Warning-----Induction heating

[MJDamiani]

John,
I long time ago I was A Detective with a city police department. I was the new guy so, I got all of the stolen cars, and the cases that nobody wanted. I was assigned to work with Fire Marshal's Office. That where the education started. The first seminar that I was sent to was an electrical arson seminar. At the time I was a licensed electrician in my state and I had never heard of fire by inductive wiring. The instructor showed us the results of induction. Here it goes... You run 100' of extension cord to a Christmas thee in your front yard, the tree is only 50' from the house, you leave a COIL of wire half-way out to the tree. The next time it snows all of the cord is covered with snow, except the coil which so hot it melts the snow. Example #2 You have an extension cord (of any type) that is too long. You plug the cord into the wall and run the cord to the Christmas Tree, the excess cord is COILED UP and left under the edge of the rug. a thermocouple was placed in each of the coiled sections and enough heat was generated by the coil of wire to start a fire and it did start a fire in these classes. Most people know about basic electricity but, (CAPS INTENTIONAL) INDUCTION HEATING IS SOMETHING TO BR RECKONED WITH AND WHILE I AM AT IT. 12-2 ROMEX IS RATED FOR 20AMPS IN FREE AIR, IF YOU RUN ROMEX THROUGH CONDUIT IT LOOSES IT'S HEAT LOSS POTENTIAL AND THEREFORE IT'S CURRENT CARRYING CAPACITY

CHECK YOUR LOCAL CODES, THEY ARE ALL BASED ON THE NEC (NATIONAL ELECTRICAL CODE) AND THEY MAY & WILL SAVE YOUR LIFE

[awright]

Well, MJDamiani, I'm reluctant to challenge your instructor at the fire marshall's office, but I don't think what you saw was the result of inductive heating. When two wires carrying the identical current in opposite directions (as is the case in an extension cord unless there is a short to ground out at the end) and run parallel to each other and close together, the inductance and the external magnetic field are cancelled out except in very close proximity (less than the wire spacing) to one of the wires. If the wire pair is coiled, there is still no significant inductance because the opposing currents cancel each other out, magnetically.

Additionally, in the absence of some magnetic material whose molecules can be excited into vibration by the magnetic field of the coil, there would be no heating even if there was some significant unbalanced current in the cord and, as a result, a strong magnetic field.

I think what you were observing was the heating effect of the wire carrying current. When coiled, the heating effect of the wire is multiplied by the number of wires bundled together, and when covered with a rug, that excess heat cannot be dissipated to the air. The temperature HAS to build up until the heat loss to the air is in balance with the heat being generated by the coiled wire. If the heat escape is restricted, the temperature increases until the temperature differential is large enough that the rate of heat loss to the air balances the energy input from the cord OR the whole thing starts to smolder, then burn.

Despite these technical quibbles, I strongly endorse your calling attention to the danger of restricting the cooling effect of air on an extension cord by covering it or the danger of keeping an extension cord carrying any significant current coiled up so all the heat is trying to excape from a small area. Cords are definitely rated for being used extended and in free air.

awright

[MJDamiani]

Well, MJDamiani, I'm reluctant to challenge your instructor at the fire marshall's office, but I don't think what you saw was the result of inductive heating. When two wires carrying the identical current in opposite directions (as is the case in an extension cord unless there is a short to ground out at the end) and run parallel to each other and close together, the inductance and the external magnetic field are cancelled out except in very close proximity (less than the wire spacing) to one of the wires. If the wire pair is coiled, there is still no significant inductance because the opposing currents cancel each other out, magnetically.

Additionally, in the absence of some magnetic material whose molecules can be excited into vibration by the magnetic field of the coil, there would be no heating even if there was some significant unbalanced current in the cord and, as a result, a strong magnetic field.

