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This paper proposes a new method for determining locations of single-phase-to-ground faults in ungrounded distribution system using pre-fault and during-fault fundamental-frequency measurements. The pre-fault measurements along with the load profiles are used to determine load demands for each individual load in a feeder section. The during-fault measurements are used to determine possible faulted area and location. The faulted feeder and faulted feeder section are first determined based on the reactive power factors of residual powers determined for the feeder breakers and switches with sensors. The line segments in the faulted feeder section are then tested for a sign change of phase-to-ground voltage on the fault phase to determine the faulted line segment. At last, the location of the fault is determined by finding a location along the faulted line segment having a zero phase-to-ground voltage on the faulted phase. Numerical examples are given to demonstrate the effectiveness of proposed method.