在三相四线电路中，三相电流的相量和等于零，即Ia+Ib+IC=0 如果在三相四线中接入一个电流互感器，这时感应电流为零。当电路中发生触电或漏电故障时，回路中有漏电电流流过，这时穿过互感器的三相电流相量和不等零，其相量和为：Ia+Ib+Ic=I(漏电电流）这样互感器二次线圈中就有一个感应电压，此电压加于检测部分的电子放大电路，与保护区装置预定动作电流值相比较，如大于动作电流，即使灵敏继电器动作，作用于执行元件掉闸。这里所接的互感器称为零序电流互感器，三相电流的相量和不等于零，所产生的电流即为零序电流。
In three-phase four wire circuit, the phasors of three phase currents are equal to zero. That is to say, if Ia+Ib+IC=0 is connected to a current transformer in three phases and four lines, the induced current is zero. When the electric shock or leakage fault in the circuit, a leakage current flowing through the loop, then through the three-phase current transformer phase and the phase ranging from zero, and the amount of: Ia+Ib+Ic=I (current transformer) so that the two secondary coil has an induced voltage, the voltage detection part of the electronic circuit, and reserve device compared predetermined action current values, such as greater than the current action, even if the sensitive relay action, acting on the actuator switch off. The mutual inductor attached here is called zero sequence current transformer. The phase amount of the three-phase current is not equal to zero, and the current produced is zero sequence current.