Grounded fault selecting and protecting method for grouded system with small current

The method includes following steps: the zero sequence voltage, phase voltage and phase current are monitored on line; when the zero voltage is more than setting value, the system is determined in failure; the difference current between phases is defined as the 0.5 difference between this phase current variation and sum of other two phases current variation; the calculating resistance is defined as the ratio of the phase voltage after failure to difference current between phases based on this phase current; the calculating resistance of feed line is calculated; it is decided if the failure will be happened according to the value of calculating resistance; the calculating resistance value equals resistance value of earth failure; the failure line sends out failure signals.