FMEA - Example FMEA Sheet - Risk Priority Number (RPN)

Risk Priority Number (RPN)

- RPN play an important part in the choice of an action against failure modes. They are threshold values in the evaluation of these actions.

After ranking the severity, occurrence and detectability the RPN can be easily calculated by multiplying these three numbers: RPN = S × O × D

However, the practice of prioritizing work on the basis of RPN has no theoretical basis, but is an informal heuristic which fails for many cases, due to the "nonsensical notion that you can multiply rankings together".

This has to be done for the entire process and/or design. Once this is done it is easy to determine the areas of greatest concern. The failure modes that have the highest RPN should be given the highest priority for corrective action. This means it is not always the failure modes with the highest severity numbers that should be treated first. There could be less severe failures, but which occur more often and are less detectable.

After these values are allocated, recommended actions with targets, responsibility and dates of implementation are noted. These actions can include specific inspection, testing or quality procedures, redesign (such as selection of new components), adding more redundancy and limiting environmental stresses or operating range. Once the actions have been implemented in the design/process, the new RPN should be checked to confirm the improvements. These tests are often put in graphs for easy visualization. Whenever a design or a process changes, an FMEA should be updated.

A few logical but important thoughts come in mind:

• Try to eliminate the failure mode (some failures are more preventable than others)
• Minimize the severity of the failure
• Reduce the occurrence of the failure mode
• Improve the detection