Sources of sags and interruptions:
A sudden increase in load results in a corresponding sudden drop in voltage.
Any sudden increase in load, if large enough, will cause a voltage sag in:
o Faults cause the voltage sag.
o Switching operation
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 voltage sag.
When a line-to-ground fault occurs, there will be 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 voltage sag as a result.
Sudden load changes or excessive loads can cause 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 residential homes, we usually see voltage sags when the refrigerator, air-conditioner or furnace fan starts up.