Abstract: The substation bus in the 10 kV distribution network is ungrounded, grounded through intelligent small reactance or directly grounded through small resistance. The fault isolation modes and accuracy of various grounding modes are different in case of grounding fault of medium voltage line. During the period of power failure or planned maintenance of main network equipment, there will inevitably be a special operation mode arrangement of power transfer between the 10 kV lines of two substations with different 10 kV grounding systems. The adaptability of small reactance grounding and small resistance grounding to ground fault is different. In this paper, the fault cause analysis is carried out for a ground fault event of medium voltage distribution line extended to the loss of voltage of the whole section of 10 kV bus due to improper operation mode arrangement. In order to achieve smaller fault influence range and higher fault isolation efficiency, suggestions on the arrangement of switching power supply operation mode are given. It has important reference significance for the dispatching operation mode arrangement of medium voltage distribution network.