Single-phase grounding is the most common fault in medium-voltage distribution networks. Although the system can continue to operate with faults after grounding occurs, due to the increase of non-fault phase-to-earth voltage, if it is not handled in time, it may develop into non-fault phase insulation damage. The threat of short-to-phase short circuits. The timely and accurate determination of the ground loop is the basis for quickly eliminating single-phase ground faults. The protection device that realizes the determination of a ground fault loop is often referred to as a low-current grounding line selection device, but early line selection devices often have incorrect selections and missed selections. Unsatisfactory. The small current grounding line selection device is a supporting product of a new generation substation automation system independently developed by Hunan Liling Huayi Electronic Technology Co., Ltd. This product uses the comprehensive line selection theory based on the comparison of transient zero-sequence current amplitudes, and uses high-speed sampling to extract and analyze the moment of grounding occurrence.