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Thread: ARC FLASH investigation

  1. #1
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    ARC FLASH investigation

    http://ecmweb.com/ops_maintenance/de...101/index.html

    I read the above article posted by another member with great interest. Seems the guy was probing with a standard DVM, and it failed, causing the arc flash. I still don't get how this caused the incident because a DVM has rather thin leads, so any fault current would be limited by the probe, and guage of the prove lead.

    Even if he had an aligator clip to ground, usually they are not the greatest connections.

    Am I missing something ???

    needless to say, I don't like going into 480/277v panels any more....

  2. #2
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    Without casting dispersions, generally when these kind of things happen there are multiple experts involved who invariably all have ideas on what actually happened that generally seem to minimize the exposure to liability of the guy that paid them.

    I don't know if his theory of the accident is any better than anyone else. I suspect there is little chance of an arc flash that is conducted through the small diameter wires of the test probes. It would not be real hard for someone with access to the software to see just what the arc flash potential would be. Maybe someone that has such access would be willing to run the numbers.
    Bob

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    The DMM did not have any CAT rating, let alone the CAT III rating required for this type of work.

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    Quote Originally Posted by zog View Post
    The DMM did not have any CAT rating, let alone the CAT III rating required for this type of work.
    I know a guy that had one of those cheap non rated meters blow up on him when he was checking 240 volts.
    Cheers and Stay Safe,

    Marky the Sparky

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    Quote Originally Posted by K8MHZ View Post
    I know a guy that had one of those cheap non rated meters blow up on him when he was checking 240 volts.
    I was at a customers site last week (Paper mill) where a guy was using a non CAT rated meter on 480V, a transient occured from MCC switching, his meter failed, he died 8 hours later in the burn center. The starter he was checking was still sitting in the corner of the room all burned up.

    Fluke has a great feee video you can download that discusses transients, CAT ratings, testing, etc......
    http://www.fluke.com/fluke/usen/trai...tm?trck=safety
    Last edited by zog; 10-07-11 at 06:18 PM.

  6. #6
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    Quote Originally Posted by the blur View Post
    ... I still don't get how this caused the incident because a DVM has rather thin leads, so any fault current would be limited by the probe, and guage of the prove lead.

    Even if he had an aligator clip to ground, usually they are not the greatest connections.

    Am I missing something ???

    needless to say, I don't like going into 480/277v panels any more....
    You are ASSuming that the catastrophic arc flash happened AT the probe leads. What can, and does, happen is that something relatively innocuous happens first, like the probe leads vaporizing or the meter exploding, and THAT spews out metallic "mist" and/or ionized gasses, which then propagates a LARGER arc flash event. All of this can happen in a tiny fraction of a second.
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    Quote Originally Posted by Jraef View Post
    You are ASSuming that the catastrophic arc flash happened AT the probe leads. What can, and does, happen is that something relatively innocuous happens first, like the probe leads vaporizing or the meter exploding, and THAT spews out metallic "mist" and/or ionized gasses, which then propagates a LARGER arc flash event. All of this can happen in a tiny fraction of a second.
    Exactly, we had a tech troubleshooting a 480 volt lighting contactor cabinet, his meter was Cat rated for what he was working on, but shorted the probe end between the 277 volt circuit on the contactor, and the 120 volt control circuit. this started a chain reaction arc flash that melted five three pole contactors before it stopped. luckily he was wearing his PPE, and was unhurt.

  8. #8
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    Quote Originally Posted by zog View Post
    The DMM did not have any CAT rating, let alone the CAT III rating required for this type of work.
    So what? Does not mean that it could sustain enough current to start an arc flash incident.

    If I take 10 feet of #16 wire and hook it across the terminals of this switchgear and turn it on, will it cause an arc flash? That is the real question here.
    Bob

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    Quote Originally Posted by petersonra View Post
    So what? Does not mean that it could sustain enough current to start an arc flash incident.

    If I take 10 feet of #16 wire and hook it across the terminals of this switchgear and turn it on, will it cause an arc flash? That is the real question here.
    It might.
    With great power comes great resistance - times current squared, of course.

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    Quote Originally Posted by petersonra View Post
    So what? Does not mean that it could sustain enough current to start an arc flash incident.

    If I take 10 feet of #16 wire and hook it across the terminals of this switchgear and turn it on, will it cause an arc flash? That is the real question here.
    What your missing here is that once an arc is initiated the plasma will provide a low impedance path to keep it going, this is the same effect that happens when a welder strikes an arc and pulls the rod away from the work but with a current limited welder it can only go so far as the current can't keep increasing to extend the energy amount, but a service can have many thousands of amps available that this arc current can keep growing till something gives, I once watched a buss duct run burn till it got to the end of it and the wire feeders fell apart, all that was left was melted copper all over the floor under it, the explosions blew the steel casing apart as it progressed through the plant.

    So all that is needed is a small arc initiated to start a much larger arc, and once it has created enough plasma it can continue till there is to much distance or not enough energy to maintain the arc.

    Over the years I have seen a few times where these cheep analog meters were being used in industrial maintenance and in more then 3 times the operator of the meter made the mistake of not paying attention to where he put the probe into the meter placing them in the amp reading holes which has a shunt across it, and yes it blew up in their hands when they tried to take a voltage reading.

    Also the one thing we do when surprised is we grasp a big gulp of air, we inhale like we are going to have to hold our breath for a few, it is an automatic response we all do, and in the event of an arc flash we inhale the hot gaseous air and sever lung damage is the results.

    Just some food for thought.
    Last edited by hurk27; 10-08-11 at 06:55 PM.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

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