I think what you were observing was the heating effect of the wire carrying current. When coiled, the heating effect of the wire is multiplied by the number of wires bundled together, and when covered with a rug, that excess heat cannot be dissipated to the air. The temperature HAS to build up until the heat loss to the air is in balance with the heat being generated by the coiled wire. If the heat escape is restricted, the temperature increases until the temperature differential is large enough that the rate of heat loss to the air balances the energy input from the cord OR the whole thing starts to smolder, then burn.

Despite these technical quibbles, I strongly endorse your calling attention to the danger of restricting the cooling effect of air on an extension cord by covering it or the danger of keeping an extension cord carrying any significant current coiled up so all the heat is trying to excape from a small area. Cords are definitely rated for being used extended and in free air.

awright

Hi California,
It is 0418hrs and +12 degrees here on the east coast. I was surprised at this school and I have been more than careful since attending. I thought that I learned a lot at all of the bomb schools! In the course they ( the Instructors) took an extension cord with a 100w light bulb in the lamp and approx 20 feet on lamp cord. They placed the lamp on the end table and the plug into the wall, coiling all of the un-needed cord under the cushion on the sofa along with a temp probe. the sofa burst into flames in 15 minutes and we all watched the temp. probe climb faster than we could believe. Seeing is believing, no more coiled cords at my home.

[steve45]

Hmmm... Something to think about! I never dreamed that you could get that much heating from a coiled up extension cord.

On a different note, our sofa almost caught on fire when a crystal ornament my wife had sitting on a table focused sunlight on it. I just happened to walk through the room and smelled something burning. The sofa now has a burn mark about 1/4" wide.

[wallythacker]

I agree with the induction/constrained area warnings. I had more than one coiled extension cord holder melt down when under load. The cord was not fully unwound and when all the portable computers powered up, meltdown.

Funny thing, I was working for Ontario Hydro at the time and the engineers thought the cord reels/cords were defective and sent them off for electrical inspection, which they passed.

It was a fluke I remembered about inductance and fully unwound the cords and we never had a problem again.

[Sober_Pollock]

I would first like to support MJDamiani. One simple remedy to this situation is what I do: NO EXTENSION CORDS ARE ALLOWED IN MY HOUSE!

Any place where my wife or I need to use an extension cord is simply an indication that I need to install an electrical outlet in that location, so that's what gets done.

Having said that.....

I would also like to support what awright has said:

The heating that took place is not likely to be caused by induction.....

Inductive heating is a very real and very powerful phenomenon. We use it in the mills to heat pieces of steel from ambient temperature to red hot in a matter of minutes by passing them through tuned coils. Inductive heating works by inducing electrical current in the object it is heating. This electrical current is what heats the object. The coil must be able to induce current in the object to be heated, hence the object must be able to conduct electricity. Inductive heating does not work on insulators (non conductive materials).

Further, the two current carrying conductors in an extension cord are in close proximity to each other, and if you coil them up together, you cancel out any electromagnetic field that might be produced. Now, if you were to "Peel" apart that extension cord, and coil one conductor by its' self, you would have a good coil with which to perform inductive heating with.

The heating that took place in the cited demonstrations was likely caused by, as awright said, resistive heating not inductive heating.

Non of this would be any comfort when a house burns down, and in my opinion (Journeyman electrician with an Electrical Engineering degree from Purdue) extension cords are downright dangerous for more reasons than we have talked about here.

Extension cords are for temporary use only. As I said earlier, anywhere an extension cord is being used on a long term or permanent basis, is a location that needs to have a proper electrical outlet installed.

[MoonRise]

Inductive heating? Nope.

Is there inductance in a coil of wire? Yup.

Is there heating in some wire coils buried under some insulating foam? Yup, yup, yup.

But it's not inductive heating. A home-made inductive heater can heat an M12 (about a 1/2 inch) nut to 780C (1436F, white hot!) in 45 seconds or so using about 400W of power and a frequency of 171 kHz. See http://www.richieburnett.co.uk/indheat.html

Inductive heating works to heat something via magnetic hysteresis and eddy-current skin effect. Magnetic hysteresis only applies to ferromagnetic materials (typically iron-containing metals, but many stainless steel alloys are not ferromagnetic). Eddy-current skin effect heating applies to an electrically conductive material, and the effect depends greatly on the AC frequency of the current with higher frequencies producing more effect.

