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IEEE C37.93 2004 Edition, March 25, 2004
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Guide for Power System Protective Relay Applications of Audio Tones Over Voice Grade Channels
Includes all amendments and changes through Reaffirmation Notice , September 30, 2010
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This guide contains information and recommendations for applying audio tones over voice grade channels for power system relaying, including transmitting and receiving equipment, leased voice grade channels, application principles, installation, and testing. Reflected in this guide 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 figures in 4.3 are used for illustrative purposes only and do not represent the preferred protection under all conditions.


The primary purpose of this document is to guide the power system user in applying, installing, and operating audio-tone protective relaying systems over voice grade 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.