| sum of durations of all interruptions of supply to individual customers served during a time interval, divided by the product of the total number of customers served 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 NS the total number of customers served, then the service unavailability per customer served is given by
Note 2 to entry: Only sustained interruptions are considered.
Note 3 to entry: If the duration of the time interval is expressed in years, the quantity is called the "average yearly downtime per customer served".
Note 4 to entry: The term "customer" represents the end user and not some intermediate distribution system or company.
Note 5 to entry: Service unavailability per customer served can be used to describe past performance or to estimate expected future performance.
Note 6 to entry: This entry was numbered 191-27-03 in IEC 60050-191:1990.