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A blackout is a total loss of electrical power. A blackout is usually defined as a drop in line voltage below 80 volts (V) (the normal voltage is 120V), since most electrical equipment will not operate below these levels. A blackout may be due to a planned interruption, such as limiting of loads during power shortages by rotating power shutoffs through different areas, or due to an accidental failure caused by human error, a failure of generating or transmission equipment, or a storm. Blackouts can cause losses of industrial production, disturbances to commercial activities, traffic and transportation difficulties, disruption of municipal services, and personal inconveniences. In the summer of 1977, a blackout caused by transmission line losses during a storm affected the New York City area. About nine million people were affected by the blackout, with some areas without power for more than 24 hours. The blackout was accompanied by looting and vandalism. A blackout in northeastern United States and eastern Canada due to a switching relay failure in November of 1965 affected 30 million people and resulted in improved electric utility power system design.

A brownout is a condition (usually temporary, but which may last longer, i.e., from periods ranging from fractions of a second to hours) when the alternating current (AC) electrical utility voltage is lower than normal. If the brownout lasts less than a second, it is called a sag. Brownouts may be caused by overloaded circuits, but are sometimes caused intentionally by a utility company in order to reduce the amount of power drawn by users during peak demand periods, or unintentionally when demand for electricity exceeds generating capacity. A sag can also occur when line switching is employed to access power from secondary utility sources. Equipment such as shop tools, compressors, and elevators starting up on a shared power line can cause a sag, which can adversely affect other sensitive electronic equipment such as computers. Generally, electrical utility customers do not notice a brownout except when it does affect sensitive electronic equipment.

Measures to protect against effects of blackouts and brownouts include efficient design of power networks, interconnection of power networks to improve stability , monitoring of generating reserve needs during periods of peak demand, and standby power for emergency needs. An individual piece of equipment can be protected from blackouts and brownouts by the use of an uninterruptible power source (UPS). A UPS is a device with internal batteries that is used to guarantee that continuous power is supplied to equipment even if the power supply stops providing power or during line sags. Commonly the UPS will boost voltage if the voltage drops to less than 103V and will switch to battery power at 90V and below. Some UPS devices are capable of shutting down the equipment during extended blackouts.

[Judith L. Sims ]



Curvin, R., and B. Porter. Blackout Looting! New York City, July 13, 1977. New York: Gardner Press, 1979.

Dugan, R.C. Electrical Power Systems Quality. New York, McGraw-Hill, 1996.

Kabisama, H.W. Electrical Power Engineering. New York: McGraw-Hill, 1993.

Kazibwe, W.E., and M.H. Sendaula. Electric Power Quality Control Techniques. New York: Van Nostrand Reinhold, 1993.