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Thread: Arc Flash

  1. #1
    Join Date
    Jan 2004
    Posts
    231

    Arc Flash

    I had an arc flash 120/208 200 amp 3 phase in a meter can it was a feeder tap off a 800 amp disconnect 500 kva pad mount 4-5 percent impedance. service conductors parallel 500 mcm cooper 100 f tin length 10 ft feeder tap to meter can. transformer high side 12470 40 amp fuse no clf just outside substation fence available fault current high side was 5000 amp l-n. The fault occurred when I pulled meter burnt a hole in my keeper and my rubber glove nothing tripped it took a substation crew to trip breaker( inst trip blocked) to clear fault 10 min later. Caught building on fire now I have to meet with safety. My safety manual states 8 cal is all need if it is higher I will be told. Can this arc flash be calculated and isn't over 8 Cal?

  2. #2
    Join Date
    Mar 2013
    Location
    Rutland, VT, USA
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    225
    There are several ways to look at this. The problem with 208V arc flash hazards is that the IEEE 1584-2002 standard was only able to sustain an arc at 208V in one test specimen. That led to the exception for 208V services supplied by transformers less than 125kVA.

    Since this is a meter can, this would most likely fall under NESC rules used by utilities. For a 208V self contained meter, the clothing system required is 4 cal/cm2. There is a footnote to that which states "Industry testing on this equipment by two separate major utilities and a research institute has demonstrated that voltages 50V to 250V will not sustain arcs for more than 2 cycles, therby limiting exposure to less than 4 cal/cm2."

    Now, doing a quick model calculation on the system you describe with some assumptions and using 2 cycles, the incident energy is 0.6 cal/cm2.

    What I find interesting is that you state a hole was burnt thru the keeper and rubber glove. Arcwear has done testing on leather keepers as well as rubber gloves. What they found is that almost all the rubber gloves, in combination with the keepers, had a ATPV of 40.

  3. #3
    Join Date
    Jan 2004
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    231
    The meter room is only 200 ft from the substation so im guessing fault current was high and nothing tripped high side or secondary the fault actually welded the customers main disconnect closed. This fault continued for minutes not cycles.

  4. #4
    Join Date
    Jan 2004
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    231
    The fault burnt a 2 inch hole in a breaker enclosure that was 2ft away, the transformer was a 500 kva so would that increase arc to a level that is that catastrophic

  5. #5
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    Mar 2013
    Location
    Rutland, VT, USA
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    true, so more information is needed to accurately model the system to determine what the incident energy actually was.

  6. #6
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    Jan 2004
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    231
    Thanks for your help

  7. #7
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    30,364
    Quote Originally Posted by newt View Post
    I had an arc flash 120/208 200 amp 3 phase in a meter can it was a feeder tap off a 800 amp disconnect 500 kva pad mount 4-5 percent impedance. service conductors parallel 500 mcm cooper 100 f tin length 10 ft feeder tap to meter can. transformer high side 12470 40 amp fuse no clf just outside substation fence available fault current high side was 5000 amp l-n. The fault occurred when I pulled meter burnt a hole in my keeper and my rubber glove nothing tripped it took a substation crew to trip breaker( inst trip blocked) to clear fault 10 min later. Caught building on fire now I have to meet with safety. My safety manual states 8 cal is all need if it is higher I will be told. Can this arc flash be calculated and isn't over 8 Cal?
    Had to read this a couple times, but eventually figured you must have been pulling a 200 amp meter (maybe for separate tenant in multiple occupancy application?). Still don't know why if there was a 800 amp disconnect ahead of this why there wouldn't have been an overcurrent device associated with it that likely shouldn't have held for 10 minutes? If not a bolted fault, 208 volts just can't sustain an arc for very long at all without feeding more conductive material into the arc like is done when "welding".

  8. #8
    Join Date
    Jan 2004
    Posts
    231

    Pictures of actual incident

    Aftermath
    Attached Images Attached Images  

  9. #9
    Join Date
    Jan 2004
    Posts
    231

    Burnt Glove Pic

    Leather glove with rubber glove underneath.
    Attached Images Attached Images  

  10. #10
    Join Date
    Mar 2003
    Location
    New England
    Posts
    12,760
    A wirebrush and some spray paint and that will be as good as new.

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