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Old 05-20-2013, 03:10 AM   #1
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Oven element "thermite"

My oven's broiler element just burnt out. I shut it off and the rest of the coil cooled off, but the blowout spot stayed bright-white hot, and began to travel along the length of the coil a couple of inches per minute, burning through the element's casing, sputtering and throwing sparks like a slow motion fuse. I googled this and found someone else's video --strangely enough the same model of oven as mine: There was little or no flame and I was unable to put it out even when I sandwiched the hot spot between two saturated sponges. I'm not a chemist but I'd swear this was some kind of thermite reaction: two compounds in the broiler element were reacting with each other at this super high temperature and didn't need any outside oxygen (or got it from the water in my sponges). Towards the end of the element, the remaining 6 inches began to glow (the regular red, not white), just like the video. Nevertheless I am sure the switch was off, and it's very unlikely the switch and element failed together. What strange chemistry goes on here?


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Old 05-20-2013, 04:57 AM   #2
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there are reports of a thing called a "RUNAWAY STOVE" ...you can google it ...
if you work on this make sure the breaker is off not just the stove ..


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Old 05-20-2013, 05:18 AM   #3
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Possible explanation (some models only). The element is fed 240 volts (via both hot legs of the 120/240 volt circuit) but the knob switch cuts only one of the legs. A fault developed between the still energized nichrome wire inside the sheath and the (grounded) sheath.
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Last edited by AllanJ; 05-20-2013 at 09:15 AM.
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Old 05-20-2013, 05:54 AM   #4
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Always unplug or shut off the breaker to the oven before messing around with the elements, especially with 2 wet sponges.
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Old 05-20-2013, 07:46 AM   #5
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Originally Posted by COLDIRON View Post
Always unplug or shut off the breaker to the oven before messing around with the elements, especially with 2 wet sponges.

hmmmm, sounds like a fun time
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Old 05-20-2013, 02:36 PM   #6
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The nichrome wire in the element is insulated from the metal shell by a powdered material which is usually magnesium oxide. MgO is a great insulator and melts at an extremely high temperature. But an arc is hot enough to melt and vaporize it, and melted or vaporized MgO is conductive. So once an arc starts between the nichrome and the metal shell of the element, the overheated MgO insulator becomes a conductor and allows the fault to continue. There had to still be power involved - as AllanJ said, 120V may still exist between one end of the element and the sheath when the stove is off. A thermite reaction is not possible with an MgO insulator, because neither the steel shell nor the nichrome wire has enough reducing potential to strip oxygen from magnesium. If the insulating powder were something else, it might be possible.


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