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  1. #27
    Join Date
    Oct 2001
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    I was reading your post few times to make sure that I would not miss any part of it. I still don’t see how your theory can be correct.
    Do you have a link to article or document talking about this issue?
    I, specially don’t get part about” fault” traveling from source to transformer and back.
    Circuit breaker reacts to over current flowing through it’s bi-metal element. It does not care if this current travels through neutral or grounding conductor.
    Breakers are electro-mechanical devices. They are not fail proof.
    I have seen 20A breaker been tripped by 15A current.
    I also have seen 20A breaker holding 60A for quite few seconds before going off.
    In this example where 200A main tripped before 50A, it is possible that 50A breaker was faulty. We have to remember that 200A panel was carrying other loads from the house so it needed much less then 200A to trip.

  2. #28
    Join Date
    Apr 2003
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    I can chime in a little regarding why 20amp Circuit Breaker would not trip at the end of the circuit, but the 200amp main might.

    They key is the "Inductance" of the wire.

    You have the main feed coming to the house which terminates at the main disconnect (200amp). From here, there could be a loooooong wire to the 20amp CB as disconnect and to the equipment.

    You short out the wire at the equipment. It will cause an abrupt spike in current which travels though the wire. Longer the wire, larger the inductance. When inductance is involved, and when abrupt change in current ocurrs, the inductance generates a current in opposit direction (ie. dead short) This is called "Faraday's Law" in electrical theory text books. Farther from the change, larger the inductance, larger the opposit current.

    Thus, the main disconnect might see a larger current than the near by disconnect.

    HOWEVER, in relative short distance of the wire, it should really not make that much difference.

    Another possiblity is the reaction time of the circuit breaker. There are minimum spec for the reaction time, but there is nothing regulating how fast they *could* react.

    I think it is more of a possibility.

  3. #29
    Join Date
    Jul 2003
    Charleston, SC, USA
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    I only slightly understand

    I only slightly understand this vlotage back up, circuit trip up line theory.
    But I thought I'd give some clarifiing info.
    New house 4 years ago.
    There are two panels in the house side by side 100A each
    Breaker was tested and is working fine
    wire is 3-8 w G
    10 Kw in AH
    40 ft panel to disconnect.
    Be good to your fellow man, not nice. There is a difference.

  4. #30
    Join Date
    Oct 2001
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    Re: I only slightly understand

    Originally posted by answer
    There are two panels in the house side by side 100A each
    Breaker was tested and is working fine

    There is a big difference between 100A and 200A main breaker
    Which breaker was tested and how?

  5. #31
    Join Date
    Jul 2001
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    My .02 cents

    All of this goes back to available fault current, protection hiarchy "?spelling", and impedance. It is not as simple as breaker ratings. I am not even going to begin to try and explain all of this. "I can't" but I have been through a lot of training at various times and this can and will happen in the right conditions. Breakers, OL, and fuses all have different ratings. Fault current inturpting ratings and OL ratings and response times are not all the same.

    For what it's worth. DaleP

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