A phenomenon in which a failure due to lightning strike, etc. occurs in power system such as transmission line constituting the power system, lasts for 0.5 cycles to 1 second and the effective voltage drops within 0.1 to 0.9 pu. until the failure is removed from the power system.

Mostly for less than 10 CYCLES, more than 60% Voltage Sag
- Instantaneous voltage drop and instantaneous power outage in industrial sites often occur
- Switching of short-circuit accident, lightning strike and large capacity load in the system
- Mostly the duration is less than 10 cycles, and the size is over 60%

- A sudden and momentary drop in supply voltage (10% - 90%), typically for the duration of 0.5 – 50 cycles
- Electrical networks in developed countries are vulnerable to voltage sags rather than voltage collapses due to high integrity of grid system
- Example, in Singapore, majority of voltage disturbances are voltage sags
- Performance measured in SARFI Index
- Majority of voltage sags fall within the 60% magnitude & 300ms duration (typical fault clearing duration ~100ms)