[6] The interruptions of the power supply affecting the customers can be either momentary (short, usually defined as less than 1 or 5 minutes[7]) or "sustained" (the longer ones).
[11] Electric utilities came into existence in the late 19th century and since their inception had to respond to problems in their distribution systems.
[3] In the US, the interest in reliability assessments of generation, transmission, substations, and distribution picked up after the Northeast blackout of 1965.
[13] Notably, SAIDI, SAIFI, CAIDI, CAIFI, ASAI, and ALII were defined in a Guide For Reliability Measurement and Data Collection (1971).
[16] By 1998 64% of the utility companies were required by the state regulators to report the reliability (although only 18% included the momentary events into the calculations).
[20] It is assumed that if the cases of demand exceeding the generation capacity are sufficiently rare and short, the distribution network will be able to avoid a power outage by either obtaining energy via an external interconnection or by "shedding" part of the electrical load.
[21] The reliability indices for the electricity generation are mostly statistics-based (probabilistic), but some of them reflect the rule-of-thumb spare capacity margins (and are called deterministic).