电压骤降是指短时间内方均根电压下降并回复正常。电压骤降的美式英语是Voltage Sag，英式英语是Voltage Dip。

根据欧洲标准EN50160，电压下降至标准值90%以下，然后回复到90%以上，持续时间10ms到60s。

因此，电压下降超过1分钟，才算是停电。

电压以标准值计算，当电压由标准值92%再下降3%，便是11%电压骤降。

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".