Join Date: Jan 2013
SCARY oven fire! Why did it burn?
I'm pretty sure this is not a true fire. It rather was an electric arc. Notice that when you unplugged the power, it immediately went out. Here's how I think it happened, and let me start from the beginning.
A stove has 4 prongs on the plug. These are Hot (0deg phase), Neutral, Hot (180deg phase), and Ground. You see the power transformer on the utility pole has 3 outputs, Hot-0deg, Neutral, and Hot-180deg. A grounding stake is driven into the ground near the pole and a cable goes from the neutral connector to the grounding stake. Also 3 lines run from the transformer to your house. At at your house there is another grounding stake or a metal water pipe is used, and a wire goes from that to all the ground contacts in your outlets. The neutral wire from the goes from the wire inlet in your house to all your outlets. The Hot-0deg wire goes from your wire inlet in your house to a breaker/fuse box, and hot wires for all the outlets in the house go out from there. The Hot-180deg wire goes to the breaker/fuse box and from there you have hot-180deg wires that go to just the 240volt outlets (such as your stove). At a 240v outlet, Neutral to Ground should be 0 volts. Hot (either one) to Neutral (or Ground) should be 120 volts, and Hot-0deg to Hot-180deg should be 240v. The heating element in your oven is Hot to Hot so it runs on 240v. A heating element consists of a resistive wire (nichrome or tungsten). A tube of ceramic insulation surrounds the resistive wire. A steel tube surrounds the ceramic insulator. Each end of the resistive wire should be connected to a Hot wire. The steel sheath should be connected to the metal chassis of the stove, which itself should be connected to the Ground wire.
As the heating element ages, cracks can develop in the resistive wire, thinning it at spots these spots will look brighter than the rest of the heating element. The current flowing through thinner spots will heat them more, eventually exceeding the melting point of the wire, causing it to "burn out". At this moment though an arc can strike across the small gap. the arc will be extremely hot and consume more power than the rest of the element. This will make this spot glow quite bright, and the rest of the heating element will get significantly dimmer than it was before. If you cut the power on your oven's switch at this point the arc will be extinguished. If you are unaware it is arcing, and it goes on for a long enough time, the heat from the arc can crack the ceramic insulation. Then the arc also will rise like a "Jacob's ladder" arc until it touches the steel sheath at a point that the ceramic insulation was compromised by cracking. At this point you will have 2 arcs, one from Hot-0deg to Ground and one from Hot-180deg to Ground. The heat from the arc will now melt the steel sheath. Since it isn't shielded by argon gas (like in arc welding or plasma cutting) though, the steel sheath will oxidize from the heat of the arc and won't be conductive (iron oxide rust is an insulator) so will fall away and not fill the gap where the arc is (which could stop the arc if it was conductive and did fill the gap). As a result, the arc on each side of the break in the heating element will keep arcing from Hot to Ground, slowly destroying the heating element (looking sort of like a candle wick slowly burning downward, but a LOT brighter, and it isn't actually a burning flame like a candle, but rather an electric arc). The more it goes on the less resistive wire there will be, so the more current will flow, and the brighter and hotter the arc will be. If allowed to "plasma cut" its way all the way back to the contacts where the heating element connects to the socket, it will then have NO resistance from the resistive wire (which previously was acting like a ballast for a mercury arc lamp). With no current limiting it will basically be a short circuit and your arc will turn into an "arc flash". However your circuit breaker will respond to an excess of current way before then (like 30A or whatever the oven circuit's circuit breaker in your house's breaker box is set for) and trip, stopping the arc.
However the moment you notice the arc you should turn off the power by turning off the knob on your stove. However if the arc is already proceeding to melt back the heating element on both sides, this may or may not work completely. If the switch only cuts off the Hot-0deg line, then the arc will still be going from Hot to Ground on one side of the element but will be stopped from the other side. This will require you to unplug the stove from the outlet, or manually switch off the oven's breaker in your house's breaker box if your oven is hardwired or the outlet is inaccessible. If the oven's setting knob switch cuts off both Hot lines, then turning the knob to "off" is all you will need to do to stop the arc on both sides of the element.
You should NOT throw anything onto the burner. It is not on fire. It is an electric arc. And whatever you do, NEVER throw flour. Flour burns, and when thrown it floats in the air, which if then it contacts the electric arc it can cause a mini "grain silo explosion" in your oven. VERY DANGEROUS. Also do not stare at a strong electric arc like the one from an oven element failure. It may look no brighter than a lighbulb glowing, but an electric arc in the air emits UV rays (air is mostly nitrogen, and the emission spectrum of nitrogen gas has a few strong lines in the in the UV region), and UV rays are dangerous to the eyes, the same reason you aren't supposed to look at a welding arc without proper eye protection.
You shouldn't necessarily need to call the fire department though, as it's not a fire, and unless there is evidence that the arc started a fire, there is no reason to call. If something in the stove is burning though, and ESPECIALLY if there is an electrical failure that causes the wall to feel hot somewhere or the electrical breaker box feels hot (possible indication of a fire in the wall or a short that might lead to a fire in the wall), then it's time to call the fire department. Only the fire department can properly handle a building on fire (flames inside the wall or other place in a building that's not accessible to the ordinary person with a fire extinguisher). If there's a fire in the oven though, a proper ABC fire extinguisher will put it out, and with there being no where else the fire could POSSIBLY have gone (being contained inside the all-metal oven), there's no possibility it spread to be inside a wall or something. So a simple fire put out with an extinguisher isn't in and of itself a reason to dial 911. But if there's reason to believe the fire spread (possibly inside a wall), then you ABSOLUTELY SHOULD call 911 IMMEDIATELY.
Last edited by Ben321; 01-20-2013 at 06:57 PM.