SOLUTION

A single-line-to-ground (LG), fault occurs when one phase conductor breaks and falls to the ground or when one phase conductor comes in contact with a neutral conductor.

When the 3-ϕ system is not grounded and if the single line to ground fault occurs then the voltage increase of the unfaulted phases during a single line-to-ground fault.

In a single line to ground fault in an ungrounded power system, voltages of healthy phases increase which may cause well in those phases.

Under a single line-to-ground fault, the voltage of the faulty phase (line) becomes equal to ground voltage and the voltages of the two remaining healthy phases (lines) with respect to ground rise from their normal phase to neutral voltages to full line value (i.e., √3 times the normal phase value). This causes stress on the insulation.