NOTE 1 – This quantity is computed as follows: 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 is N_S, then

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

This quantity can be used to describe past performance or to estimate expected future performance.

NOTE 2 – Usually, only sustained interruptions are considered.

NOTE 3 – The term “customer” represents the ultimate customer and not some intermediate distribution system or company.