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Thread: grounding

  1. #1
    Join Date
    Mar 2003
    Location
    Pennsylvania
    Posts
    2

    grounding

    I am an industrial electrician for a corrugating plant in Pennsylvania. We have a delta three phase, three wire system. I was wondering as to why manufacturers want a grounding rod driven in the ground beside their equipment and tied to the grounding of the service. One piece of equipment was an air compressor with a small onboard computer. At first I thought how is this grounding rod going to protect people when it is approximately 60 yards from the three phase service grounding rod? I also thought as this is to help in lowering high transients affecting the controller. There has been a couple more grounding rods driven into the ground by other equipment that doesn't have computer controls. Can you please let me know why these manufacturers want this and for what specific reason.

    Thank you,
    Scott

  2. #2
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    8,808

    Re: grounding

    The only reason a manufacturer would ask for a seperate ground rod at their equipment is: Ignorance of proper grounding pratices. At least they asked for it to be bonded to the building grounding system/grid.

    Every service should have a single grounding system (ideally a ground grid in non-residential buildings). It make take multiple ground "rods" to create the system but that should be part of the design not simply because a machine manufacturer wants one.
    Just because you can, doesn't mean you should.

  3. #3
    Join Date
    Feb 2003
    Location
    Plano, TX
    Posts
    3,773

    Re: grounding

    I have to agree with Jim here, there is not a real good reason other than lowering the high frequency impedance to earth. The controllers do not need a low HF impedance to earth, they need low impedance to what they are controlling and the power source. From a safety performance point of view, there is little to be gained.

    By stating it has to be bonded back to system ground at least makes it compliant. In the long run it does not do any harm either.

  4. #4
    Join Date
    Feb 2003
    Posts
    3,172

    Re: grounding

    This is the same situation I have been squacking about, concerning a transformer being called a separately derived system.

    Transient current, plus load current, will appear on the equipment ground.

    The building steel is probably not bonded, it is likely fed. This makes two ground points.

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