To improve power supply capacity and save land resources, a large number of single-circuit distribution lines have been transformed into double-circuit distribution lines. The influence of their complex coupling relationship on the characteristics of non-solidly grounding faults and the applicability of existing line selection methods are not yet clear. An equivalent model of small grounding fault in double-circuit line from different buses is established, and the influence of the line-to-line coupling on the fault characteristics of the grounding system and the applicability of the line selection methods are analyzed by it. The results show that: for the system with a grounding fault, the line-to-line coupling does not affect its zero-sequence voltage and current characteristics, and the existing line selection methods are still applicable; for the system without a grounding fault, the double-circuit line shows the characteristics of the faulty line, and the existing line selection methods are no longer applicable. Simulation is used to verify the results.

With the repeated grounding of the PEN line, load fluctuation, and other factors, there is no available leakage protection for the low-voltage TN-C system, which is very dangerous when a leakage fault occurs. And TN-C grounding mode is still widely used, and the reformation is slow. The single-phase leakage fault and load fluctuation characteristics of the TN-C system are analyzed, and a leakage protection method for the TN-C system based on the amplitude ratio of characteristic currents is proposed. This method takes the amplitude of the residual current sudden change as the starting criterion, and constructs the action criterion according to the amplitude ratio of load current sudden change and residual current’s sudden change, so as to realize the discrimination of leakage fault and load fluctuation. The proposed method can overcome the influence of load fluctuation and other factors, suitable for TN-C systems with high sensitivity and reliability. The effectiveness of the protection is verified by simulation.