Mouth of the dragon's lair (the secret revealed)

[rjlittlefield]

I have no idea what this stuff around the hole is, but I wanted to take a closer look at it. Shot at 7X on sensor, cropped and resized here to 70% of actual pixels. Optics were Nikon CFI 10X NA 0.25 on a Canon 100mm f/2.8L USM macro plus a Kenko 1.4X teleconverter. Illuminated by flash diffused through paper. Focus stepped by EOS Utility, started off as 1 ">>" then switched to 2 ">>". (Can't see any difference in the sections.)



Any last guesses about the dragon's lair, before I spill the beans?

--Rik

[Pau]

They seem growths of crystals, but no idea of the mouth itself

[Cactusdave]

I keep looking at this and coming to a different conclusion each time. The low mag. picture looks like a branch partly covered in some kind of concretion like hardened mud. The little spigot on top with the accumulation of what look like crystals around it looks like hot mineral laden water or steam has come though it. I wonder if it is a twig, partly burnt from the fire. The only thing I can recall seeing that was at all similar was a tree branch that had been in one of those volcanic steaming mud pools at Yellowstone, and I doubt you have one of those in your kitchen.

[DQE]

Well...I think it's definitely a scene from a movie like "Aliens", in which the invaders from outer space have attached themselves to a twig. The aliens think (correctly, I fear) that twigs are the most intelligent life form on our planet, and they are therefore associating themselves with twigs.

In the meantime, the aliens have to eat and breathe, hence the visible mouth on the photo. Be careful about putting your finger or some other valuable body part near the alien's mouth!!!

I freely admit that this theory may be incomplete or in other ways imperfect (grins).

For example, there may be more than one alien, or the alien may soon discover that twigs aren't the most advanced life form, but gerbils are (as happened in "The Restaurant at the End of the Universe" by Douglas Adams, from distant memory).

Anyway, this is the best I can do. Don't bet too much money on this theory! (more grins)

[rjlittlefield]

Many good suggestions in this thread!

I especially like the twig theory, but that reminds me of a story in which the alien invaders were able to imitate anything they wanted. Their plan was to infiltrate one of their members aboard a spaceship returning to Earth, escaping our vigilant defenses by substituting as one of the spaceship's actual components. Unfortunately the component chosen to substitute was a chunk of electrical wire in the circuit that opened the spaceship's door. Everything went fine until victory was in sight, and then "Poof!" What a way to lose!

And that brings us to the actual solution for the current puzzle:



The subject is a hole blown in the side of a stove top heating element by an electrical arc that initially developed inside the element.

As mentioned, I was stirring a pot of soup at the time. The first evidence of a problem was bright red coming out from under the pot. At first I thought something in the drip tray had caught fire. But then it quickly brightened and became blue-white. Perhaps 5 seconds elapsed before I could figure out what was going on and turn off the power.

I haven't cut the thing open, but all indications are that the arc extended for quite some distance inside the outer tube. There is another smaller hole blown through the tube 1-1/4 turns away, a full 12 inches along the arc of the coil.

I suppose that the crystals are some combination of chemicals formed when the arc ripped apart the insulation layer, melted the outer metal shell, and then everything interacted with the air. As I said, I'm not sure to what extent oxidation was involved. Certainly iron rusts very quickly when it gets hot.

This is another one of those "casual" high mag photography setups. The camera is mounted on a tripod in its lowest posture, with the subject sitting on a pile of phone books, all this on top of the counter and the stove itself. (You can see another heating element in the background.) The blue band near the base of the objective is masking tape. At one point I didn't have any way to do RMS-to-M52, so I made one by hacking apart the cap of an objective storage vial and threading it into an M26-M52 adapter that I did have. The bit of tape was needed to make the threads fit as tight as I wanted.

--Rik

[rjlittlefield]

Well, I've always been curious about what's in one of the stovetop heating elements. So of course I couldn't resist cutting one open. Turns out it wasn't easy to find out even then.

Here's my first attempt, cutting straight across the element at an undamaged point:



That pretty much ruled out a simple rod, but I still couldn't tell what shape the actual element had. Some sort of triangular coil, perhaps?

Second cut, slanting across the element. This one had me really confused!



Unable to figure it out from cross sections, I decided to take apart a longer section. That cleared up the mystery.



Finally, here's a closer view:



I have no idea why these guys use a double helix. Chalk up yet another coil design!

--Rik

[rjlittlefield]

By the way, there is a triangular element to the coil design:



I suspect that the coil and its surrounding metal shell is manufactured to form a long cylinder, then coiled to form the stovetop element and finally flattened on the top surface. The flattening process would simultaneously deform the outer shell, the granular ceramic insulation, and the inner coil of nichrome heating wire.

