In the distribution system, under normal circumstances the zero sequence current value is very small, when the single-phase ground fault happens, the ground phase current will rapidly increase. Therefore, we uses over value zero sequence current method as our earth fault criterion. When the zero sequence current exceeds the previous set-up value and exceeds the set time, the indicator will indicate the fault, the earth fault probe sends out a fault signal through the optical fiber separately to the panel and to the electrical automation device outputs . The set value (according to the user's request) is preset in the factory.