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Another ground question

Discussion in 'Electrical Engineering' started by Brian Dugas, Mar 17, 2005.

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  1. Brian Dugas

    Brian Dugas Guest

    Ok, I've been doing some reading to help myself out on all this theory
    stuff. :) Most of the macro-level concepts are making sense. I am trying
    to learn just because I am curious by nature.

    Anyway, my question is in regards to the grounding wire. The
    grounding(green) wire of an appliance is usually connected to the case.
    This way, if the hot wire comes loose in a device and touches the case, the
    grounding wire carries the current back to the breaker panel where the
    grounding wire is bonded to the neutral, and a short circuit is allowed to
    trip the breaker. During the short time the grounding wire is energized, do
    the rest of the grounding wires in the house carry a current (or voltage)
    until the breaker trips? I mean, all the grounding wires are connected in
    the breaker panel right?

    My instincts tell me that the current will take the path of least
    resistance, which would be up the neutral wire back up to the transformer on
    the pole. But, let's say an earthquake hits and breaks the neutral wire
    coming into my house. At the same time an appliance falls off a shelf,
    breaks internally, and now the hot wire is touching the case. Well, the
    circuit can't complete back to the transformer, and from what I understand,
    a ground rod at the main service entrance is not gonna carry enough amps to
    trip a breaker. So are all the grounding wires now energized?
    This may not seem like a real practical hypothetical, but it illustrates my
    point a little better.
    Under this hypothetical, if all of the grounds are now carrying a voltage,
    then doesn't that mean any appliance I touch with a metal case will zap me?
    Thanks to anyone who wishes to enlighten me on this.
    Brian
     
  2. Guest

    Not necessarily. It depends on the voltage on the appliance
    you are touching with respect to ground, and the resistance
    through you to ground. The voltage on the appliance you are
    touching could range anywhere from 0 to 120 volts, and will
    be different on various appliances throughout your house,
    depending on where they are on the circuits in your house,
    and their resistance to ground via the equipment grounding
    conductor and the grounding electrode system, as well as
    the resistance to ground of the broken appliance that contains
    the short.

    Ed
     
  3. Rich256

    Rich256 Guest

    That will happen only if the is no earth ground at the meter. Assuming the
    returns are still grounded though the ground stake at the meter, the breaker
    will trip.
     
  4. Brian Dugas

    Brian Dugas Guest

    Are you saying that given my original scenario, the breaker will trip with
    no neutral connection to the transformer, and only the ground rod at the
    meter? I read somewhere that the ground rod wouldn't even conduct 1 amp.
    How would it conduct enough to allow a 15A breaker to trip? Guess if you
    had some really rich soil it would work.
     
  5. Beachcomber

    Beachcomber Guest

    If you have a good ground (low resistance), the dangerous voltages at
    the fault point will be near zero. The voltage drops distribute
    themselves by Kirchoff's Voltage Law which sums to zero and individual
    drops depend on the individual resistances in the circuit. The key
    resistance point must be set low enough to allow enough current to
    flow to trip the breaker or blow the fuse. In the USA for residences,
    the code says that this ground resistance must measure 25 ohms or
    less.

    If you have a high resistance or an open ground, you have a highly
    dangerous situation in that the case of your appliance will now be
    elevated to near or full line voltage (and possibly use you as a
    ground if indeed you are standing barefoot on a damp floor or touching
    a water pipe or faucet).

    A good ground system is best of all and much preferred to a system
    without a ground.

    A faulty or open ground is worse than no ground at all.

    Beachcomber
     
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