A description is given of the general arrangement of the overall feeder protection systems used by the CEGB. It is shown how this arrangement is only designed to accommodate protection relay failure when this failure leads to nonoperation under power-system fault conditions. The increasing importance of ensuring relay failures do not cause incorrect tripping is highlighted and existing methods of preventing this are discussed. A new approach to overcome this problem is described; that of computerised maintenance testing. A computerised protection test set specificaly developed for this purpose is described. Details are given of the hardware, software and mode of operation of the equipment, and test results produced by it are used to demonstrate the tests which the equipment can perform.