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Thread: Current on main ground

  1. #1
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    Current on main ground

    Hi,

    I would like to know if you have 277/480V, 3 phase system at 2000A. What is the maximum current you can have on your main ground system? I know it should be zero (ideal case) but my understanding is all large buildings there will be few amps on the ground. I was unable to find this value anywhere and wanted to see if there is a rule of thumb or code that I am not aware of indicating the maximum? please let me know if you have any questions or if I am not explaining this properly.

    Thanks

  2. #2
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    There is no NEC mandated max, but more than a few would probably indicate a problem.

    Have you measured, and if so, what did you read?
    "Electricity is really just organized lightning." George Carlin


    Derek

  3. #3
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    Of interest would be the voltage drop from ground system to remote earth. The current is a possible indication of a compromised POCO or feeder neutral wire.

    Sent from my XT1585 using Tapatalk

  4. #4
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    Quote Originally Posted by GoldDigger View Post
    Of interest would be the voltage drop from ground system to remote earth. The current is a possible indication of a compromised POCO or feeder neutral wire.

    Sent from my XT1585 using Tapatalk
    Can you elaborate?

    Thanks

  5. #5
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    Schedule an outage and pull the neutral disconnect link and megger the downstream neutral to the service bonded neutral.
    Brian John
    Leesburg, VA

  6. #6
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    I should have added hard to say what a typical building is but in Washington DC 12 story office building we have measured 2-12 amps.

    If you have multiple services some of the current is due to multiple ground connections.
    Brian John
    Leesburg, VA

  7. #7
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    Quote Originally Posted by MechEdetour View Post
    Can you elaborate?

    Thanks
    There are two potential sources of current in the GES:

    1. Actual leakage, which would probably trip a ground fault breaker if one was present. This should generally be tracked down to faulty equipment and corrected.
    2. Current which is returning to the main panel (where the ground/neutral bond is located) but is for some reason returning to the POCO secondary neutral through the earth path instead of through the POCO wire neutral.
    This will always occur because the GES to earth to POCO ground path is in parallel with the wire neutral. There will be a current divider effect which is related to the ratio between the earth path resistance and the wire neutral resistance. Anything more than maybe 1% of the total neutral current would seem to me to suggest that the wire neutral resistance is higher than it should be. However if there is a water pipe ground electrode and a metallic water main system the "earth" path resistance may be of the same order of magnitude as the wire path resistance.

  8. #8
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    so the system on one switchboard rises to 304A on ground wire from Fluke meter readings and drops to 26A. I am trying to see if I should even worry about the 26A on ground? This is a 13 floor, million square feet office building. there is no IEEE or any document indicating a percentage that could be on ground? like harmonics have certain threshold percentage. Thanks in advance to all who respond.

  9. #9
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    Quote Originally Posted by rfazeli55 View Post
    so the system on one switchboard rises to 304A on ground wire from Fluke meter readings and drops to 26A. I am trying to see if I should even worry about the 26A on ground? This is a 13 floor, million square feet office building. there is no IEEE or any document indicating a percentage that could be on ground? like harmonics have certain threshold percentage. Thanks in advance to all who respond.
    Exactly what point of the system are you measuring? The grounded service conductor, a grounding electrode conductor, an equipment grounding conductor?

    Those first two will vary depending on load conditions and the resistance of the grounding electrode and can be "normal".

    The last one I mentioned is likely a result of improper neutral to ground bonds somewhere in the building.

  10. #10
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    Quote Originally Posted by kwired View Post
    Exactly what point of the system are you measuring? The grounded service conductor, a grounding electrode conductor, an equipment grounding conductor?

    Those first two will vary depending on load conditions and the resistance of the grounding electrode and can be "normal".

    The last one I mentioned is likely a result of improper neutral to ground bonds somewhere in the building.
    good point, so from local Utility 25kV switchboard it goes to 2 transformers that are 2400kVA to bring down 25kV to 480V. the secondary of each transformer goes to a main-tie-main switchboard rated at 2000A. we put a fluke meter on Phases A, B, C and Netural and ground bar of each side of the main switchboard and on one side we are reading those measurements. Please note that the tie breaker is open.

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