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    5 ohms or less

    we are currently constructing a cell tower site. we are installing the ground ring and need to take ohms reading for close out package my requirements require me to get less than 5 ohms when testing, The master ground bar ties directly to building steel and the rest of the equipment ties to Master ground bar. I get 2-3 ohms at Building steel connection. i get 12-14 ohms at Master ground barconnection from Building steel, but i get 2-3 ohms from to master ground bar to each piece of equipment. do you guys have any experience with this type work? why would i get 12-14 ohms at master ground bar ground coming from building steel but get lower ohms reading at equipment and building steel connections. why wouldnt i get less than 5 ohms at master ground bar. what do you guys think im missing??

    #2
    How are you measuring the resistance?
    Moderator-Washington State
    Ancora Imparo

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      #3
      Additionally, what is the complete wording of the 5-ohm specification?
      [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

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        #4
        Originally posted by MBLES View Post
        i get 12-14 ohms at Master ground barconnection from Building steel, what do you guys think im missing??
        Ground resistances of very close places usually do not differ much. There may be contact resistance at Master ground bar connection from building steel. Disconnect and remake the connection after cleaning the contact surfaces.

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          #5
          Do you have a drawing you are working from? Could you provide that?
          Kirchoff and Ohm...the only laws that make sense

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            #6
            Mid-point

            From what you describe you are only reading half the circuit. If the start is building then ring then equip, you would read from building to equip and get a total the two readings. As said redo the building to ring connection

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              #7
              5Ω is a very common spec for the resistance of a grounding-electrode system when measured to remote-earth.

              When checking the quality of bonding the spec is often 0.5Ω.

              Be careful about what the specs actually call for and what test you are performing: The electrode system will need to be certified with a fall-of-potential apparatus.

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                #8
                Originally posted by big john View Post

                Be careful about what the specs actually call for and what test you are performing: The electrode system will need to be certified with a fall-of-potential apparatus.
                Which can be almost impossible with an extensive ground electrode system surrounded by other structures.
                The distance to the closest fall of potential electrode should be at least several times the scale distance of the GES.

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                  #9
                  Good primer on the subject http://www.weschler.com/_upload/site...owntoearth.pdf

                  as mentioned the far probe should be 5-7 times the largest dimension of the ges
                  the mid 62% of that distance
                  there is a table in the manual

                  A common issue is too close
                  the farther the lower the R
                  as d increases area A grows as the square of d
                  and since R = p d/A if A>d R gets smaller

                  a guy was measuring a mine bed (2 beds actually, station and ngr)
                  by law R<4
                  he was getting 7-8
                  added casings, etc, no change
                  I told him use the manual guidelines
                  got down to 2

                  keep a copy on my phone

                  Comment


                    #10
                    Originally posted by GoldDigger View Post
                    Which can be almost impossible with an extensive ground electrode system surrounded by other structures.
                    The distance to the closest fall of potential electrode should be at least several times the scale distance of the GES.
                    Alternative to fall of potential method and solution:
                    https://www.hioki.com/en/products/de...oduct_key=5627

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                      #11
                      Originally posted by Sahib View Post
                      Alternative to fall of potential method and solution:
                      https://www.hioki.com/en/products/de...oduct_key=5627
                      Junk

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                        #12
                        Originally posted by iwire View Post
                        Junk
                        Is it the product or the concept?

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                          #13
                          Originally posted by Sahib View Post
                          Is it the product or the concept?
                          It has the same application problems as a traditional fall-of-potential test: The readings will be negatively affected by anything in the zone-of-influence of the electrode.

                          The trick is that while there are steps you can take to verify your FOP test, there's nothing you cab do to double-check those clanps. Also, they are not a recognized test method per IEEE.

                          Comment


                            #14
                            The tester (also available from Fluke and other makers) can do a reasonable job of measuring the earth resistance of a single electrode as long as you understand its limitations.
                            But you cannot combine the single electrode measurements to get a complete GES value if the electrodes are within each other's sphere of influence.

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                              #15
                              This is geared towards linemen.

                              My emphasis:

                              "Note: For multi grounded systems only. In a multi-grounded system, the larger the number of grounding poles, the more accurate the measured value."

                              If the grounding rods are installed in another's (of the same GES) sphere of influence then they aren't really installed "correctly" anyway. Simple single-point/one-step test equipment wouldn't work as well.

                              Of course, the NEC leaves this up to the electrician:

                              250.53(A)(3) Informational Note: The paralleling efficiency of rods is increased by spacing them twice the length of the longest rod.
                              Kirchoff and Ohm...the only laws that make sense

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