Voltage sag

A voltage sag (U.S. English) or voltage dip (British English) is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors. A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute. Some references defines the duration of a sag for a period of 0.5 cycle to a few seconds, and longer duration of low voltage would be called a "sustained sag".

Related notions

The term "sag" should not be confused with brownout which is the reduction of voltage for minutes or hours.

The term "transient" as used in power quality is an umbrella term and can refer to sags, but also to swells, dropouts etc.

Swell

Voltage swell is the opposite of voltage sag. Voltage swell, which is a momentary increase in voltage, happens when a heavy load turns off in a power system.

Causes

There are several factors which cause a voltage sag to happen: