Electric reliability indices measure the reliability of a given electric grid. They often are used by regulators to measure utility performance and by utilities, transmission companies, and system operators to internally monitor reliability levels. The indices may be used to measure system-wide reliability or may be applied to determine the reliability of specific portions of a system such as each electric feeder or a set of electric feeders on a distribution system.
Various indices are used:
System Average Interruption Frequency Index (SAIFI)
SAIFI is the average number of interruptions per consumer with a sustained duration (five minutes or greater) over a given time frame. SAIFI is commonly stated in terms of interruptions per year.
SAIFI = (the total number of sustained interruptions) / (the total number of consumers)
System Average Interruption Duration Index (SAIDI)
SAIDI is the average duration of sustained interruption per consumer over a given time. SAIDI is commonly stated in terms of hours per consumer per year.
SAIDI = (the total duration of sustained interruptions) / (the total number of customers)
Consumer Average Interruption Frequency Index (CAIFI)
CAIFI is the average number of sustained interruptions for consumers who experience such interruptions during a given time frame. This is used to evaluate whether a limited number of consumers are experiencing repeated outages since SAIFI may hide the fact that a few consumers may experience a disproportionate number of outages.
CAIFI = (the total number of sustained interruptions) / (the total number of consumers that experienced one or more sustained interruptions)
Consumer Average Interruption Duration Index (CAIDI)
CAIDI is the average duration of sustained interruptions over a given time. Similar to CAIFI, it is used to determine whether a limited number of consumers are experiencing long periods of outages.
CAIDI = (the total duration of sustained interruptions) / (the total number of sustained interruptions)
Momentary Average Interruption Frequency Index (MAIFI)
MAIFI is the average number of momentary (less than five minutes) interruptions per consumer over a given time. It's been used in recent years to recognize the momentary interruptions that can cause issues for consumers given the amount of electronics now prevalent in consumer facilities.
MAIFI = (the total number of momentary interruptions) / (the total number of consumers)
Average Service Availability Index (ASAI)
ASAI is the ratio of the total number of customer hours that service was available during a given time to the total customer hours demanded. This is sometimes called the service reliability index.
ASAI = (the total number of customer hours with service availability) / (the total number of hours that service was desired by customers)
The indices are typically calculated over a period of one year, although other time periods can be used. SAIFI and SAIDI are the most used, and typical levels in North America are 1.1 interruptions per customer per year for SAIFI and 1.5 hours per customer per year for SAIDI. Note that SAIDI is often calculated to exclude major events such as extreme weather. If major events are included, average SAIDI has ranged from 3.5 to as high as 8 hours in recent years.