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".
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 the 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.
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