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Tree limbs create a short circuit in electrical lines during a storm that spawned two tornadoes.

A power outage (also known as power cut, power failure, power loss, or blackout) is the loss of the electricity supply to an area.

The reasons for a power failure can for instance be a defect in a power station, damage to a power line or other part of the distribution system, a short circuit, or the overloading of electricity mains. While the developed countries enjoy a highly uninterrupted supply of electric power all the time, many developing countries have acute power shortage as compared to the demand.

Some developing countries and newly-industrialized countries have several hours of daily power-cuts in almost all cities and villages because the increase in demand for electricity exceeds the increase in electric power generation. Wealthier people in these countries may use a power-inverter (rechargeable batteries) or a diesel/petrol-run electric generator at their homes during the power-cut. The use of standby generators is common in industrial and IT hubs.

A power outage may take one of three forms:

Blackout

where power is lost completely. "Load shedding" or a rolling blackout is a common term for a controlled way of rotating available generation capacity between various districts or customers, thus avoiding wide area total blackouts.

Brownout

where the voltage level is below the normal minimum level specified for the system. Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. Some brownouts, called voltage reductions, are made intentionally to prevent a full power outage.

Dropout

where the loss of power is only momentary (milliseconds to seconds).

Power failures are particularly critical for hospitals, since many life-critical medical devices and tasks require power. For this reason hospitals, just like many enterprises (notably colocation facilities and other datacenters), have emergency power generators which are typically powered by diesel fuel and configured to start automatically, as soon as a power failure occurs. In most third world countries, power cuts go unnoticed by most citizens of upscale means, as maintaining an uninterruptible power supply is often considered an essential facility of a home.

Power outage may also be the cause of sanitary sewer overflow, a condition of discharging raw sewage into the environment. Other life-critical systems such as telecommunications are also required to have emergency power. Telephone exchange rooms usually have arrays of lead-acid batteries for backup and also a socket for connecting a diesel generator during extended periods of outage.

Power outages may also be caused by terrorism (attacking power plants or electricity pylons) in developing countries. The Shining Path movement was the first to copy this tactic from Mao Zedong.

In power supply networks, the power generation and the electrical load (demand) must be very close to equal every second to avoid overloading of network components, which can severely damage them. In order to prevent this, parts of the system will automatically disconnect themselves from the rest of the system, or shut themselves down to avoid damage. This is analogous to the role of relays and fuses in households.

Under certain conditions, a network component shutting down can cause current fluctuations in neighboring segments of the network, though this is unlikely, leading to a cascading failure of a larger section of the network. This may range from a building, to a block, to an entire city, to the entire electrical grid.

Modern power systems are designed to be resistant to this sort of cascading failure, but it may be unavoidable (see below). Moreover, since there is no short-term economic benefit to preventing rare large-scale failures, some observers have expressed concern that there is a tendency to erode the resilience of the network over time, which is only corrected after a major failure occurs. It has been claimed that reducing the likelihood of small outages only increases the likelihood of larger ones. In that case, the short-term economic benefit of keeping the individual customer happy increases the likelihood of large-scale blackouts.

Restoring power after a wide-area outage can be difficult, as power stations need to be brought back on-line. Normally, this is done with the help of power from the rest of the grid. In the total absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation. The means of doing so will depend greatly on local circumstances and operational policies, but typically transmission utilities will establish localised 'power islands' which are then progressively coupled together. To maintain supply frequencies within tolerable limits during this process, demand must be reconnected at the same pace that generation is restored, requiring close coordination between power stations, transmission and distribution organizations.

See also: Uninterruptible power supply

It has recently been argued on the basis of historical data^[1] and computer modelling^[2] that power grids are self-organized critical systems. These systems exhibit unavoidable^[3] disturbances of all sizes, up to the size of the entire system, and attempts to reduce the probability of small disturbances only increase the probability of large ones^[4]. This has immediate policy implications^[1]. The following are the relevant quotations from the sources cited:

As expected from studies of general self-organised critical systems, ... apparently sensible efforts to reduce the risk of smaller blackouts can sometimes increase the risk of large blackouts ^[4]

...the NERC blackout data suggests that the North American power system has been operating near criticality. ...It would be better to analyze this tradeoff between catastrophic blackout risk and loading instead of just waiting for the effects to manifest themselves in the North American power system! ^[1]

[The models'] PDF of the blackouts size has the same power dependence that have been found from the analysis of NERC data for the North American power grid over a period of 15 years. ^[2]

First and perhaps most striking is the intrinsic unavoidability of cascading events in such a system when driven near its operational limits. ^[3]

A complex network-based model to control large cascading failures (blackouts) using local information only was proposed in A. E. Motter, Cascade control and defense in complex networks, Phys. Rev. Lett. 93, 098701 (2004).

Energy Portal

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• A. E. Motter and Y.-C. Lai, Cascade-based attacks on complex networks, Physical Review E (Rapid Communications) 66, 065102 (2002)
• Ontario Electricity articles
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• First-Hand Account of NYC's 2003 Blackout
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