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, or transient which is the reduction of voltage for a very short duration (less than a half cycle).
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 falling an 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 happening voltage sags.
- Voltage sags can arrive from the utility but most are caused by in-building equipment. In residental homes, we usually see voltage sags when the refrigerator, air-conditioner or furnace fan start up.
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