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The primary purpose of this document is to guide the power system user in applying, installing, and operating audiotone protective relaying systems over telephone channels. Secondly, it is to provide a reference for equipment manufacturers engaged in the design and application of relaying equipment and for telephone personnel engaged in providing telecommunications channels for audio-tone protective relay schemes. The guide has been prepared not only for those considering audio-tone relaying for the first time, but as a reference for the experienced.


New IEEE Standard - Superseded. Superseded.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.

Document History

  1. IEEE C37.93-2004

    IEEE Guide for Power System Protective Relay Applications of Audio Tones Over Voice Grade Channels

    • Most Recent
  2. IEEE C37.93-1987


    IEEE Guide for Power System Protective Relay Applications of Audio Tones over Telephone Channels

    • Historical Version