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Thread: 25 ohm or less resistance test with multiple ground rods

  1. #1
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    25 ohm or less resistance test with multiple ground rods

    I have a school board that is requiring a 25 ohms or less ground rod resistance test with a system that is already CAD welded together with (3) 5/8" X 10' ground rods tied together with 2/0 and bonded to a ufer, structural steel, and (2) meters. This is for a small 70A service that has a shadeport structure for solar. Eventhough the inspector has said I have complied with 250.56 they still require a ground test. When we tested the 2/0 bare that came off of the ground rod we got 1 ohm or less per ground rod. When they used their independent tester (handheld clamp on tester) they came up with anywhere from 250-350 ohms when they got below the cadweld and just tested the ground rods. I understand that the clamp tester is not the best in the case of tied together system but the school boards engineers have approved their greenlee clamp on tester and are needing us to comply with the 25 ohms or less for just the ground rod. I need to know if I added a second rod next to the other ground rod and tied them together with a #6 will I get the lower ohm ratting we are looking for? Any other suggestions or comments would be appreciated.


  2. #2
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    Quote Originally Posted by innersurffer View Post
    When we tested the 2/0 bare that came off of the ground rod we got 1 ohm or less per ground rod.
    I highly doubt that is accurate, how did you conduct this test?


    Quote Originally Posted by innersurffer View Post
    When they used their independent tester (handheld clamp on tester) they came up with anywhere from 250-350 ohms when they got below the cadweld and just tested the ground rods. I understand that the clamp tester is not the best in the case of tied together system but the school boards engineers have approved their greenlee clamp on tester and are needing us to comply with the 25 ohms or less for just the ground rod.
    Not the proper test equipment for this test, this has been discussed many times here, you can find by doing a search. Here is one of many threads. http://forums.mikeholt.com/showthrea...ighlight=clamp
    Last edited by zog; 02-08-13 at 11:36 AM.

  3. #3
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    If your system is setup as you stated then I don't know what the engineers are wanting. You are way beyond what is required and if you are tied to a ufer, etc you are probably getting as good as you can get.

    The question that I don't understand is why there is such a difference in the testing of the grounding electrode system. If you did a 3 point test correctly than why would a clamp on meter be so far off. Something does not seem right. Adding rods IMO is a waste of time and money at this point.

  4. #4
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    Detail picture of grounding detail

    I have a correction, the ground test that was initially done was with a clamp on ground meter on the 4/0 bond loop.
    (2) meters and (2) columns in the equation. Exampled below.
    Name:  ground detail.jpg
Views: 1996
Size:  62.5 KB

  5. #5
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    (3) 3/4" ground rods and 4/0 bare

    Correction on my submission. The ground rods are 3/4" and the tie together is with 4/0 bare copper.

  6. #6
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    on what basis are they requiring the test?

    does the spec require it?

    does the spec require you meet 25 Ohms or less? or does it just require the test?

    the other question i have is who at the school board has any authority to order such a thing? is this coming from the GC? the engineer? the school system itself?
    Bob

  7. #7
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    Volusia County schools has their own inspection department. They require all ground rods be tested for 25 ohms or less. I understand with my grounding detail this is way beyond normal measures. I'm just trying to get this completed.

  8. #8
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    Quote Originally Posted by innersurffer View Post
    Volusia County schools has their own inspection department. They require all ground rods be tested for 25 ohms or less. I understand with my grounding detail this is way beyond normal measures. I'm just trying to get this completed.
    If you read the details in the Fluke clamp-on tester description at http://support.fluke.com/find-sales/...15_ENG_A_W.PDF , you will see that this test is capable of measuring the resistance of individual ground electrodes, as long as the overall ground system has a low enough resistance.
    The tester automatically determines the ground
    loop resistance at this ground rod. If there is
    only one path to ground, like at many residential
    situations, the Stakeless method will not
    provide an acceptable value and the Fall-of-
    Potential test method must be used
    In your case I would guess that your 1 ohm measurement, if accurate, is being influenced mostly by the Ufer ground or even the utility ground if that was not disconnected for your test, and that the rods may well have a soil problem that is making them high resistance.
    If you agreed to do the job to their specification, you may need to use a chemical kit or other ground enhancer.
    But first I would make sure I understood how the Greenlee tester is being used by them to verify that their measurement is correct. They can also use the same method to measure the Ufer (steel bond) portion of the ground system for comparison.

  9. #9
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    Best advice yet. I guess I'll use the old epson salt trick.

    thanks

  10. #10
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    Quote Originally Posted by innersurffer View Post
    Volusia County schools has their own inspection department. They require all ground rods be tested for 25 ohms or less. I understand with my grounding detail this is way beyond normal measures. I'm just trying to get this completed.
    once again i ask is this in the spec? they can ask for anything they want. they can't make you do it if it is not in the spec.
    Bob

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