To simplify the circuit and increase the fault signature for single-phase grounding faults in neutral ungrounded distribution networks, a single-phase grounding fault detection method based on zero sequence current transformer with large secondary resistance is proposed. The behavior of zero sequence current transformers with large secondary resistance are investigated and verified by simulation and experiment results. Design considerations of the secondary load resistance is given. 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 simulation results. Compared with the traditional approaches, the proposed method makes the detection circuit simpler and increases the fault signature for single-phase grounding line selection in neutral ungrounded distribution networks.