Brownout (electricity)

A brownout is an intentional or unintentional drop in voltage in an electrical power supply system. Intentional brownouts are used for load reduction in an emergency. The reduction lasts for minutes or hours, as opposed to short-term voltage sag (or dip). The term brownout comes from the dimming experienced by incandescent lighting when the voltage sags. A voltage reduction may be an effect of disruption of an electrical grid, or may occasionally be imposed in an effort to reduce load and prevent a power outage, known as a blackout.

In some media reports the term brownout refers to an intentional or unintentional power outage or blackout of some areas rather than to a drop in voltage.

Different types of electrical apparatus will react in different ways to a sag. Some devices will be severely affected, while others may not be affected at all.

The heat output of any resistance device, such as an electric space heater, is equal to the true power consumption, which is an increasing function of the applied voltage. If the resistance stays constant, power consumption is proportional to the square of the applied voltage. Therefore, a significant reduction of heat output will occur with a relatively small reduction in voltage. An incandescent lamp will dim due to lower heat creation in the filament, as well as lower conversion of heat to light. Generally speaking, no damage will occur but functionality will be impaired.

Commutated electric motors, such as universal motors, will run at reduced speed or reduced torque. Depending on the motor design, no harm may occur. However, under load, the motor may draw more current due to the reduced back-EMF developed at the lower armature speed. Unless the motor has ample cooling capacity, it may eventually overheat and burn out.

An induction motor will draw more current to compensate for the decreased voltage, which may lead to overheating and burnout. If a substantial part of a grid's load is electric motors, reducing voltage may not actually reduce load and can result in damage to customers' equipment.

An unregulated direct current supply will produce a lower output voltage for electronic circuits. The output ripple voltage will decrease in line with the usually reduced load current. In a CRT television, the reduced output voltage can be seen as the screen image shrinking in size and becoming dim and fuzzy.

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