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Calculation
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Description
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RPN
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The overall risk of a particular failure mode occurring in your system. The RPN (risk priority number) is calculated as:
Severity * Occurrence * Detection
Where:
• Severity is an assigned value that indicates the severity of the effect of a particular failure mode.
• Occurrence is an assigned value that designates how frequently that particular failure mode is likely to occur.
• Detection is an assigned value that indicates how often that particular failure mode can be detected.
While RPN values are computed automatically as you enter severity, occurrence, and detection values, they are computed again when you perform FMEA calculations to ensure that all RPN values are current.
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RPN result
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An RPN value calculated after design changes are implemented to address failure modes with high RPNs. The RPN result is calculated as:
Severity Results * Occurrence Results * Detection Results
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RPN improvement percentage
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The percentage of improvement between the original RPN (before any corrective actions have been taken) and the RPN result (after corrective actions have been taken).
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Criticality rank
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A value that provides an assessment of the failure mode’s severity and probability of occurrence. A failure mode with a rank of 1 is the most severe with the highest likelihood of occurring. The result is a categorized breakdown of failure modes based on risk. This approach is described in the SAE FMEA 5580 document.
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Item failure rate
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The number of failures of a specific item over a time period, usually defined in units of FPMH (failures per million hours) or FITS (failures per billion hours).
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Mode failure rate
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The number of occurrences of a specific failure mode over a time period, usually defined in units of FPMH (failures per million hours) or FITS (failures per billion hours).
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Mode criticality
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How critical the failure mode is under a particular severity classification. This criticality value is calculated as described in MIL-STD-1629, Procedures for Performing a Failure Mode, Effects and Criticality Analysis:
Mode Failure Rate * Mode Operating Time * Failure Effect Probability
Where:
• Mode Failure Rate is a value representing the number of occurrences of a specific failure mode over a time period.
• Mode Operating Time is the duration of applicable mission phase(s), usually expressed in hours or number of operating cycles.
• Failure Effect Probability is the probability that the failure occurs.
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Detection percentage
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The probability that the failure mode is detected.
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Detection failure rate
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A value representing the number of detectable occurrences of a failure mode over a time period, usually defined in units of detectable failures per million hours or detectable failures per billion hours.
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Isolation percentage
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The percentage of probability that the failure mode can be attributed to a specific part.
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Isolation failure rate
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A value representing the number of occurrences of a failure mode over a time period that can be attributed to a specific part, usually defined in units of isolation failures per million hours or isolation failures per billion hours.
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