Note 1 to entry: If the power supply to customer j is interrupted n_j times during a time interval of duration T, and the total number of customers served is N_S, then interruption frequency per customer served is given by

${\displaystyle f_{\text{CS}}=\frac{{\sum }_j{n}_j}{N_{\text{S}}T}}$

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.