| number of supply interruptions experienced by the individual customers affected at least once during a time interval, divided by the product of the number of affected customers and the duration of that time interval|
Note 1 to entry: If NNS is the number of customers affected and the supply of customer j is interrupted nj times during a time interval of duration T, then the interruption frequency per customer interrupted is given by
When determining NNS, each customer affected must be counted only once, regardless of the number of interruptions experienced during that time interval.
Note 2 to entry: Usually, 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: Interruption frequency per customer interrupted can be used to describe past performance or to estimate expected future performance.
Note 5 to entry: This entry was numbered 191-27-02 in IEC 60050-191:1990.