Sag refers to a phenemonon in which RMS voltage decreases within 0.1 to 0.9 pu during one seconf of 0.5 cycle in power supply system. Comprehensively, it can be said to be a phenemonen in which a fault caused by a lightning store occues in a device constituting power system and the effective value voltage is reduced to 0.1 to 0.9pu for 0.5 cuctem to 300 ms while removing the fault.

Some of the most common causes of Sag is often caused by either lightning, typhoon, rainfall, snowfall, wildfire or others. When this damages happens and causes a Sag, it often leads to loss pf product during production process, damage to major production equipment, product failure rate arises, loss of production opportunity and most important, damage to corporate image.