Abstract: In the resonant grounding system, the accurate judgement of the arc-extinction time has a guiding significance for the line selection of a grounding fault, the evaluation of the arc suppression effect, and the extraction and utilization of the transient information after an arc extinction. Aiming at the single-phase grounding fault of the resonant grounding system, this paper analyzes the electrical characteristics of the fault before and after an arc extinction. By using the bus zero-sequence voltage and the Petersen's coil current, the two characteristic parameters are constructed by obtaining the 1st-order derivative of its signal and the second-order derivative of its squared signal. It is found that the characteristic parameters are constant before the arc extinction, while change significantly after the arc extinction, which can be utilized to form the corresponding basic criterion for the arc extinction time. Considering the influences of various factors in the actual operation of the system, an additional criterion for the arc extinction time are given, and finally a method for judging the arc-extinction time of the resonant grounding system based on the difference of electrical characteristics is proposed. The validity and accuracy of the method for judging the arc-extinction time are verified through the MatLab/Simulink simulation and a field test.