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Help Solve a Forensic Investigation of a Fire

  1. Mar 2, 2007 #1
    Hello,

    I am a Forensic Investigator, and I am seeking help solving a case involving a possible (and probable) extension cord failure. This failure caused a fire and very large lawsuite. I need help determining the heat release potential of this extension, given the load on the end of it. Here are the details:

    The extension cord:

    • 18 AWG / 2 stranded copper conductor cables insulated with SVT. (2 copper conductors no ground and not polarized)
    • The cord was approximinately 25 ft
    • The cord was energized with standard 120 volts AC.
    • The load on the end of the extension cord was 1500 watts.
    • The resistivity of 18 AWG copper is 6.510 Ohms/1000ft *http://www.interfacebus.com/Copper_Wire_AWG_SIze.html*

    This cord is only rated at 10 amps. It was pulling in the neighbourhood of 12.5 amps, I need to know if this is a capable ignition source for nearby combustibles.

    Thank you for your time.
     
    Last edited: Mar 2, 2007
  2. jcsd
  3. Mar 2, 2007 #2
    Most of the time extension cords fail (especially when overloaded) at the ends. There is really no way of telling whether or not the heat that may have been generated at the connection where the cord plugs into something else was enough to start a fire. That would depend on the condition of the end before the fire started. If the end of the cord is in good shape, then little heat will be generated provided that it isn't too severely overloaded to begin with. A poor connection usually gets poorer when running hot. The end quite possibly was worn out or damaged from overload before the fire started. I would seriously doubt that the fire started midway down the cord unless there was a spot on the cord that was damaged beforehand.
     
    Last edited: Mar 2, 2007
  4. Mar 2, 2007 #3

    berkeman

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    Yeah, where did the fire start exactly?
     
  5. Mar 2, 2007 #4
    Under a matress, where it was most likely insulated and not allowed to dissipate heat.
     
  6. Mar 2, 2007 #5

    berkeman

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    The extension cord was run under a mattress? Like, between the box springs and the mattress, or do you mean on top of the carpet under the bed? What was the condition of the rest of the cord? Was it likely that the cord was pinched or had the bed's supports on top of it?
     
  7. Mar 2, 2007 #6

    berkeman

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    It doesn't sound right that running the cord at 125% of its rating would cause a fire. Cords are not derated that badly.
     
  8. Mar 2, 2007 #7
    If the cord was coiled then increased heat would be generated, saying that I would be surprised if the cord failed catastrophically on a 25% overload. From my experience of seriously overloading a cable the cord got hot, enough to soften the insulation, but not enough to cause breakdown. That was pulling 40A on a 100m 32A 3ph cable left on a drum, which I would say was much more severe a test.
    Also any pre-existing damage on the cable whilst not being enough to be significant under normal conditions could become significant in an overload situation although the flex over rating should account for most fair wear and tear.
    If this is for a forensic examination it would be best to do some experiments with flex in different coil configurations running at 125% rating. There are too many unknowns about the specifics set-up to come up with a definitive answer, especially when there is money and law suits attached to it.
     
  9. Mar 2, 2007 #8
    i'd say yes, 18 gauge at 1500 watts. the jacket is rated at 140F and you have to consider that you have what amounts to two hot wires right next to each other. if i did the math right (cheated with online calc for temp coefficient cause book is :yuck: )somewhere around 1200-1300 watts something like secondary breakdown happened as the wire heated up.
     
    Last edited: Mar 2, 2007
  10. Mar 2, 2007 #9

    berkeman

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    At 125% rated load, you anticipate an insulation breakdown? That's hard to believe. I guess as Panda suggested, it's time for a CSI lab reproduction of the situation.....

    Who are youuUuuu! :biggrin:
     
  11. Mar 3, 2007 #10
    wouldn't it be 150% rated load, i think with a 10ohm resistor and a varaic this should be easy to reproduce.
     
  12. Mar 3, 2007 #11

    russ_watters

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    That's only 25 wats of heat dissipation over the entire length of the wire, or 1w/ft. That's nothing - there is no way that could have caused the wire to fail. There must have been a flaw in it somewhere (if that's where the fire started).

    My electric blanket burns significantly more energy than that. The controllers say 360W, though I don't know if that is per side. There are 12 strands on each side (the wire loops around) at 6 feet each, so that's 5w/ft if that's 360w per side or 2.5w/ft if it is 360 total.

    As berkeman suggests, though, it would be a piece of cake to re-create (even easier to simulate) the conditions of the wire and measure what happens.
     
    Last edited: Mar 3, 2007
  13. Sep 10, 2007 #12
    This may help...

    http://www.keeva.net/publications/TR2005-053.pdf

    Abstract—Guidance for predicting the ampacity of small-gauged
    conductors is not readily available and not addressed in
    the authoritative National Electric Code. While the Code and
    manufacturers provide definitive tables of ampacities, an
    analytical method is helpful. Increasingly, reliability and safety
    personnel are interested in ampacities of appliance and extension
    cords. The interest may be the result of product design,
    insurance, legal or forensic involvement. Because fire is a
    potential result, the First Law of Thermodynamics is an excellent
    starting point for a useful analytical tool. This paper explores
    one such analytical method derived as a result of a forensic
    engineering examination into the cause of a fatal fire.
     

    Attached Files:

  14. Sep 11, 2007 #13

    Danger

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    Welcome to PF, both Mike and Phorensics. (Excellent post, P.)
    I don't have time right now to read that link, but I will say that I've never seen a 10A rated cord fail at less than 15A unless there was something wrong with it before-hand.
    As Averagesupernova mentioned, it's also extremely unlikely that the failure would be midway down the cord. It's almost exclusively at the connection points. I have no idea of where you are, though. Alberta is rat-free, but is there a chance that rodents might have been chewing on the cord?
    All of this, of course, is purely speculative since we don't have first-hand knowledge of the scene or the circumstances.
    Could there, for instance, have been deliberate tampering with the cord? If so, why would someone do that? Insurance... attempted murder... pyromania...
    I'm not trying to cloud the issue here... just covering a few bases.
     
    Last edited: Sep 11, 2007
  15. Sep 11, 2007 #14

    NoTime

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    Interesting paper.
     
  16. Sep 11, 2007 #15

    berkeman

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    Welcome to the PF, phorensics. We generally do not allow linking to unpublished papers like yours here on the PF. Have you submitted it for publication yet? Can you say which journals you have submitted it to?

    We do have a forum here specifically for as-yet-unpublished (unrefereed) papers -- it is the Independent Research forum. Here is a link to more info:

    https://www.physicsforums.com/showthread.php?t=82301

    I may need to have you post your paper there instead, depending on the status of your submissions to refereed journals. I did find your paper interesting, BTW. I think that with some changes, it is worthy of publication.
     
  17. Sep 12, 2007 #16
    Please remove any offending links (and content) - and accept my apologies. I should have taken out the link after I scrolled down and noticed that a file could be attached.

    The paper was presented for publication to and refereed by the IEEE; however, there was 'no theoretical advance' and the 'contribution of this paper to researchers is limited', therefore it was dismissed. Rather than lose the effort, it was published at http://www.ceejpublishing.com/ for PDH credit. It may interest others to know that the paper was solicited based on the abstract - but, ultimately rejected...so your time can be spent in vain.

    BTW, the point of the paper is ratings (10A or otherwise) are based on an assumed operating environment; change the environment and the rating becomes almost meaningless.

    Thank you for the kind remarks. All the best, P
     
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