Failure Modes and Effects Analysis (FMEA) – part 3

After the above steps, Risk Priority Numbers (RPN) are computed. Once the S, O, and D are ranked, then RPN is arrived at by multiplying the three numbers: RPN = S x O x D.

RPN pinpoints the areas that are of greatest concern. RPN should be done for the entire life-cycle, so that failure modes with highest RPN are set right. There are some failure modes which may be lesser in severity, but greater in frequency and less detectable. After allocation of values, the next step is to note the recommended actions with targets and dates of implementation. These actions may include testing, specific inspection, quality procedures, redesign, limiting environmental stresses, etc.

After implementation, it is necessary to check the new RPN for confirming the improvements. The tests can be noted in a graphical form. In a word, in the face of failure modes, you have to first eliminate the failure mode, then minimize its severity. Then take steps to reduce its frequency and increase its detection possibility.

The use of FMEA is limited by the experience of committee members regarding previous failures. In the event they cannot identify the failure mode, then they have to approach consultants for solving this. In FMEA, documentation is an important factor for implementation. If FMEA is used as a top-down tool, it may identify only major failure modes in a system, when compared to Fault Tree Analysis (FTA).

It can complement FTA when used as a bottom-up tool. FMEA does not have the capacity to identify complex failure modes which involve multiple failures within a subsystem. It also cannot predict failure intervals of a particular failure mode up the upper level system/subsystem.

There is also a risk that the multiplication of S, O, and D rankings may end up in rank reversals, so that a less severe failure mode is shown as a more severe one. The reason is that the rankings are numbers based on ordinal scale, which only compare one ranking to another, but does not measure the degree of difference.

Many specific national and international standards require that FMEA should be used in the evaluation of product integrity.