--Rik

[Rusty]

Rik
Should the element not have tripped the earth leak protection system when arcing to the outside of the element (ground)?
Maybe wise to check for safety sake ?

[rjlittlefield]

Should the element not have tripped the earth leak protection system when arcing to the outside of the element (ground)?
Maybe wise to check for safety sake ?

Good question.

All installed safety devices are working fine, but this house does not include ground or arc fault circuit interrupters except for GFCI's on certain plugin outlets (kitchen, bathrooms, garage, yard). All the other circuits have only overload protection (classic circuit breakers). It's unclear how much of the current was to ground, but in any case the arc would have been relatively low current because of its location near the middle of the element, where the ends of the element would have provided current limiting resistance. The circuit breaker didn't flip because there was no overload -- the amperage was well within normal bounds for the stove, which under normal operating conditions could have all four burners and the oven heating at once. Probably a ground fault interrupter would have tripped near the end of the event, when the arc grew to include the outer shell. An arc fault interrupter would have tripped near the beginning of the event, as soon as the arc across the break in the heating coil lasted too long to be a normal switching event. But last I checked, arc fault interrupters are not required in range circuits even in the most recent standards. A quick check of local repair parts suppliers showed no AFCI's with ratings above 20 amps, well below the 50 amps needed for this range circuit.

Thanks for asking!

--Rik

[Planapo]

Rik, one question, as I have problems to understand how you could watch that "Poof":
Wasn't there something like the old fashioned solid cast iron plates or a more fancy Ceran glass ceramics plate between the bottom of your pot and that heating spiral? I first thought it must have been transparent glass ceramics where you could look through to watch the "Poof"?!
But then.. with such a Ceran plate one can always see "red coming out from under the pot", it's red shining through from the heating element beneath the glass ceramics that is always emitting red light when heating in normal operation, and besides, the heating spirals look totally different from yours...

--Betty

[rjlittlefield]

Wasn't there something like the old fashioned solid cast iron plates or a more fancy Ceran glass ceramics plate between the bottom of your pot and that heating spiral?

No, these are classic tubular sealed elements ("Calrod" style), and the pot sits directly on the outer shell of the element. There is ceramic insulation, but only sandwiched between the actual heating coil and the outer shell.

Here's a picture of the new burner and the pot in question, slid sideways so you can see how it all fits together.



--Rik

[Rusty]

Rik Wrote:

All installed safety devices are working fine, but this house does not include ground or arc fault circuit interrupters except for GFCI's on certain plugin outlets (kitchen, bathrooms, garage, yard

Ok...that makes sense , we have a different setup where all the individual current breakers works via a main breaker (60 amps) that trips as soon as it senses more than 20milli amps total combined
leakage to ground from the whole house circuit
It can be a real nuisance in very damp conditions ,most of my kitchen appliances are "vintage"

[ChrisR]

Some off-the-cuff musings...

I'm not happy with:

I suppose that the crystals are some combination of chemicals formed when the arc ripped apart the insulation layer,

I feel they'd have taken a lot longer than that to form.

I'm really pretty sure I've seen split outer sheaths on heating elements, where the heater is still working. [One might guess that the sheath is made of recycled car bodies, ie mild steel, which is reasonably ductile and corrosion resistant, but nothing special. That might be unfair, maybe there's a little of something in there. Try a grindwheel-spark test to get an idea.] In service it gets red hot of course. It's been work hardened and deformed, so a crack isn't all that surprising.

As Rusty suggested, there's often an earth-leakage /residual-current safety device involved, and these do often "trip". These protection devices are by no means universal in the UK though.
I see a lot of immersion heater failures. They're different in that they're as implied, submerged in water.
Sometimes the "trip" activates, but the element still measures the right resistance. Water between element and sheath, then, they give way too.
Sometimes the element fails, but if the owner leaves it on long enough, the residual current through the water, between "live" and earth and/or neutral ( in case of no trip) heats the water over a period. ( rather alarmingly...)

I'm almost sure I've seen a cooker ring like Rik's produce "steam" when first turned on, because water has worked its way into a crack, and then of course become adsorbed(?) by the sand in there. It figures though for Rik’s case I think - the corrosion is on the underside of the ring, where the water would collect. The "sand" would have likely contain have various salts, which would dissolve in the hot water, and precipitate back on the outer surface of the ring from the vapours. Temperature/humidity/vapour concentration gradient just outside the evacuation point would tie up with the direction /placement of the coloured banding and crystal build-up we can see.
Even if the water were not conducting enough to self-heat, it would be getting hot and evaporating while the heater was working.
I don't know much analytical chemistry, but I'd expect the crystals would show themselves to be of aqueous-soluble compounds. Maybe something exciting like a spinel?

