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F.A.Q.#19.

What is a ground rod?...
   A ground rod is a metal shaft used for grounding. With plastic pipe now being used for water systems these rods are being used for services and other devices to be grounded. These rods are to be driven in the ground at least 8 ft. These rods when made of iron or steel shall be at least 5/8 inches thick. Non ferrous rods should be free of paint or any other non coductive material shoud be listed and not less than 1/2 inches thick.
  Most codes call for a ground system of 25 ohms or less. While, as a practical matter, you can't get to zero ohms, you certainly can get to 25 ohms if the ground rods are properly installed. A single electrode consisting of rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified above. Where multiple rod, pipe, or plate electrodes are installed to meet these requirements, they shall not be less than 6 feet apart.
 The resistance of this code ground is intended to be 25 Ohms or less, but is rarely tested. To verify the resistance of ground, it is most often tested with instruments using the fall-of-potential method by a trained technician.

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