When an unloaded transformer is connected, the inrush current at unfavorable switching instants such as for voltage zero, may have values that exceed the transformer nominal current several times over. It takes some time for the current to assume its small stationary value. Since the high inrush current flows on the connected side only, the tripping characteristic of the P632 differential protection may give rise to a trip unless stabilizing action is taken. The fact that the inrush current has a high proportion of harmonics having twice the system frequency offers a possibility of stabilization against tripping by the inrush current.

The P632 filters the differential current. The fundamental wave and second harmonic components of the differential current are determined. If the ratio exceeds a specific adjustable value in at least one measuring system, tripping is blocked optionally in one of the following modes:

There will be no blocking if the differential current exceeds the set threshold DIFF: Idiff>> PSx.