The paper describes a new permanent-magnet moving-coil relay which overcomes the non-linearities associated with electromagnetic differential relays.Its application to balanced-current protective systems is described, and the factors affecting the use and interpretation of through-current bias are discussed.Other applications of the relay to impedance, pilot-wire and busbar protective systems are described as well as a new scheme of instantaneous differential protection for power transformers.The appendices indicate that the performance of protective gear has been reduced to a calculable basis.