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January 12, 2007 A Prism Business Media Publication Vol. V No. 1

250.56 Resistance of Ground Rod Electrode

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Code Q&A

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    Top 50 NEC Rules

    250.56 Resistance of Ground Rod Electrode

    By Mike Holt
    When the resistance of a single ground rod is more than 25 ohms, an additional electrode is required to augment the first ground rod electrode, and it must be installed not less than 6 feet away. However, no more than two ground rods are required -- even if the total resistance of the two parallel ground rods exceeds 25 ohms.

    You can measure the resistance of a grounding electrode with a ground resistance clamp meter or a 3-point fall of potential ground resistance tester. Let's review the concepts behind each test method.

    • Ground clamp meter. The ground resistance clamp meter measures the resistance of the grounding (earthing) system by injecting a high-frequency signal via the grounded neutral conductor to the utility ground, and then measuring the strength of the return signal through the earth to the grounding electrode being measured.
    • Fall of potential ground resistance meter. The 3-point fall of potential ground resistance meter determines the ground resistance by using Ohm's Law: R = E / I. This meter divides the voltage difference between the electrode to be measured and a driven potential test stake (P) by the current flowing between the electrode to be measured and a driven current stake (C). The test stakes are typically made of 1/4-inch diameter steel rods, 24 inches long, driven two-thirds of their length into the earth.

      The distance and alignment between the potential and current test stakes, and the electrode, is extremely important to the validity of the ground resistance measurements. For an 8-foot ground rod, the accepted practice is to space the current test stake (C) 80 feet from the electrode to be measured.

      The potential test stake (P) is positioned in a straight line between the electrode to be measured and the current test stake (C). The potential test stake should be located at approximately 62% of the distance that the current test stake (C) is located from the electrode. Since the current test stake (C) is located 80 feet from the grounding (earthing) electrode, the potential test stake (P) will be about 50 feet from the electrode to be measured.

      The 3-point fall of potential meter can only be used to measure one electrode at a time. Two electrodes bonded together cannot be measured until they have been separated. The total resistance for two separate electrodes is calculated as if they were two resistors in parallel. For example, if the ground resistance of each electrode were 50 ohms, the total resistance of the two electrodes bonded together is about 25 ohms.
    If the electrode to be measured is connected to the electric utility ground via the grounded neutral service conductor, the ohmmeter will give an erroneous reading. To measure the ground resistance of electrodes that aren't isolated from the electric utility (such as at industrial facilities, commercial buildings, cell phone sites, broadcast antennas, data centers, and telephone central offices), a clamp-on ground resistance tester would better serve the purpose.

    The resistance of the grounding electrode can be lowered by bonding multiple grounding (earthing) electrodes that are properly spaced apart or by chemically treating the earth around the grounding (earthing) electrode. There are many readily available commercial products for this purpose.

    One final note on soil resistivity. The earth's ground resistance is directly impacted by the soil's resistivity, which varies throughout the world. Soil resistivity is influenced by the soil's electrolytes, which consist of moisture, minerals, and dissolved salts. Because soil resistivity changes with moisture content, the resistance of any grounding (earthing) system will vary with the seasons of the year. Since moisture becomes more stable at greater distances below the surface of the earth, grounding (earthing) systems appear to be more effective if the grounding electrode can reach the water table. In addition, having the grounding electrode below the frost line helps to ensure less deviation in the system's resistance year round.

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    Code Challenge
    What's Wrong Here?
    By Joe Tedesco
    Think you know how this installation violates the NEC?

    Visit EC&M's Web site to see the answer.

    Hint: Bad bend or lack of protection?

    Code Q&A
    By Mike Holt
    Q. What are the NEC requirements for using electrical metallic tubing (EMT) to physically protect NM cable?

    Visit EC&M's Web site to see the answer.

    Code Quiz
    By Steven Owen
    Q. When sizing an equipment grounding conductor for a solar photovoltaic system, which also has ground-fault protection for equipment, what is the minimum size equipment grounding conductor required for a circuit that is protected by a circuit breaker rated at 30A?
    A) The ground-fault protection equipment is all that is required. An equipment grounding conductor is not required.
    B) The minimum size of the equipment grounding conductor shall be based on 125% of the photovoltaic-originated short-circuit currents in that circuit.
    C) A 10 AWG conductor, as noted in Table 250.122.
    D) An 8 AWG conductor, as noted in Table 250.66.

    Visit EC&M's Web site for the answer and explanation.

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    Shows and Events
    Get In-Depth Information on Code Changes & Standards
    Are you a residential, industrial, commercial, or institutional electrical contractor? Do you specify equipment or review construction plans? Do you maintain electrical systems in a facility? If you answered "yes" to any of these questions, then you have to make plans to attend the Electric West conference program in Long Beach, Calif. Attend one of 12 fact-filled seminars -- three of which offer CEUs. Or make plans to attend one of two update sessions on the 2005 NEC, led by top trainer Mike Holt. These Code sessions will also offer a sneak preview of proposed changes to the 2008 NEC. Check out this event's 40+ seminars and make plans to meet 200+ leading suppliers. Or register now.

    Attend Free Live Conferences
    at the EC&M e-Tradeshow

    Don't miss these informative live conferences scheduled for January 23, 2007:

    • 9:00 a.m. (Eastern and Pacific): Distributed Generation: Products, Trends, Benefits, presented by John DeDad, senior director, Editorial and EC&M Development. Learn about the various small-scale power generation technologies currently available, such as reciprocating engines, micro-turbines, combustion gas turbines, fuel cells, photovoltaics, and wind turbines. See respective technology applications such as customer generation, co-generation, peak shaving, and selling power to the grid.
    • 10:00 a.m. (Eastern and Pacific):
      Arc Flash Calculations and Distribution Analysis"
      , presented by Bob Moore, president, Z Meters. Learn about the beginnings of arc flash consideration, stemming from the basic theory promoted by Ralph Lee. Find out how impedance affects the amount of energy released from an arc flash, and how calculations and measurement tools can help you perform your own system analysis.
    Also, enter the "Become an Electrical Inspector Contest" for a chance to win a $100 American Express gift certificate by proving your knowledge of the National Electrical Code and citing every Code violation appearing in an actual electrical installation.

    Visit for information on accessing the EC&M e-Tradeshow, a virtual online exhibition and live conference center.

    And take a look at the archive of various past conferences, such as Claim Litigation and Harmonics: Causes, Symptoms, Remediation Techniques.

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