Abstract: With the gradual maturity of intelligent terminal measurement technology in the low-voltage distribution network, achieving holographic perception on the low-voltage side while also possessing the ability to monitor multiple points of the medium-voltage distribution network. This paper proposed a method of single-phase grounding fault detection for an un-effectual grounding system based on the low-voltage side's negative-sequence characteristics. The characteristics of the negative sequence signal at the low-voltage side after a single-phase grounding fault occurred on the medium-voltage side are analyzed; the negative-sequence current can distinguish the fault and non-fault lines. after setting the ratio coefficient of negative-sequence voltage and positive-sequence voltage, multi-point collaborative calculation and comparison can be used to determine the section of the fault point. The unbalanced load at the low-voltage side and the special case of a fault point at one end of the line are considered. Through the simulation of medium-voltage and low-voltage combined distribution systems in MATLAB software and dynamic model experiment, it is verified that the proposed method has good robustness and accuracy. Monitoring the status information of the medium-voltage distribution network through the smart distribution transformer terminal to realize the fault detection of neutral un-effectual grounding systems has great potential in practical application and implementation.