The physical means by which electric energy is received from the electric utility is known as an electric interconnection. The primary elements include transmission or distribution circuits, transformers, and switching devices in the form of circuit breakers, fuses, and isolating disconnecting switches. The secondary elements include sensing devices to obtain information to operate the primary equipment intelligently, and relays to initiate removal of equipment from service automatically and quickly when an electric fault or disturbance occurs. It is important to remember that the physical laws of nature which govern the operating behavior of an electric sys...

The scope of this guide does not include the subject of lightning protection for electrical systems as provided by shielding or lightning arrester devices. Lightning is, of course, often the cause of short circuits on power systems, but only the clearing of these short circuits through the use of protective devices is within the scope of this guide.

This guide has been prepared to aid in the effective and uniform application of fuses, relays, and their associated switching equipment located at the point of interconnection between the utility and the consumer electrical systems. Descriptions of the various devices, definitions of terms, and references to other technical publications have been included to make the guide useful not only to relay engineers, but also to other technical people who may not be intimately familiar with the art of relaying, but who nevertheless have a responsibility for the installation and operation of such interconnections. It includes examples for relaying of typical installat...