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    #31
    Re: Subpanel grounding

    Bob: Please note I am not declaring the earth alone will clear a fault. I am declaring it will facilitate the clearing of a fault.

    Please read all of section 250.2 Pay special attention to the examples in the FPN. Ground fault current "paths".

    There is also ground fault protection devices that will operate very effectively on almost all ground electrode paths through the earth.

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      #32
      Re: Subpanel grounding

      I firmly stand by my statements that ground rods, and the earth, are part of the ground fault clearing path. This applies to out buildings and parking lot lighting.

      A ground rod should be driven at each light pole, and connected to the pole and equipment ground.

      The math is basic Ohm's law of parallel resistance.

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        #33
        Re: Subpanel grounding

        bob,
        Just apply ohms law. A 25 ohm fault return path on a 120 volt circuit won't even trip a 15 amp breaker. A 120 volt circuit will only flow 4.8 amps through a 25 ohm load. A 480 volt circuit will only flow 19.2 amps through a 25 ohm load. Look what happens when the voltage is increased. If the voltage is 4.16kV, then 166 amps will flow and if the voltage is 13.8kV, 552 amps will flow. This is why a grounding electrode can play a big part in clearing utility distribution faults, but almost no part in clearing ground faults in building system voltages.
        don
        Don, Illinois
        (All code citations are 2017 unless otherwise noted)

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          #34
          Re: Subpanel grounding

          I agree with Don, the earth alone will not clear a fault on a thermal/magnetic breaker.

          There is residual and zero sequence ground fault devices that will see a low level fault and initiate a trip.

          Some earth paths are not too high of an impedance. This amount in parallel with a solid conductor will facilitate the clearing activity.

          Any amount of impedance in parallel is a positive.

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            #35
            Re: Subpanel grounding

            Sorry I had to have you keep hitting me over the head with this but I like some others have a hard time letting go of the long held belief that the ground rod played the major role of fault dispersal. Thanks All for clearing this up for me.
            Glad I found this sight, well look forward logging on in the future.
            Thanks again,
            Bob

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