DFMEA – Chain of events

The loss of dielectric is an example of a failure mode resulting in a low withstanding voltage, which in turn results in the assembly sorting out (the effect). Using a chain of events is a good technique if it helps you better explain the failure modes and their effects. Look for the outcomes or consequences of the failure mode on the part, assembly, other parts, end-user, etc.  You should also include safety or regulatory non-compliance outcomes.
Other examples include unpleasant odor, unstable, regulatory non-compliance, intermittent operation, or poor appearance.
Assuming that the failures have occurred, give specific descriptions of the ways in which customers could observe each failure. Use the perspective of the external or internal customer, but it isn’t necessary to use the customer’s terminology here.
Note that there could be more than one effect for a given failure mode, or, an effect could be the result of several failure modes. Don’t forget to refer back to your FTA. Note: MTBF stands for mean time between failures.

The severity or estimated consequence of the effect on a 1 to 10 scale, where a 1 is a minor nuisance and a 10 indicates a severe total failure of the system.

When weighing the consequence (effect) of the failure, ask “How serious would the effect of this failure be to the customer, assuming it has appended?”

To reduce the severity of the effect of a product failure mode, a part design action is usually required.

The part design action to reduce a high rating may include design additions or revisions that mitigate the resultant severity of the failure, e.g., seat belts in a car. Severity is the first of the 3 Risk Priority Numbers (RPN) rating criteria. Think of the estimated consequence or seriousness of the effect, assuming it does exist*, on the external and/or internal customer. If the effect is critical, the severity is probably high. *Do not consider the likelihood that the defect will occur in scoring this criterion; this will be evaluated in the second criterion (Occurrence).

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