action (instruction) doesn't work will be better.
And also my question is: during the PFMEA compiling, should we consider the operator making an "intentionally" fault? Because we have already have the preventive action which is Working Instruction, but if the operator doesn't obey the instruction, the fault will still happen. Maybe I'm in the wrong way of thinking.