NOTE 1 – This quantity is computed as follows: if t_ij is the duration of the ith interruption of supply to customer j and n_j is the number of times customer j is affected during a time interval of duration T, then

${\displaystyle T_{\text{I}}=\frac{{\sum }_i{\sum }_j{t}_{ij}}{{\sum }_j{n}_j}}$

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

NOTE 2 – Only sustained interruptions are considered.

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