Distribution System Reliability Metrics

Electric reliability is the ability of the distribution system to withstand events such as severe weather. Three metrics are commonly used to assess distribution system reliability:  SAIDI, SAIFI, and CAIDI.

These metrics are calculated in two ways:  1) “All Weather” which includes the entire year year and 2) “Excluding Major Event Days” which excludes events when the distribution system is experiencing major impacts. Excluding Major Event Days reduces the number of days used in the calculation and the resulting metric will be lower than the All Weather numbers.

Each year, the Institute of Electrical and Electronics Engineers surveys utilities across the country to gather electric reliability metrics.  IEEE uses the data provided to divide the survey respondents’ data into 4 groups (quartiles) to create benchmark electric reliability quartile data for utilities. (Benchmarking | IEEE PES Distribution Reliability Working Group)  Data in the 1^st quartile indicates the highest electric reliability and data in the 4^th quartile represents the lowest. While the utilities participating in IEEE’s annual benchmark study are kept anonymous, the benchmark data provides a useful comparison.