Earth Fault Detection Method for TN-C Systems Based on Comparison of Current Deviations

The existing low voltage (LV) earth fault detection techniques using residual current measurements cannot be effectively applied to the multi-earthed TN-C system. When an earth fault occurs on the multi-earthed TN-C system, the fault current passes through the combined protective and neutral (PEN) conductor back to the source, resulting in a low residual current. However, unbalanced load switching also generates residual current through the same path back to the source, thus making fault discrimination very difficult. To address this problem, the four-wire multi-earthed TN-C system was analyzed by decoupling the mode 0 and the mode N of the equivalent circuit so that the current characteristics can be clearly studied. A new earth fault detection method is developed which compares the residual current deviation with the PEN conductor current deviation. The scheme uses amplitude ratio and phase angle difference between these two measurements to discriminate between earth faults and unbalanced load switching. The feasibility of the proposed method was verified by digital simulation and by physical tests in an actual LV distribution network. Compared to approaches based only on residual current, the proposed method demonstrates better sensitivity and reliability.