剩余电流监测原理是在低压配电TN-S系统无故障的正常回路中的la+Ib+Ic+In=0，在互感器磁芯内没有磁通，线圈内的电动势为零。当出现接地故 障时，故障电流穿过磁芯流向故障点，但却经大地或经TN系统的保护线返回变压器中性点。使得穿过磁芯的线路中的电流因此不再平衡 Ia+Ib+Ic+In≠0，电流差值在磁芯内产生了磁通，导致线圈内产生出感应电动势。
The residual current monitoring principle is la+Ib+Ic+In=0 in the normal circuit of the low-voltage distribution TN-S system. There is no flux in the core of the transformer, and the electromotive force in the coil is zero. When a grounding fault occurs, the fault current flows through the core to the fault point, but returns to the neutral point of the transformer through the earth or the protection line of the TN system. The current through the core line so no longer balance Ia+Ib+Ic+In = 0, the difference between current magnetic flux in the core, resulting in the coil produces induced electromotive force.