In the three-phase power supply system in mining wells,the method to detect the single-phase breakdown earth phase in the system of neutral point unconnected to earth is the next to the phase with the highest voltage in the three-phase voltage is the breakdown phase.

This Paper analyzes the principle of various measure of electric shock safeguard in LV three-phase power system,elaborates various wiring formats,pay attention to the problems when the leakage current operated protective device is used in different power system.