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Thread: Sparks; Breaker Didn't Trip

  1. #1
    DIY Senior Member molo's Avatar
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    Default Sparks; Breaker Didn't Trip

    The hot wire got nicked on a metal junction box. Sparks flew and the wire was split in two. However, the breaker didn't trip.

    Also, there is one wire of 12-2 romex coming into the box where it happened (ceiling light). The ground was not screwed to the light fixture adapter plate, but wrapped around it.

    Should the breaker have tripped and if so what prevented it from tripping?


    Thanks,
    Bill
    "Any American who is prepared to run for President should automatically, by definition, be disqualified from ever doing so."
    Gore Vidal.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    A CB is more like a time-delay fuse - it takes time for it to trip depending on the current drawn and the time. It's possible that the breaker is bad, but it's also possible that the current drawn wasn't high enough, or the duration wasn't long enough. The curves of how and when a CB trips can be tailored, and each one is somewhat different, but it's not a simple fuse replacement. When you get a short, it can liquify the conductor, and that may have meant that it no longer was making contact, and thus was no longer shorted. If that took less time or used less current than the trip curve of the breaker, it won't trip.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Electrical Contractor/Instructor jwelectric's Avatar
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    A good rule of thumb is that a circuit breaker will draw six times its rating for .033 second (two full cycles) and not open.
    In other words a 20 amp breaker will allow 120 amps to pass for two cycles and a 15 amp breaker will allow 90 amps to pass for the same amount of time.
    Through unofficial test I have witnessed a 20 amp Square D breaker that allowed 30 amps to pass for over 2 full minutes and did not trip. The Edison based fuse would open at 135% of its rating in two seconds.

    Also a 5 horse power 240 volt single phase motor will draw 28 amps but when it first starts up it will draw six times this current or 168 amps. Check out the draw in 430.248 and the locked rotor in 430.251 of the NEC.
    Last edited by jwelectric; 09-10-2011 at 03:30 AM.

  4. #4
    Electrician ActionDave's Avatar
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    Quote Originally Posted by molo View Post
    The hot wire got nicked on a metal junction box. Sparks flew and the wire was split in two. However, the breaker didn't trip.

    Also, there is one wire of 12-2 romex coming into the box where it happened (ceiling light). The ground was not screwed to the light fixture adapter plate, but wrapped around it.

    Should the breaker have tripped and if so what prevented it from tripping?


    Thanks,
    Bill
    What brand of breaker was it?

  5. #5
    Moderator & Master Plumber hj's Avatar
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    If it can create an arc, or sparks, then it is NOT a true short circuit, which is probably why it did not trip the breaker.
    Licensed residential and commercial plumber

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