The self-heating of a coil of wire is mostly due to I2R heating of the wire. Trap that heat under a nice thick foam pad (a pretty good thermal insulator) and the temperature can and will rise. But it's not inductive heating.

[Joe H]

Interesting thread...

So anyone ever built their own small induction forge?

[Rick Moran]

Ditto Sober & Awright -

Assoc Science Degree - Electircal Technology major.

Resistance vs Inductance heating. Ext. cords for temp use only & never piggy back outlet strips for computer/tv/periphreals. Seen this too many times.

[MJDamiani]

Resistive heating / inductive heating,

The whole point is--- I started some productive dialog that may keep someone alive and safe, and that was the object of my post. The engineers can argue the terminology all day long.

[Rick Moran]

You are right there MJ, and that IS a positive thing to get out the word on safety related stuff we are all guilty of doing without thinking sometimes. Particularly at Christmas. Probably need to resurrect this post again next year. :laugh:

[awright]

Yes, JoeH. Not me, but many others. It takes some sophistication in ultrasonics/low RF(Radio Frequency) electronics, coil design, shielding, etc. It is a widely used industrial technology.

And yes, MJDamiani, you started an interesting thread. Thanks.

My interesting little book, "High-Frequency Induction Heating," by Frank Curtis, 1944 (another find in my local independent used book store, for $3) confirms that the heating effect is primarily the result of eddy-currents in a conductive workpiece, not the magnetic heating I erroneously referred to in my original post. Additionally, a less used but important field is dielectric heating applied to such things as curing glue lines, curing plastics, etc. This is the molecular excitation I was thinking of, but is still not dependent upon magnetic properties, so I was doubly wrong.

Curtis presents an introductory table of frequencies used for various types of work, from 60 Hz for "low temperature heating and annealing" through 1000 Hz for "Deep-Heat-penetration, melting" to 1,000,000 Hz for surface heating of small objects.

It is a very useful technology, but would take money for a fairly powerful sonic/ultrasonic or RF generator and some study and sophistication for the average metal worker to apply to his specific project. It is also more suited to a production situation, rather than a one-off project situation, since you must select a frequency and design and fabricate a coil electrically and physically suited to your workpiece. Kind of as if you had to fabricate a special welding torch for each of your welding jobs.

You also have to be sure that your coil properly terminates the transmission line from the generator, since a serious impedance mismatch can reflect too much energy back into the generator, frying it.

Interesting, though.

awright

[MJDamiani]

You are right there MJ, and that IS a positive thing to get out the word on safety related stuff we are all guilty of doing without thinking sometimes. Particularly at Christmas. Probably need to resurrect this post again next year. :laugh:
Rick,
It looks like you have the background to understand this. The next experiment they did in the school (Titled- "Fatal Fires") was to burn down a structure that was build just for this purpose. The build was wired with the old standard BX armored cable. A dead short was created between the hot wire and the steel armor of the cable. It looked like a toaster in approx. 5 minutes, the armor was glowing red hot and the entire building (wooden shed) was up in flames. They let the building burn to the ground surrounded by fire trucks (controlled burn) before hosing it down.

Life saving Tip # 2 replace old BX cable


Mike

[Rick Moran]

I believe BX is still allowed in the code and is used, or at least found, on old recessed lighting. I had some on my old ventilation fan hood over the stove. Evidently, the breaker failed, or they didn't use the correct rating to lock-in the short for demo purposes. If everything is correct, it shouldn't happen. Of course, that's what makes it an accident. Usually because people don't know what they're doing by putting in a higher amp breaker because the old one was popping when the microwave was on high.....

