This device is mainly used for isolated neutral or via extinction coil grounding of power system (low current system) when the single phase to ground fault occurred, grounding fault line is automatically selected when PT ferroresonance occurs or eliminate resonance. When you single phase grounding fault in small current system, zero-sequence current is capacitive current of distribution, numerical fault characteristics are small and barely visible, vulnerable to the impact of operation mode, this is the route of difficulties of the reason for that. Some of the existing fault line selection device due to limitations on line selection scheme and hardware problems in the circuit itself, onsite operation conditions far from ideal. Address the problem of the past, when we use the failure inherent in the system of zero-sequence/harmonic characteristics developed the line selection device, by the power industry power equipment and instrument quality test center of testing and commissioning on site demonstrated operation reliable, advanced actions with high accuracy, line selection scheme, the capture of small current system single-phase earth fault line detection accuracy problems of low. This device is suitable for conventional substation, and comprehensive automatic substation of power plant auxiliary power system.