The application and performance of carrier-current relaying

THE GENERAL idea of pilot relaying is quite old. It has been employed for several decades in the Mertz Price system where pilot wires are employed for balancing the currents at the two ends of the line and has met with considerable success in Europe, and particularly in England. A simpler pilot scheme in which the pilot wire is used merely to conduct an impulse, and the conditions of the line at the two ends of the section are indicated by the relative positions of a series of power directional relays, has been employed and has been described elsewhere.¹ Neither of the above schemes, however, has ever met with any great favor in the United States on account of the prohibitive cost of pilot wire circuits, and particularly on long lines, and because in the long run the scheme itself is no more reliable than the pilot wire. Experience has shown that the reliability of the latter is not quite at the 100 per cent level desired and mandatory for proper service.