Development of high permeability cores for earth leakage protection devices

Nickel-iron differential transformer cores are used in residual current devices (RCDs) for the prevention of electric shock caused by a leakage of current to earth through the human body. Essential to the effectiveness of an RCD is the magnetic performance of this nickel-iron core. The Teaching Company Scheme programme at Telcon Ltd., in collaboration with the University of Brighton, UK, has been concerned with improving core performance by modifying the intrinsic magnetic properties of the alloy and improving the core fabrication and heat-treatment processes required to realise these properties. This paper describes techniques that have been developed through applied research in order to achieve these aims.