Abstract：This paper describes that in the power system how to use DC testing methods to detect whether there is a bus or slip fault, and accurately calculate the value of the ground resistance. The method is that the positive and negative of the DC bus through the balance bridge and the non-equilibrium bridge connect to the ground, thus the whole voltage were imposed directly on current system by all of these (or a) bridges, according to Ohm's law, used to collect the voltage of the positive and negative bus and the two resistance values of the bridge, to establish a Equations, then get the value of the grounding resistance; At the same time, in every slip on the power supply devices has a Hall current sensor, so that all the wire of the positive and negative slip through the Hall sensor, respectively, according to the leakage current which the sensors detect, can determine the slip fault, and in accordance with Sensors detected the leakage current value and acquisition value to the bus voltage, we can calculate grounding resistance value of the slip. With the traditional detection method, the method of DC system without any adverse impact from the impact of capacitance, detection accuracy and sensitivity is higher, and does not need a signal device and reduce the cost of the product, but also reduces design The difficulty, greatly reducing the development cycle.