当被测系统发生单相接地且零序电压达到设定值（可在5V～100V之间任意选取，一般选为30V）时，装置将启动选线程序并采集被测线路的零序电流和故障母线电压，然后进行数字滤波以消除干扰。因为故障线路的零序电流为非故障线路零序电流之和，所以，理论上故障线路的零序电流采样值应为大，但由于CT误差、信号干扰以及线路长短差别悬殊等原因, 故障线路的零序电流采样值在排序时可能不在位，但不会超出前三位，因此在步排序时采用“相对幅值”原理，取前三位数值。
When single-phase grounding occurs in the tested system and the zero sequence voltage reaches the set value (it can be arbitrarily selected between 5V and 100V, generally 30V), the device will start the line selection program and collect the zero sequence current and fault bus voltage of the tested line, and then carry out digital filtering to eliminate interference. Because the zero sequence current of the fault line is the sum of the zero sequence current of the non fault line, theoretically, the sampling value of the zero sequence current of the fault line should be large, but because of CT error, signal interference and the great difference in the length of the line, the sampling value of the zero sequence current of the fault line may not be in place when arranging the sequence, but it will not exceed the first three bits, so the "relative amplitude" is used in the step sequencing. Principle, take the first three values.
然后利用“相对相位”概念，即各电流之间的方向或电流与电压之间超前与滞后关系，来进一步确定前三个之中的哪一个为故障线路，或者是母线故障。由于采用了“相对幅值”和“相对相位”的双重判据，因而克服了因系统运行方式变化、接地时电阻大小及线路长短对选线的影响，并且不需整定。
Then we use the concept of "relative phase", that is, the direction of each current or the lead and lag relationship between current and voltage, to further determine which of the first three is the fault line or bus fault. Because the double criteria of "relative amplitude" and "relative phase" are used, the influence of system operation mode change, grounding resistance and line length on line selection is overcome, and no adjustment is needed.