| sum of durations of all supply interruptions to individual customers during a time interval, divided by the product of the number of customers affected and the duration of that time interval|
Note 1 to entry: If tij is the duration of the ith interruption of supply to customer j during a time interval of duration T, and NNS the number of customers affected, then the service unavailability per customer interrupted is given by
Note 2 to entry: Only sustained interruptions are considered.
Note 3 to entry: The term "customer" represents the end user and not some intermediate distribution system or company.
Note 4 to entry: Service unavailability per customer interrupted can be used to describe past performance or to estimate expected future performance.
Note 5 to entry: This entry was numbered 191-27-04 in IEC 60050-191:1990.