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Thread: Checking 25 ohms ground rod

  1. #1
    Join Date
    Jun 2003
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    Checking 25 ohms ground rod

    What is the easiest and cheapest means of checking the resistance in a ground rod driven into the ground?

  2. #2
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    Re: Checking 25 ohms ground rod

    The easiest method is using a ground loop tester such as a AEMC 3730. But it requires the ground rod to be connected to a service with a utility grounded neutral. The other problem with the method is it always reads higher than the actual impedance because it reads the series resistance of the rod under test plus the utility ground.

    The other more accurate method is a megger three point test.

    I have also used just a 20-amp amp breaker feeding 120 volts to the rod, but I do not suggest you use this method.

  3. #3
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    Re: Checking 25 ohms ground rod

    One method is to connect a 5-amp fuse in series between a 120-volt "hot" conductor and the electrode itself. If the resistance is below 25-ohms, the 5-amp fuse will blow.

    Simple Ohm's Law: 120V/5A = 24-ohm.

    Obviously, special considerations for safety should be made before performing this type of test and should only be performed by a qualified person.
    Bryan P. Holland, MCP

  4. #4
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    Re: Checking 25 ohms ground rod

    Not that I would do any self promoting, BUT......... if this is a single project (one time test) hire a company that specilizes in electrical testing.
    Brian John
    Leesburg, VA

  5. #5
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    Re: Checking 25 ohms ground rod

    Originally posted by bphgravity:
    One method is to connect a 5-amp fuse in series between a 120-volt "hot" conductor and the electrode itself. If the resistance is below 25-ohms, the 5-amp fuse will blow.
    However, like the ground loop tester that Dereck mentioned, this method tests the resistance of the entire circuit, including the utility company's ground. If the fuse blows it does mean that your ground is less than 25 ohms, but if it doesn't blow, it doesn't necessarily mean your ground is over 25 ohms. It just means that the sum of the resistances of your ground, the utility company's ground, and the soil in between is 25 ohms or more.

  6. #6
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    Re: Checking 25 ohms ground rod

    After giving this some more thought the easiest and cheapest method is to drive two ground rods and walk away, your done.

  7. #7
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    Feb 2003
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    Bremerton, Washington
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    Re: Checking 25 ohms ground rod

    The technical term for this is "drive two and go home". For most installations, the resistance of the grounding electrode system is not that important, as it is for lighting, lighting doesn't really care about 25 ohms, it just wants a path to follow to ground.

    But some locations need a 5 ohm or even 1 ohm ground, such as cellular and radio sites.

    [ June 11, 2003, 12:12 AM: Message edited by: tom baker ]
    Moderator-Washington State
    Ancora Imparo

  8. #8
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    Apr 2003
    Location
    Westchester County, New York
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    Re: Checking 25 ohms ground rod

    I agree with Tom, unless the specs or situation requires a specific value, "drive two and go home". Remember these ground rods in most instances are SUPPLEMENTAL.

    Pierre

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