Abstract: Leakage is one of the common faults caused by the reduction of insulation level of IT power supply system in coal mine. A DC detecting leakage protection circuit was designed, which converts leakage current reflecting insulation degree into voltage value for insulation performance detection. When various leakage faults occur in the power grid, the calculation of the action resistance of the protective circuit was given in combination with the setting of the insulation resistance of the grid to the ground.