Running voltage of chinese underground grid system was 6～10 kV. In order to prevent large grounding current occurring in single phase earth fault, neutral non-direct grounding system was adopted. But different operation modes had different requirements for leakage protection. 10 kV high voltage power supply network was taken as research object in this paper. Different manifestations of steady state characteristics after occurring single phase to ground fault under two operation modes were studied, which provided reliable theoretical basis to line selection method.