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Standard home 240 - neutral to ground

Discussion in 'Electrical Engineering' started by Kaz, Nov 1, 2007.

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  1. Kaz

    Kaz Guest

    Im curious, in a standard US home 240 neutral split connection, when you
    break the circuit at the feed/meter box, you are simply removing both
    hots, right? So the neutral is still connected through. Same for when
    a breaker is tripped and you might do some wiring, neutral is still
    connected. So my question is, will there be any unsafe potential
    difference from neutral to ground, circuits energized or not?
     
  2. Guest

    | Im curious, in a standard US home 240 neutral split connection, when you
    | break the circuit at the feed/meter box, you are simply removing both
    | hots, right? So the neutral is still connected through. Same for when
    | a breaker is tripped and you might do some wiring, neutral is still
    | connected. So my question is, will there be any unsafe potential
    | difference from neutral to ground, circuits energized or not?

    When energized, there can be voltage on the neutral. The neutral of 120
    volt circuits will have a voltage about equal to the "voltage drop" of
    the wire from the point of test back to the main panel, plus any ground
    voltage differences. For a 4-wire 120/240 volt circuit, this will be the
    case for however much of the circuit is out of balance (different levels
    of current on the two hot wires).

    Whether energized or not, any voltage differences in the ground itself can
    be seen in the neutral as a difference between it and ground when at some
    distance from the panel where neutral is bonded to ground. This is rare,
    but can come from things like lightning strikes (as a surge).

    Another source of neutral voltage can occur when the "medium voltage"
    utility distribution system is operating improperly. If it's own neutral
    is broken somewhere, the return current on that neutral will seek ground.
    It will uses every ground path it can in parallel to reach back to the
    source, usually through a nearby pole with a grounding to get back to the
    neutral on the other side of the breakage.

    The problem with thie above is that the neutrals of both sides of the
    pole (or pad) transformers are connected together. That means that if
    it is operating that way where it is using a ground return, it will even
    use the ground return on the 120/240 volt side. Current will flow to
    your house via the neutral and reach ground via the ground bond in the
    panel. At some distance away from that point in ground, it is still
    possible to see voltage potentials between the neutral and ground as a
    result of this, especially if that point in ground is closer to where
    the return current can find a pole to get back onto the neutral on the
    other side of the break. If the ground wire on the transformer pole is
    also broken or has a bad electrode, these voltages can be substantial.
    These situations, however, are extremely rare. But they do happen at
    times.
     
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