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Thread: PDU Grounding in DATA Center

  1. #1
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    PDU Grounding in DATA Center

    Hi this is my first post. Will try to be as short and accurate as possible. Am doing some work in a data center. The data center floor is a raised floor and has multiple Power Distribution Units (PDU's) on it feeding the computer equipment.The PDU's are made by a company (obviously) and each are fed by 2 different city utility feds coming in at 480V. The PDU's each have 2-75 KVA transformers-480V Delta primary and a 208/120V Y secondary. The static transfer switch, switches the power at the 480V level. Only one of the utility sources powers the PDU and both transformers at the same time. The directions from the manufacture show the X0 from both transformers tying together on a single ground bar and a single conductor going to the Grounding Electrode . I believe this is a single point ground which the instructions call for. This is how they are wired when installed around 2009. The Master Electrician on the job says this is wrong and we need to treat each transformer as a separately derived system. That we need to run a separate ground wire from each transformers XO to the Grounding Electrode. This seams to me to be incorrect and could cause problems and that we should stay with manufactures instructions that they were designed for, built for and UL approved. I don't know who is correct and which code section would apply here or if as some say it probably doesn't really matter either way. Hope this is clear and accurate and Thank You

  2. #2
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    Welcome to the forum.

    Look at 250.58. Your master electrician is correct that the transformers are SDSs, but that doesn't mean they can't share grounding electrodes.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  3. #3
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    Confirming: You describe PDUs with dual transformers. Each with (2) feeds. At any one time both are powered from the same source. They tie to a common output bus, which means parallel xfmrs?

  4. #4
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    The input side of the transformers are always connected to the same utility source by a common bus but the secondary side of the transformers are separate and fed there own bus and loads

  5. #5
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    Quote Originally Posted by Radty View Post
    Hi this is my first post. Will try to be as short and accurate as possible. Am doing some work in a data center. The data center floor is a raised floor and has multiple Power Distribution Units (PDU's) on it feeding the computer equipment.The PDU's are made by a company (obviously) and each are fed by 2 different city utility feds coming in at 480V. The PDU's each have 2-75 KVA transformers-480V Delta primary and a 208/120V Y secondary. The static transfer switch, switches the power at the 480V level. Only one of the utility sources powers the PDU and both transformers at the same time. The directions from the manufacture show the X0 from both transformers tying together on a single ground bar and a single conductor going to the Grounding Electrode . I believe this is a single point ground which the instructions call for. This is how they are wired when installed around 2009. The Master Electrician on the job says this is wrong and we need to treat each transformer as a separately derived system. That we need to run a separate ground wire from each transformers XO to the Grounding Electrode. This seams to me to be incorrect and could cause problems and that we should stay with manufactures instructions that they were designed for, built for and UL approved. I don't know who is correct and which code section would apply here or if as some say it probably doesn't really matter either way. Hope this is clear and accurate and Thank You
    I would ask him where in the NEC it says he can alter an Approved, Listed, piece of equipment.

    Point him to NEC 90.7. If he alters the equipment does he then have to pay to have it relisted again by a recognized third party testing laboratory?

  6. #6
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    Quote Originally Posted by Radty View Post
    which the instructions call for.
    And being that is the case send the Master Electrician to 110.3(B)

    Roger
    Moderator

  7. #7
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    Quote Originally Posted by Radty View Post
    The input side of the transformers are always connected to the same utility source by a common bus but the secondary side of the transformers are separate and fed there own bus and loads
    Is the neutral bus common between the two separate secondary busses?

  8. #8
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    Quote Originally Posted by Radty View Post
    The directions from the manufacture show the X0 from both transformers tying together
    on a single ground bar and a single conductor going to the Grounding Electrode.
    really the last thing you want is to create a ground loop in a data center.
    they are pretty neurotic about grounding in those types of facilities.

    what i'm visualizing here is a common ground bar inside the equipment, that
    you tie to your grounding system.

    why on earth (bad pun) would you want to separate a manufactured system?

    tell the master electrician you asked the internet, and the internet says nope.
    that should solve it.
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