| number of interruptions of supply to individual customers during a given time interval, divided by the product of the total number of customers and the duration of that time interval|
Note 1 to entry: If the power supply to customer j is interrupted nj times during a time interval of duration T, and the total number of customers served is NS, then interruption frequency per customer served is given by
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 served can be used to describe past performance or to estimate expected future performance.
Note 5 to entry: This entry was numbered 191-27-01 in IEC 60050-191:1990.