Earth Fault Relay is sensed by measuring the neutral current. Normally in three phase system the vector sum of three phase current are zero hence no current flows from neutral to earth. If ground fault occurs the vector sum is not zero. If we can sense the current from neutral to earth we will get the residual current which we can say as earth fault current. Sensing this current calibrated we can compare with the set point to detect the earth fault.

Earth Fault Relay is sensed by measuring the neutral current. Normally in three phase system the vector sum of three phase current are zero hence no current flows from neutral to earth. If ground fault occurs the vector sum is not zero. If we can sense the current from neutral to earth we will get the residual current which we can say as earth fault current. Sensing this current calibrated we can compare with the set point to detect the earth fault.