Fluxgate sensors measure magnetic fields by periodically saturating a piece of ferromagnetic core material in alternating directions. When an external magnetic field is present, the periodic saturation is offset and measured. Fluxgate performance is limited by intrinsic magnetic noise of the core as the core saturates.

A ground fault creates a magnetic field due to the imbalance of current through line and return current through neutral.

Magnetics are created by current going through a wire. When equal current flows in opposite directions, the sum of magnetic fields cancels out. To detect mA levels of fault current, a soft magnetic material that has a high permittivity and low coercivity is needed. The Hitachi core used for this design is FT-3K70T F2520C which is a nanocrystalline core. For questions on magnetic core selection and availability see Amorphous and Nanocrystalline at Hitachi Metals.