(1) The following service reliability indices measure the frequency and duration of service interruptions.  They are recognized as standard reliability indices for the electric utility industry and may be applied to entire distribution systems and operating areas.  For purposes of this rule, "interruption" means the loss of service for more than five minutes.

(a) "Customer average interruption duration index" is the average interruption duration for those customers of electricity service who experience interruptions during the year.  It is calculated by dividing the annual sum of all customer interruption durations by the total number of customer interruptions.

(b) "System average interruption duration index" is the average interruption duration per customer served during the year.  It is calculated by dividing the sum of the customer interruption durations by the total number of customers served during the year.

(c) "System average interruption frequency index" is the average number of interruptions per customer during the year.  It is calculated by dividing the total annual number of customer interruptions by the total number of customers served during the year.