If the transformer is loaded with a voltage in excess of the nominal voltage, saturation effects occur. Without stabilization, these could lead to differential protection tripping. The fact that the current of the protected object under saturation conditions has a high proportion of harmonics having five times the system frequency serves as the basis of stabilization.

The P632 filters the differential current. The fundamental wave and fifth harmonic components of the differential current are determined. If the ratio exceeds the set value DIFF: Ov. I(5f0)/I(f0) PSx in at least one measuring system, and if the differential current is smaller than 4·Iref, then tripping is blocked selectively for one measuring system.

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