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The object of the paper is to describe early experience in designing and testing high-speed distance relays using junction transistors, and to expose the main difficulties in this field. The paper reviews briefly the history and application of electronic relays to electrical power-system protection and considers the general advantages of junction transistors over thermionic valves for this purpose. The principles of two distance relays using junction transistors are described, and it is shown that there is no theoretical difficulty in obtaining a range of relay characteristics within the general classification of ?distance relay?, together with the properties of a straightforward directional relay. The particular susceptibility of the ?mho?-type distance relay and of the directional relay to collapse of line voltage is then discussed. The circuits of the pulse type and direct phase comparison types of relay are described, together with the relay test arrangement and a brief exposition of the method of presenting the relay characteristics. The operating characteristics of the relays, as expressed by accuracy/range curves, are explained, it being shown that the direct phase comparison type is more successful than the pulse type but that both are extremely susceptible to transient overreach; the nature of this latter phenomenon is discussed. It is concluded that junction transistors have some important operational advantages over thermionic valves where electronic protection can be justified, but that, in particular, the application of junction transistors, in common with conventional electronic circuits, to high-speed distance relays is dependent upon a satisfactory solution of the transient overreach problem; two possible ways of overcoming this latter difficulty are advocated. The rapid development of transistors is bringing their rating, power and length of life into a situation where it is thought they will be of importance in the design of new protective-gear rel- ays and systems.