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Manual for Life Cost Based FMEA

2.5. Effect of Failure

Effect of a failure on product, user, or other systems is the noticeable effect on performance, safety, and perceived quality. As a designer, this is the effect you are trying to prevent in your own design. The root cause is where you will make changes in your existing designs as will be discussed later in section 2.10.

Effect of Failure

Main function

Potential Failure mode

Cause of Failure

Root Cause

No magnet field

not(Generate Magnetic field)

Degraded field

Turn to turn Short

Insulation Thermal damage

Ground Short

Insulation Thermal damage

Figure 1 Failure Scenarios

Figure 1 shows an example of a cause-and-effect chain of events. There are many such “failure scenarios” or “event chains” possible in an FMEA and you should list each on a separate row with an associated RPN. It should be noted that failures can have many causes, and each scenario should be listed on a separate line on the FMEA worksheet.

2.6. Probability of Occurrence (O)

Occurrence is defined as how frequently the specific failure cause is projected to occur and result in the “failure mode”. The literature prescribes that Occurrence is assigned to the cause (and failure mode) and has nothing to do with the probability of the end effects. However, we recommend associating the Occurrence rating with the entire failure scenario, since some causes could have many different effects.

Occurrence should refer to the probability of cause a particular failure mode a particular effect/event. In mathematical terms:

Probability of failure = (Probability of cause) × (Probability of failure given the cause)


FMEA MANUAL By S. Rhee and C.M. Spencer


January 2009

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