Differential transformer cores for ground fault interrupting circuit breakers

The magnetic requirements of differential transformers used in ground fault interrupters (GFI) for the sensing of 5 mA ground faults over the temperature range from -35°C to +66°C have been established. Ring laminations made of vacuum-melted permalloy and annealed so that the magnetocrystalline anisotropy
K_{1} \sim 0
just above the maximum operating temperature meet these requirements. Such heat treatment will produce rings with low remanence in which the low-flux-density permeability is stable with temperature and changes little from the demagnetized to the remanent state. The processing of rings and the influence of impurities are briefly discussed.