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Fault current path for High Resitance Ground

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    Fault current path for High Resitance Ground

    I had a bad VFD which created a ground fault that was picked up by a high resistance ground located in the room. The HRG knocked the fault current down to 8 Amps which prevented any OCPD from opening which is all good. I understand that electricity seeks its source which is the switch board feeding the VFD. So my question is how does the fault current find it’s way to the HRG? There is a TVSS located in the switchboard. Does a TVSS shunt fault current in a way that it would migrate to the HRG. All the equipment in the switchgear room is bonded by a common ground 4/0. I am trying to understand how this HRG which is physical located 10’ from the switch board was able to pick up the fault current. I don’t understand how the HRG operates?

    #2
    Simple concept of HRG is to place a resistor in series with grounded conductor and the neutral point of the source. Grounding elecgrode system is on the "load side" of this resistor the neutral of the transformer isn't grounded.

    Any time a ground fault occurs there is voltage across the resistor, the fault monitoring equipment kicks in when it senses voltage across the resistor. Monitoring equipment may be just a light to indicate a ground fault, or may be connected to audible alarm or other items to notify someone there is a fault condition.

    By limiting the fault current no overcurrent devices immediately open. This is most useful for operations where a sudden unexpected shut down on conventional grounded systems may introduce more problems or even hazardous conditions, and allows user to acknowledge there is a problem and shut things down in an orderly fashion. Disregarding that fault however, can still result in sudden shutdown if another fault should occur on another phase, because then you essentially have a line to line fault, as all the grounded objects are at same potential as the first faulted phase right before the fault occurs.
    I live for today, I'm just a day behind.

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      #3
      This is the key:
      Originally posted by mjc1060 View Post
      I understand that electricity seeks its source which is the switch board feeding the VFD.
      This is correct, though I shudder to think how many people still think electricity is seeking a path to ground. That said, however, electricity will not hesitate to use planet Earth as a conductive medium, if by so doing it can find its way back to its source. The current path is, switchboard to VFD to fault point to dirt (with perhaps some intervening points before it gets to the dirt), then through dirt to the grounding electrode system, and then up the HRG back to the switchboard.

      Charles E. Beck, P.E., Seattle
      Comments based on 2017 NEC unless otherwise noted.

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        #4
        [COLOR=#333333] Originally Posted by mjc1060 [/COLOR]
        [COLOR=#333333]I understand that electricity seeks its source which is the switch board feeding the VFD.[/COLOR]
        Charlie is right except for one thing that needs addressed yet.

        Switchboard is not the source. It is just a part of the conductors between the source and whatever point on the circuit one may be addressing.

        Source is the transformer ahead of the switchboard either a separately derived system or the POCO's transformer.

        Source could be a generator or other on site power production unit also.
        I live for today, I'm just a day behind.

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          #5
          Originally posted by charlie b View Post
          This is the key: This is correct, though I shudder to think how many people still think electricity is seeking a path to ground. That said, however, electricity will not hesitate to use planet Earth as a conductive medium, if by so doing it can find its way back to its source. The current path is, switchboard to VFD to fault point to dirt (with perhaps some intervening points before it gets to the dirt), then through dirt to the grounding electrode system, and then up the HRG back to the switchboard.

          Effectively the resistor is connected between the grounding system and the neutral point of the transformer. The VFD fault could have traveled over a green grounding conductor or even the conduit system without ever touching dirt. But, regardless of the path it takes, it must go through the HRG to get back to its source.
          Just because you can, doesn't mean you should.

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            #6
            Originally posted by jim dungar View Post
            Effectively the resistor is connected between the grounding system and the neutral point of the transformer. The VFD fault could have traveled over a green grounding conductor or even the conduit system without ever touching dirt. But, regardless of the path it takes, it must go through the HRG to get back to its source.
            Even if current traveled through dirt as part of the path, there can be no grounding between the resistor and the source neutral point or even at the source neutral, or you may not get desired performance out of it. You want any stray current to have to find a return path to the resistor and not go directly to the source for best performance of this sort of a system
            I live for today, I'm just a day behind.

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              #7
              Thank you

              My mistake was not looking past the switchboard to the actual source which was the transformer. Thank you for the explanation I understand now!

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