Arcing horns are not to be confused with grading rings, which are ring shaped conductors surrounding the high potential end of an insulator string on some high voltage transmission lines, attached to the line. The purpose of grading rings is not to provide a spark gap but to even out the potential gradient across the string. The electric field across a string of insulators is not distributed evenly along the string but is concentrated at the ends, so with an overvoltage the end insulator units will break down first. By distributing the electric field more evenly, grading rings increase the breakdown voltage of the string. However, sometimes grading rings have a secondary usage as arcing terminals, combined with arcing horns on the ground side of the insulator.
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