The five largest electric utilties annually report industry standard measures of the duration and frequency of outages in order to evaluate performance and identify problem divisions and circuits.
Annual Reliability Reports of the major electric utilities include the following information:
- Duration and frequency of sustained and momentary outages using SAIDI, SAIFI, and MAIFI, with and without excludable major events for the past 10 years.
- The top ten power outage events based on customer-minutes, excluding events such as weather, declared emergencies, or disasters affecting over 10% of the utility's customers.
- Circuits in which customers have experienced greater than twelve sustained outages in a reporting year.
System statistics are computed including transmission, substation, and distribution outages but exclude intentional outages for example for line maintenance. Some of the remaining outages such as storm damage are considered beyond control of the utilities. Statistics are reported both including and excluding the storm outage data.
- SAIDI (System Average Interruption Duration Index) is the system-wide total number of minutes per year of sustained outage per customer served.
- SAIFI (System Average Interruption Frequency Index) is how often the system-wide average customer was interrupted in the reporting year.
- MAIFI (Momentary Average Interruption Frequency Index) is the number of momentary outages per customer system-wide per year.
As used above sustained outages are defined as 5 minutes or longer and momentary outages are less than 5 minutes long.
Utilities must maintain this data for each circuit (a circuit serves roughly 2,000 customers) and to make data available to any interested person on request. See Appendix A. of D.96-09-045
After Major Storms Utilities Must Restore Service in Less Than 12 Hours on Average
CAIDI was added to reports of Measured Events (an event affecting between 10% simultaneous and 40% cumulative of all customers). The Commission presumes utility performance is unreasonable when the total minutes of interruption during the event divided by the total interruptions exceeds 570. See D.00-05-022.
PG&E reports its divisions where reliability indices vary beyond certain triggers including: See D.04-10-034.
- Division indices excluding major events vary by 10% from a 5-year rolling average.
- A weather-related excludable event that causes Customer Average Interruption Duration Index (CAIDI) to vary by 25%, or only 10% from the system benchmark.