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Rolling blackout refers to an intentionally-engineered electrical power outage, caused by insufficient available resources to meet prevailing demand for electricity.
Though the term did not enter popular use until the California electricity crisis of the early 2000s, such outages had occurred previously, almost always triggered by unusually hot temperatures during the summer, which cause a surge in demand due to heavy use of air conditioning.
California
Most of California is divided into 14 power grids, each containing approximately 7% of electricity customers in the state. This adds up to 98%; the remaining 2% are placed on a separate grid (examples of facilities on this latter grid include hospitals, police stations, etc.) which is exempt from ever having its power deliberately cut off.
When a Stage 3 power emergency is declared, electricity to one of the grids is shut off for a fixed period of time, which can range from 60 minutes to 2½ hours (in a Stage 1 emergency only a general call for voluntary conservation is issued, while a Stage 2 emergency results in power being temporarily cut off to certain large users, primarily industrial concerns, who have agreed to this arrangement in exchange for lower rates). If after this period of time the Stage 3 emergency still exists, power is restored to this grid but then the next grid in the sequence is blacked out, and so on, until the situation is stabilized — the blackout thus "rolls" from one grid to the next.
In California, each customer's electric bill includes the number of the power grid (from 1 to 14) that customer belongs to; this gives customers at least some advance notice of when their electricity might be turned off in the event of a Stage 3 emergency. The grids are set up in such a manner as to ensure that a large percentage of customers in the same neighborhood would not be blacked out concomitantly, which could invite looting and related problems.
Elsewhere
Not all states are on this system; in many East Coast states (such as New York State and New Jersey), "brownouts" rather than rolling blackouts are implemented during power emergencies: In this scenario, instead of the power being cut off altogether to a certain percentage of customers, the voltage is reduced by a certain percentage to all customers — the resulting dimming of electric lights being the origin of the term "brownout."
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