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Manual for Life Cost Based FMEA

Probability of Failure

Occurrence Ranking

1 in 2 1 in 3 1 in 8 1 in 20 1 in 80 1 in 400 1 in 2,000 1 in 15,000 1 in 150,000 1 in 1,500,000

10 9 8 7 6 5 4 3 2 1

Table 2 Example of Occurrence criteria [AIAG, 1995]

Probability of Failure

Very High: Failure is almost inevitable

High: Repeated failures

Moderate: Occasional failures

Low: Relatively few failures

Remote: Failure is unlikely

Occurrence scores are generated on the basis of an industry- or company-specific mapping from probabilities to a 1-10 scale (see Table 2).

2.7. Severity of effect (S)

Severity is typically defined as an assessment of the seriousness of the potential “end

effects,” and is assessed independent of the causes.

However,

assessing Severity to the Severity is estimated on a 1

entire failure scenario (causes, to 10 scale (see Table 3).

failure

we recommend modes effects).

Effect

Severity of Effect

Severity Ranking

Hazardous without warning Hazardous with warning very high High Moderate Low very low Minor very minor None

when a failure mode affects safe device operation without warning when a failure mode affects safe device operation with warning device inoperable: loss of primary function device operable: at a highly reduced level of performance device operable: at a reduced level of performance device operable: at a slightly reduced level of performance device operable: defect noticed by most customers device operable: defect noticed by average customers device operable: defect noticed by discriminating customers almost no effect

10 9 8 7 6 5 4 3 2 1

Table 3 Severity of effect scores [AIAG, 1995]

2.8. Detection difficulty (D),

Detection, sometimes called detectability, has no standard definition. There is some confusion surrounding this index, since different definitions exist for this term. If the team does not have a good understanding of this index, we recommend using a value of “1” for all fields and the team can fill it in later if time permits. The most common interpretation of detection is an assessment of the ability of the “design controls” to identify a potential cause or design weakness before the component, subsystem or system is released for production. Detection scores are generated on the basis of

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

7/26

January 2009

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