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

${\displaystyle U_{\text{CS}}=\frac{{\sum }_i{\sum }_j{t}_{ij}}{N_{\text{S}}T}}$

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

NOTE 2 – Only sustained interruptions are considered.

NOTE 3 – If the duration of the time interval is expressed in years, the quantity is called the “average yearly downtime per customer served”.

NOTE 4 – The term “customer” represents the ultimate customer and not some intermediate distribution system or company.