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FMEA RPN
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What is FMEA?
How FMEA is Done
Types of FMEA
Benefits of FMEA
FMEA Books
FMEA Software
FMEA Examples
Other Resources
Risk Priority Number (RPN) is a measure used when assessing risk to
help identify critical failure modes associated with your design or process.
The RPN values range from 1 (absolute best) to 1000 (absolute worst). The
FMEA RPN is commonly used in the automotive industry and it is somewhat
similar to the criticality numbers used in Mil-Std-1629A. The graphic below
shows the factors that make up the RPN and how it is calculated for each
failure mode.
help identify critical failure modes associated with your design or process.
The RPN values range from 1 (absolute best) to 1000 (absolute worst). The
FMEA RPN is commonly used in the automotive industry and it is somewhat
similar to the criticality numbers used in Mil-Std-1629A. The graphic below
shows the factors that make up the RPN and how it is calculated for each
failure mode.
Severity (S) - Severity is a numerical subjective estimate of how severe
the customer (next user) or end user will perceive the EFFECT of a
failure. Examples of Severities.
Occurrence (O) - Occurrence or sometimes termed LIKELIHOOD, is a
numerical subjective estimate of the LIKELIHOOD that the cause, if it
occurs, will produce the failure mode and its particular effect. Examples
of Occurrence values.
Detection (D) - Detection is sometimes termed EFFECTIVENESS. It is
a numerical subjective estimate of the effectiveness of the controls to
prevent or detect the cause or failure mode before the failure reaches
the customer. The assumption is that the cause has occurred.
Examples of Detection values.
the customer (next user) or end user will perceive the EFFECT of a
failure. Examples of Severities.
Occurrence (O) - Occurrence or sometimes termed LIKELIHOOD, is a
numerical subjective estimate of the LIKELIHOOD that the cause, if it
occurs, will produce the failure mode and its particular effect. Examples
of Occurrence values.
Detection (D) - Detection is sometimes termed EFFECTIVENESS. It is
a numerical subjective estimate of the effectiveness of the controls to
prevent or detect the cause or failure mode before the failure reaches
the customer. The assumption is that the cause has occurred.
Examples of Detection values.
Assessing Risk
Some words of caution when using the RPN value to assess risk - RPNs have
no value or meaning in themselves. Although it is true that larger RPN values
normally indicate more critical failure modes, this is not always the case. For
example, here we have three cases where the RPNs are identical, but clearly
the second case would warrant the most attention.
As a general rule, any failure mode that has an effect resulting in a severity 9
or 10 would have top priority. Severity is given the most weight when assessing
risk. Next, the Severity and Occurrence (S x O) combination would be
considered, since this in effect, represents the criticality.
Some words of caution when using the RPN value to assess risk - RPNs have
no value or meaning in themselves. Although it is true that larger RPN values
normally indicate more critical failure modes, this is not always the case. For
example, here we have three cases where the RPNs are identical, but clearly
the second case would warrant the most attention.
As a general rule, any failure mode that has an effect resulting in a severity 9
or 10 would have top priority. Severity is given the most weight when assessing
risk. Next, the Severity and Occurrence (S x O) combination would be
considered, since this in effect, represents the criticality.
Below is another RPN example reminding us that we need to be careful not to
assess risk purely based on the RPN values. Here, the failure modes with the
lowest RPN values are actually the most critical. Be careful to not just establish
"threshold values" for RPNs when assessing risk, as this could lead you to
make costly mistakes. Below we see that #1 is most critical even though it has
the lowest RPN value, then #2, and then #3.
assess risk purely based on the RPN values. Here, the failure modes with the
lowest RPN values are actually the most critical. Be careful to not just establish
"threshold values" for RPNs when assessing risk, as this could lead you to
make costly mistakes. Below we see that #1 is most critical even though it has
the lowest RPN value, then #2, and then #3.