The FMEA for power distribution systems amounts to the determination and listing of those component outage events or combinations of component outages that result in an interruption of service at the load point being studied according to the interruption definition that has been adopted.
This analysis must be made in consideration of the different types and modes of outages that components may exhibit and the reaction of the system’s protection scheme to these events.
The primary result of the FMEA as far as quantitative reliability evaluation is concerned is the list of minimal cut-sets it produces. A minimal cut-set is defined to be a set of components that, if removed from the system, results in loss of continuity to the load point being investigated and that does not contain as a subset any set of components that is itself a cut-set of the system.
In the present context, the components in a cut-set are just those components whose overlapping outage results in an interruption according to the interruption definition adopted.
An important nonquantitative benefit of the FMEA is the thorough and systematic thought process and investigation that it requires. Often weak points in system design will be identified before any quantitative reliability indexes are computed. Thus, the FMEA is a useful reliability design tool even in the absence of the data needed for quantitative evaluation.
The FMEA and the determination of minimal cut-sets are most efficiently conducted by considering first the effects of outages of single components and then the effects of overlapping outages of increasing numbers of components. Those cut-sets containing a single component are termed first-order cut-sets.
Similarly, cut-sets containing two components are termed second- order cut-sets, etc. In theory the FMEA should continue until all the minimal cut-sets of the system have been found. In practice, however, the FMEA can be terminated earlier, since high-order cut-sets have low probability compared to lower-order cut-sets.
A good rule of thumb is to determine minimal cut-sets up to order n + 1 where n is the lowest-order minimal cut-set of the system. Since most power distribution systems have at least some first-order minimal cut-sets, the analysis can usually be terminated after the second-order minimal cutsets have been found.