Electric, power systems, having outgrown simplicity a long time ago, continue to require more and better protection as the demand for power increases and the need for reliability becomes greater. Protective relaying has met this challenge by providing more sensitive, higher speed relay systems that are inherently reliable. Among the more significant protective developments that furthereed the economic design of power systems are pilot, relaying schemes and direct, transfer tripping over telephone channels.
In recent years a growing need developed for more versatile wire-pilot relay systems and more secure means for direct transfer tripping. These factors, in addition to technological changes in telephone communication plants affecting availability of metallic, telephone circuits, led to the consideration of audio-tone signals over telephone channels for these functions. Since audio tones had already been employed extensively for less demanding applications—telemetering, telegraphy, dial signaling, etc.—it appeared logical that audio tones could also be employed successfully for remote tripping of circuit breakers and wire-pilot relaying.