The paper deals with the automatic isolation or indication of earth faults on systems protected by Petersen coils. After discussing the principle of operation of this type of protection, describing the operation of a wattmeter-type relay, and enumerating the electrical constants of the system to which the protection is applied, the authors describe the application of the relay to a 66-kV distribution system.The currents in the feeders are analysed and the problem of accurate current summation is discussed in detail. Laboratory tests are given n current transformers at small percentages of rated current approximating to the capacitance currents. A comparison is made between current summation by means of a summation transformer and by direct paralleling. Further test figures are given showing the effect of load current in addition to capacitance currents. The design of the relay element is described, emphasis being laid on its constructional features and operating characteristics.