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Thread: Ungrounded Delta 480V/3-Phase/3-Wire System

  1. #11
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    I volt that electrofelon and gar have it.

    There is no such thing as an ungrounded system. What we call ungrounded, if there are no faults, is really _capacitively_ grounded through all of the unintentional capacitors formed between the circuit conductors and ground.

    A wire in a conduit is a capacitor. The magnet wires in a motor and the steel of the motor? Another capacitor.

    The (very, very small) leakage through the insulation and the air around the conductors? Resistors.

    An 'ungrounded' system in perfect condition will have very small _wye_ currents to ground, through the above 'parasitic' components. If the system is in perfect condition, and you measure with a high impedance meter, then I'd expect voltages quite similar to those from a wye system.

    If you measure with a low impedance meter, or connect a load from phase to ground and measure voltage across that load, then the voltage to ground will be much lower than 277V.

    If there are faults or intentional wye resistances (for example old school ground fault detection would be with a set of lamps connected wye to ground) then you might need a very low impedance meter to get a low voltage measurement.

    -Jon

  2. #12
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    Quote Originally Posted by winnie View Post

    There is no such thing as an ungrounded system. What we call ungrounded, if there are no faults, is really _capacitively_ grounded through all of the unintentional capacitors formed between the circuit conductors and ground.
    Since an ideal capacitor acts as an open circuit when subject to DC, does what you say still apply to what we call "ungrounded DC systems"?

  3. #13
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    Quote Originally Posted by winnie View Post
    I volt that electrofelon and gar have it.

    -Jon
    Didn't I say that in the first reply?


    Quote Originally Posted by peter d View Post

    It could be that the reading was simply a capacitive measurement and they rounded it to 277 volts because that is a nominal voltage.

  4. #14
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    Quote Originally Posted by peter d View Post
    Didn't I say that in the first reply?
    That too

  5. #15
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    Quote Originally Posted by winnie View Post
    That too
    Not nearly as detailed and in depth as yours, of course.

  6. #16
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    Quote Originally Posted by Carultch View Post
    Since an ideal capacitor acts as an open circuit when subject to DC, does what you say still apply to what we call "ungrounded DC systems"?
    Not nearly as well.

    You still have the resistive leakage through the insulation, but this is probably small with modern insulation materials.

    -Jon

  7. #17
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    Didn't older ungrounded systems use lamps for ground fault detection? Wouldn't these be the resistive element to ground?
    Tom
    TBLO

  8. #18
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    Quote Originally Posted by ptonsparky View Post
    Didn't older ungrounded systems use lamps for ground fault detection?
    Yes.

    Wouldn't these be the resistive element to ground?
    Yes.


  9. #19
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    I would say that his system is fault free and working as it should then.
    Tom
    TBLO

  10. #20
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    Quote Originally Posted by ptonsparky View Post
    I would say that his system is fault free and working as it should then.
    I do not think every ungrounded system had these, I think in the past they were optional.

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