The Dish Network User's Resource
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Contrary to popular opinion, not all earth grounds are at the same potential. Either the electrical service ground, the telephone ground, or a connection to a cold water pipe may be adequate to shunt dangerous currents and voltages from nearby lightning strikes to earth, but there can be hundreds of ohms of earth resistance between the two ground points. Any current flowing between the two rods will generate voltage differences. These voltage differences are often percieved as hum bars on a TV or an audio buzz, and are caused by currents flowing through the shields of the cables. The purpose of a cable's shield is to prevent interference by screening the inner wire in a Faraday cage. If currents flow through the shield, they can actually induce interference. To avoid ground loops, use single point grounding practice where each ground from a piece of equipment connects to the actual earth ground at only one point.
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Page updated 11-23-11. Edited by BobaBird. Submissions & corrections welcome!