小电流接地系统单相接地故障的特点是，故障线路的零序电容电流为所有非故障线路的零序电容电流之和，如果同一条母线上的线路条数足够多，且各条线路的参数参差不齐，则故障线路的零序电容电流将远远大于非故障线路的零序电容电流，利用零序电流的大小就可以判断出故障线路。
The characteristic of single-phase grounding faults in low current grounding systems is that the zero sequence capacitance current of the fault line is the sum of the zero sequence capacitance currents of all non fault lines. If there are enough lines on the same bus and the parameters of each line are uneven, the zero sequence capacitance current of the fault line will be much greater than that of the non fault line. The size of the zero sequence current can be used to determine the fault line.
同时故障线路和非故障线路的零序电容电流方向不同是构成选线的另一主要判据，非故障线路的零序功率方向由母线指向线路，而故障线路的令修改功率方向则由线路指向母线，利用零序功率方向的不同也可以判断出故障线路，并且零序功率方向还不存在动作死区的问题。如果所有线路的零序功率方向都是正向，就判断母线接地。
Another main criterion for line selection is the difference in the direction of zero sequence capacitor current between the fault line and the non fault line. The zero sequence power direction of the non fault line is from the bus to the line, while the modified power direction of the fault line is from the line to the bus. By using the difference in zero sequence power direction, the fault line can also be identified, and there is no dead zone problem in the zero sequence power direction. If the zero sequence power direction of all lines is positive, it is judged that the bus is grounded.