Rolling blackout refers to an intentionally-engineered electrical power outage, caused by insufficient available resources to meet prevailing demand for electricity. For information about accidental blackouts that are not intentionally engineered, see power outage. In many African countries – e.g. Cameroon, Democratic Republic of the Congo, Nigeria and Zimbabwe – a combination of aging electricity generation infrastructure, and the inadequacy of the supply of electricity to the ever expanding demand, has made rolling blackouts a staple of daily life.
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United States
Texas
In April of 2006, parts of Texas experienced rolling blackouts due to excessive air conditioner use because of unexpectedly high temperatures. The longest power outage lasted for a period of five hours, affecting areas in the Middle to the South of Texas. The Texas power system runs on a system similar to the one in California.
California
Though the term did not enter popular use in the U.S. 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. Rolling blackouts were again imposed in late August 2005 in Southern California due to the loss of a key transmission line; the transmission line shut itself off because of a faulty sensor. Most of California is divided into 14 power grids, each containing approximately 7% of electricity customers in the state, creating a total of 98%. The remaining 2% are placed on a separate grid, where users such as hospitals and police stations are exempt from ever having their power deliberately cut off. 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. 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. 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 concurrently, which could invite looting and other related problems. Normal electricity customers can fall within the areas reserved for emergency use (if they are near a hospital or other critical infrastructure), in which case their electric bill will indicate a power grid of 99 and they will not be affected by rolling blackouts.
Elsewhere
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." Brownouts can cause significant damage to unprotected electronic equipment, but usually have no effect (other than reduced performance) on incandescent lights or some types of motors.
Republic of Ireland
On several occasions in the 1970s and 1980s trade union strikes in Ireland's power utility, the Electricity Supply Board (ESB) led to rolling blackouts. However, rolling blackouts have not occurred for this reason since 1991. For such eventualities, the ESB have a zone rota system in place. The country is divided into regions which in turn are subdivided into zones, referred to by the letter codes A, B, C, X, Y, and Z. During periods when blackouts may occur, advertisements are placed in the national newspapers informing customers which region and zone they are in, and at what times of the day they have a high, moderate or low risk of supply interruption. (Customers fortunate enough to live close to a hospital may find they are on a "priority line" and don't lose power at all). The authorities appeal to the public to conserve electricity (especially during hours of peak demand); however, if and when electricity demand exceeds available supply, supply is cut in some or all of the "high risk" zones. If there is still a shortfall once all the high risk zones have had power cut, then the "moderate risk" zones start experiencing power cuts. The level of risk in each zone changes every three hours moving from "Low" to "Moderate" to "High" and back to "Low".
United Kingdom
The Three-Day Week of January to March 1974, introduced to limit electricity consumption, and thus conserve coal supplies which were severely reduced due to industrial action, meant that non-essential commercial users were only allowed to consume electricity for three days each week. Home electricity supplies were also limited in some areas.
South Africa
Due to a lack of strategic planning by Eskom(and moreso a lack of funding from the South African Government), South Africa's national electricity producer, the capacity of the nation's electricity grid has not kept up with demand. From December 2005 until February 2006, rolling blackouts were implemented in Cape Town and other areas of the Western Cape. This was the result of one of two reactors at Koeberg nuclear power station being unavailable for this period, with the transmission lines from the coal-fired power stations in Mpumalanga having insufficient capacity to make up the shortfall. These blackouts and the accompanying brouhaha in the media resulted in Eskom and the government announcing a number of plans for new power stations, and Eskom started returning mothballed power stations to service. However, it is expected that the supply constraints will last for a number of years. In October 2007, rolling blackouts were implemented in many parts of the country, including the executive capital city, Pretoria, and the biggest city and economic hub, Johannesburg. Although Eskom and the municipalities involved in implementing the blackouts make some attempt to provide schedules, very often the information is incorrect or is only provided after the blackout has started. A number of deaths resulted from power to hospitals being cut in the Cape Town blackouts in 2006, with certain hospitals not having adequate backup facilities. As a result, an agreement was reached to not cut power to hospitals. However, during the blackouts in Pretoria in October 2007, the City of Tshwane municipality cut power to areas including hospitals. Eskom euphemistically refers to rolling blackouts as "load shedding".
