| sum of durations of all supply interruptions to individual customers during a time interval, divided by the number of these supply interruptions|
Note 1 to entry: If tij is the duration of the ith interruption of supply to customer j and nj is the number of times customer j is affected during a time interval of duration T, then the mean duration of a customer interruption is given by
Note 2 to entry: 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: Mean duration of a customer interruption can be used to describe past performance or to estimate expected future performance.
Note 5 to entry: This measure is also equal to service unavailability per customer served divided by interruption frequency per customer served.
Note 6 to entry: This entry was numbered 191-27-05 in IEC 60050-191:1990.