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Thread: Current on main ground

  1. #11
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    34,331
    Quote Originally Posted by rfazeli55 View Post
    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.
    Do you have 277 volt loads being supplied? And if so where is neutral bonded? If bonded in more then one place you will have stray neutral currents on every possible parallel path, including metal raceways, equipment enclosures, etc.

  2. #12
    Join Date
    May 2003
    Location
    Leesburg, VA
    Posts
    7,068
    You need to take current readings in multiple locations and if possible zero sequence readings on the main and/or all feeders. If you measure current with Zero Sequence on the main, you need to do some searching.


    And as noted above if indeed you have a large zero sequence on a GFPE protected switchboard you could have tripping of the GFPE protected distribution system. Additionally, verify you have no ground connection connections on the neutral in the switchboard downstream from the Neutral Ground disconnect link and GFPE Sensor (CT).
    Brian John
    Leesburg, VA

  3. #13
    Join Date
    Jan 2016
    Location
    PA
    Posts
    4,530
    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.
    you read 300 A on the ground (bonded to xfmr X0)?
    26 A steady state on only one side only?
    imo you have an issue

    here's some rules of thumb for charging current
    I would expect no more than 3-4 A total with 5 mva 1-2 A per xfmr
    https://www.megaresistors.com/produc...nce-to-ground/
    the numbers are from an old GE paper

    on a mining system with ngr 1 mva 480/3 we see <1 A
    these are long cables laid on wet earth
    8-10 ckts
    each with 125 mA gf protection
    Last edited by Ingenieur; 12-18-17 at 10:47 PM.



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