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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".
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.
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.
There are several factors which cause a voltage sag to happen:
- Since electric motors draw more current when they are starting than when they are running at their rated speed, starting an electric motor can be a reason of a voltage sag.
- When a line-to-ground fault occurs, there will be a voltage sag until the protective switch gear operates.
- Some accidents in power lines such as lightning or a falling object can be a cause of line-to-ground fault and a voltage sag as a result.
- Sudden load changes or excessive loads can cause a voltage sag.
- Depending on the transformer connections, transformers energizing could be another reason for voltage sags happening.
- Voltage sags can arrive from the utility but most are caused by in-building equipment. In residential homes, voltage sags are sometimes seen when refrigerators, air-conditioners, or furnace fans start up.