A fault protection device incorporated in a transformer having a primary winding and at least two secondary windings connected to ground, and having a sensing coil to sense the fluxes generated by each of the secondary windings and generating a flux signal when an imbalance in the fluxes in the secondaries is detected and in which the sensing coil is in the form of at least two sensing coil portions, the sensing coil portions being positioned near but spaced from the sides of the secondary windings, the flux signal being constituted by a voltage induced in the sensing coil by fluxes produced by the secondary windings, and a switch connected to the sensing coil and connected in series with a power supply and at least one terminal of the primary winding, the switch being conditioned to an open condition in response to the flux signal to isolate the primary winding from the power supply.