The paper reviews the salient features of a major co-operative research, development and site trial validation programme which was initiated in 1980 with the primary objective of accomplishing a major advance in power transmission system protection. Details of the project are given, together with the research/development methodology found necessary to transform a new approach to directional comparison protection into a next-generation product to meet stringent operational design objectives. The basic principles of the new scheme are outlined, together with details of the early computer-aided design studies which formed the basis of first prototype digital measuring equipments. Details are also given of the site-trial tests which were performed on the British 400 kV supergrid system, together with the results of investigations into the performance in long-distance power transmission applications. The paper concludes with brief details of preproduction prototype equipment and its performance.