To simplify the circuit and increase the fault signature for single-phase grounding faults detection device in distribution networks, a single-phase grounding fault detection method based on zero-sequence current transformer with large resistance is proposed. The characteristics of zero-sequence current transformers with large secondary resistance are investigated, and its correctness is verified by simulation and experiment results. On this basis, the parameter selection of the secondary side resistance is studied. The applications of the proposed methods in relay protection of the customer circuit breaker and the single-phase grounding line selection in substations are described with typical examples, the feasibility of which is verified by the digital simulation method. Compared with the traditional approaches, the results show that the proposed method makes the detection circuit simpler and increases the fault signature for single-phase grounding protection on the user side.