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Thread: Objectionable Ground Current

  1. #1
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    Objectionable Ground Current

    Article 250.6 covers objectionable ground currents on grounding conductors, but nowhere in the NEC can I find what level of current is considered objectionable.

    Any ideas?
    James Wilson PE

  2. #2
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    Re: Objectionable Ground Current

    IMO, there should be zero current on the grounding conductors unless you happen to be measuring during a ground fault event. So my answer would be anything over zero Amps is considered objectionable.

  3. #3
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    Re: Objectionable Ground Current

    There will always be leakage current on the ground. Objectional is up for interpretation. Objectional currents in dairy barns are a few mA. Circulating currents in parallel ground paths may be objectional only when over several Amps. The code leaves this up for interpretation.
    e^(i pi) = -1

  4. #4
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    Re: Objectionable Ground Current

    A process that is purely electro-mechanical will be quite tolerant of current on the grounding conductors, as a general rule.

    A microprocessor controlled electro-mechanical process, depending upon its sensitivity, will have problems created by currents on the grounding conductors.

    Dairy cattle will go off milk production from small voltages caused by current on the grounding conductors.

    I submit the level of current that is objectionable is directly related to whether the current has effects that produce objectionable results.
    Another Al in Minnesota

  5. #5
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    Re: Objectionable Ground Current

    David,
    I think if you were to use a clamp-on ammeter on any grounding electrode conductor from a service n-g bond, you will find current on most.
    Although the earth is not a great conductor, it still will see its portion of the current divider circuit offered by the GEC path and the neutral path back to the utility.
    Ron

  6. #6
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    Re: Objectionable Ground Current

    Mr. Wilson, there are no NEC limits. Some industries, like telecom, set limits on the various sub-systems. As Ron Mentioned the GEC will always have current flowing in proportion to the GES impedance. Not much you can do about that except to oversize the grounded conductor, or use isolation transformers.

    If you are talking about the feeder grounds and EGC systems, then there are some limits you can set. However in large distribution systems you are going to have capacitive leakage current through cable insulation in any unbalanced system referenced to earth, just the nature of the beast.

    There are other leakage points that are caused by the equipment, like L-N installed RFI/EMI filters, MOV's for surge protection, and lighting ballast.

    SO IMO it is objectionable if you do not want it there or if it should cause problems by data processing equipment and other communications equipment that use ground as a signal reference or path. But then again I like common mode current on grounds, I have made a fortune finding it and eliminating it.

  7. #7
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    Re: Objectionable Ground Current

    Don't forget 250.6(D) Limitations to Permissible Alterations. The provisions of this section shall not be considered as permitting electronic equipment from being operated on ac systems or branch circuits that are not grounded as required by this article. Currents that introduce noise or data errors in electronic equipment shall not be considered the objectionable currents addressed in this section.
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  8. #8
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    Re: Objectionable Ground Current

    I happen to have a clamp-on true RMS ammeter that is rated for use with variable frequency drives. One thing that I have discovered is that electromagnetic fluorescent ballast ( about 30% harmonic distortion in the current drawn from the line ) send high frequency currents back through the equipment ground in the amount of about 10% of the hot lead current. The neutral lead current is about 90% of the hot lead current.

    Therefore, there is a very good case besides the occasional broken conduit joint for using redundant grounding conductors in all metal conduits like the British do and not just in hospitals and explosionproof areas. On 22Y380 volts you most definitely need a redundant EGC and I have opened up a walls and recconected a conduit joint enough times ( once ) to know why the British require redundant grounds in all occupancies that are using metal conduits.

  9. #9
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    Re: Objectionable Ground Current

    Let me add that for current to be “objectionable,” it has to be driven by a voltage source that is capable of sustaining that current if a human touches the metal. Here is what I mean: Magnetic fields exist in the air all around us. They can induce a small current in any “loop” of conductive material. If you place a clamp-on ammeter around a (let us say empty) conduit, you might read the current. Also, if you were to measure voltage, the amount you would measure would be equal to the resistance of the conduit times the amps flowing along the conduit. But if you touch the conduit, you will not feel the current. That is because the source is not strong enough to maintain that current, and to push a share of that current through your body. You might envision the source as having a high value of internal resistance, so that as the current drawn (by whatever is drawing current) increases, the amount of voltage available to continue pushing current drops off rapidly.

    The point is that the notion of “objectionable current,” as discussed in the NEC, begins with some type of installation problems or fault conditions within an electrical distribution system.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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