Superseded. This guide contains information and recommendations for applying audio tones over telephone channels for power system relaying. Included are sections on transmitting and receiving equipment, leased telephone channels, application principles, installation, and testing. Reflected in this work is the knowledge and experience of equipment manufacturers and telephone companies as well as that of power utility users. This guide is not intended to supplant specific or general instructions contained in the manufacturers' instruction books or in any contractual agreement between a manufacturer or telephone company(s) or both and a purchaser of a given relaying system. The illustrations in 5.7 are used for illustrative purposes only and do not represent the preferred protection under all conditions.