One thing the sand does do, is take heat away from the element. I suspect the death mode was that the element locally overheated, and melted.
The element is going to be 20 Ohms, or something. If it shorts to ground partway along then sure, it could overheat, but there's still resistance in the unshorted bits, so there's not going to be a huge explosion with fireworks and exploding metal sheath. The impedance is too high, there’s no “shorting” in the accepted “bang” sense here. A 50A breaker would sustain a melting-current for quite a while,

If the ring was bright-yellow hot, it would be up around 1000 Celsius. NiChrome starts to melt at about 1350. Lower if it's made to be a bit cheaper, with iron added. Not far off.
ARC?? Not much, the UK mains voltage is only enough to produce a spark half a millimetre or so long. (10,000 Volts per inch)

If the OUTSIDE of the ring is at 1000 it's going to be a lot hotter on the inside where the wire is, with a temperature gradient across the sand, which isn't the best heat conductor in the world. So, the wire lives its life pretty close to its melting point. Removal of heat-sinking route and it melts, or touching the sheath, and the current goes up by even a factor of a couple, and it melts.

Or another possible - that the exposure to air, cleaning liquids, soup etc caused local corrosion of the wire. That would "thin" it and cause a local hotspot.
( It would be interesting to know if the inner end (closer) of the spiral, is "live" or "neutral", assuming US electrics are like UK, with Neutral near ground potential)

Melting points of Iron and Nickel are a bit above 2700C
Would they boil or just burn? I don’t know. Some of each?
Metal oxides boil at much lower temperatures (NaCL 1413C), so there are possibilities there.

I am entertained by the double helix - ingenuity suspected! Never seen one like that.
Possible reasons:
1) heat conduction away from the metal. There's more surface area in two strands. The clever extra is the way they're held apart, rather than being twisted round each other? If you use two stands of half the cross sectional area, instead of one, your SA goes up by 41%.
2) Stiffness.
You don't want a stiff wire. It's harder to make, and the wire in service is going to move around a lot, relative to its sheath. If it's stiffer then higher stresses will be raised so it's more likely to crack. Using two strands instead of one means the stiffness goes to 35% for each one.

[rjlittlefield]

Some off-the-cuff musings...

I'm not happy with:

I suppose that the crystals are some combination of chemicals formed when the arc ripped apart the insulation layer,

I feel they'd have taken a lot longer than that to form.

OK, but be aware that you're working from incomplete information.

Here are a couple more pictures of the burner.





Now bear in mind, the top and sides of this heating element are exposed in normal use. They also get inspected and cleaned frequently, like before and after every use, and I'm the individual who does that. I can assure you that the element had normal appearance before I started cooking that pot of soup. But after the event -- during which I slid the pot aside and saw blue-white stuff shooting out of the element -- the side of the element is melted out to form obvious projections, some sort of light yellow stuff has been sprayed across the next two turns of the element, and the crystals in question have suddenly appeared on the outside of a metal projection that didn't exist before the event.

While I respect your feelings about how long it would take such crystals to form, I think the evidence is pretty firmly on the side of "quickly" --- as in a few seconds, while I watched. This is a very different situation from retrieving a heating element from the inside of a water heater where it could sit unobserved for a long time.

ARC?? Not much, the UK mains voltage is only enough to produce a spark half a millimetre or so long. (10,000 Volts per inch)

Sure, but that's in ordinary air. It's pretty clear from the stuff blown around and recrystalized that there was, um, "significant opportunity" for a fairly dense plasma to form. That's the way consumer arc welders work -- high current at low voltage ("typically between 17 to 45 volts") through an arc enabled in large part by flux on the rod. In the case of this burner, it's not even necessary for the arc to be very long at any one instant. As Rusty pointed out, current need only flow from the energized coil to the nearest point on the metal shell, where it can then return through the safety ground. As the end of the energized coil burned back, it would remain only about 2 mm away from the shell, surrounded by a cloud of that recrystalized stuff and goodness knows what else. I suspect that nucleobyte's guess of "Flux coated welding/brazing rod" was pretty much dead on, from a functional standpoint.

--Rik

[rjlittlefield]

I don't know much analytical chemistry, but I'd expect the crystals would show themselves to be of aqueous-soluble compounds. Maybe something exciting like a spinel?

Or maybe something boring like MgO, boiling point 3873 K (well below blue-white), and listed by Wikipedia as being a typical insulating ceramic used in this type of element. The ceramic structure is described elsewhere in Wikipedia as being "a lattice of Mg^(2+) ions and O^(2–) ions held together by ionic bonds." I don't know much chemistry either, but to me this sounds like just the ticket to form a nice plasma at blue-white temperatures, and to quickly recrystalize on relatively cool surfaces.

--Rik