Definitions

# Failure Mode, Effects, and Criticality Analysis

Failure Mode, Effects, and Criticality Analysis (FMECA) is an extension of Failure Mode and Effects Analysis (FMEA). In addition to the basic FMEA, it includes a criticality analysis, which is used to chart the probability of failure modes against the severity of their consequences. The result highlights failure modes with relatively high probability and severity of consequences, allowing remedial effort to be directed where it will produce the greatest value.

The typical goal, when FMECA is performed as part of a design project, is to eliminate failure modes with high severity and probability, and to reduce as much as possible those with high severity or high probability. If the Criticality Analysis is performed iteratively during the design process, the charted failure modes should be seen to migrate to the left and bottom (typically) of the chart.

This enables priority ranking by means of the so called Risk Priority Number (RPN). The $RPN$ is a result of a multiplication of detectability $\left(D\right)$ x severity $\left(S\right)$ x occurrence $\left(O\right)$. Each on a scale from 1 to 10. The highest $RPN$ is $10$x$10$x$10$ = $1000$. This means that this failure is not detectable by inspection, very severe and the occurrence is almost sure. If the occurrence is very sparse, this would be $1$ and the $RPN$ would decrease to $100$. So, criticality analysis enables to focus on the highest risks.