It is pointed out that the magnetic material for cores of a differential current transformer (DCT) of a ground fault interrupter (GFI) should be characterized by high initial permeability, little dependence on temperature, and a low remanence. These parameters have been taken into consideration during selection of the most suitable annealing regime of Co-based (CoFeMnMo)/sub 77/(SiB)/sub 23/ material. It is assumed that a GFI is required to have a service life of 20-years continuous operation. Hence, the magnetic properties of the core of the DCT must be highly stable and exhibit virtually no deterioration over this period. During its operational lifetime, the core may be subjected to rapid saturation due to short circuits and earth faults, etc., as well as cyclic temperature changes. The effects of temperature aging and high current transients on the performance of the DCT were investigated. It is concluded that current shocks do not significantly alter the small-signal magnetic permeability of Co-based amorphous cores of selected annealing. The material examined exhibits degradation of magnetic properties due to aging greater than that reported for Fe-based materials. >