Both “the grid” at large and most utilities routinely operate without interruption more than 99.9 percent of the time. Such a measure of power availability alone, however, is of limited utility in evaluating reliability performance at the distribution level. To do so, utilities turn to the distribution reliability indices defined in IEEE-1366, or elsewhere, which capture the frequency and duration of system power outages at the customer meter.
While the distribution reliability indices are useful as objective measures on the reliability of distribution systems, their importance in informing and regulating the energy transformation will likely grow. As concern for environmental justice increases, new methods will need to be developed that can inform about disparities in service. As described later in the paper, the distribution reliability metrics can provide that information.