Based on long underground power supply line of Xinglongzhuang Mine, traditional line selection method and device couldn’t be used to make accurate judgement of faulty line and fault point. In order to determine line selection device precisely, current status of this device was analyzed firstly, which found the reasons of low precision rate of traditional method. Then, design requirements of new ground connection system were analyzed, and solutions were put forward, which had a guidance meaning to determination of ground connection system.