I would have liked to see the cable / burn test though. I'll bet it was glowing like a toaster wire! A short circuit will burn until it creates an OPEN circuit.
:blob2:

[MJDamiani]

I believe BX is still allowed in the code and is used, or at least found, on old recessed lighting. I had some on my old ventilation fan hood over the stove. Evidently, the breaker failed, or they didn't use the correct rating to lock-in the short for demo purposes. If everything is correct, it shouldn't happen. Of course, that's what makes it an accident. Usually because people don't know what they're doing by putting in a higher amp breaker because the old one was popping when the microwave was on high.....

I would have liked to see the cable / burn test though. I'll bet it was glowing like a toaster wire! A short circuit will burn until it creates an OPEN circuit.
:blob2:
The new armored cable is called MC. It contains an insulated (Green) ground wire along with the other conductors.

[MoonRise]

BX is fine.

MC or AC are fine.

They all can be had in different 'flavors'. Different metal outsides (aluminum or steel), different AWG conductors on the inside, different quantities of conductors on the inside(1, 2, 3, 4, or more with or without ground and/or bond), different color wires on the inside (black, red, white, green, brown, gray, orange, yellow, purple, etc), different colors of metal on the outside. The CABLE would still be AC or MC or whatever.

Bypassing a circuit breaker and putting a dead-short from a 120V hot to the grounded metal of an armored cable will of course heat things up. Putting electrical power into a resistive conductor makes heat. Enough power and enough heat and you can make fire.

Now I'm all for safety warnings. But PLEASE use the proper correct terms and don't scare-monger things.

BX is fine. An extension cord is fine. A welder is fine. They are all just fine when used and maintained properly. Overload an extension cord or coil it up underneath a foam pillow where the normal heat of the current through the wires in the cord or cable just builds up and you can make a fire. Short out a section of armored cable to the metal armor and by-pass a circuit breaker and you can make a fire. Replace a burned out fuse in a fuse panel with a penny and you can make a fire. Use a welder improperly and you can make an unintended fire or possibly cause an electrocution with an arc welder.

[MJDamiani]

To all,
This has been some very interesting dialog with some outstanding input from a variety of knowledgeable individuals. Some pro, and some con. I love a lively debate because everybody learns something. The thing to remember in a democracy, is "we all agree to disagree". What a novel concept !

Thank you again, and let the debate continue.

I need to go out the the garage and melt some metal

Mike

[steve45]

This is certainly one of the more interesting threads I've read here. Sober_Pollock, I have to agree with you, now that I think about it. It's not induction heating, but anyway...

Good to see another Boilermaker! I'm a BSME '75!

[tresi]

I bet it won't be long before see a couch or throw rug go up in flames on mythbusters.

[littlefuzz]

Where I used to work I built a set of metal spools to roll our lead up on for the portable rig. They were also removable. One day in the shop we needed a little more lead to reach the other side of the shop for the tig. Part of the roll was unwound but left some on the spool. It wasn't long until there was burning rubber smell throught the whole shop. Yup that was it, it was smoking when I went to see. It didn't ruin the leads though.

[Doolittle]

Wow, good topic. Thanks for the heads up.

[MJDamiani]

I am attaching a link to today's Providence Journal (the statewide newspaper)
http://www.projo.com/news/content/Provfire_update7_02-07-07_KP4AD8L.1b1870a.html

I have received intelligent dialog from the members of the forum. However some people in the outside world are just not up to speed regarding the proper use of electricity. I hope that you can open the link, if not, send me you e-mail addresses and I will send it to you . The news article is about a father loosing his life attempting to save his family from their burning home. The fire was caused by an extension cord that was overloaded and running under the carpet. This is not "Scare-mongering" as it was sarcastically stated by Moonrise. it is just dissemination of information learned by a lifetime of seeing other people's misfortune, and many hours, days, and weeks of schooling at the governments expense

Keep the post going, we are getting smarter with each passing minute

[MoonRise]

MJ,

Sorry to hear about the father losing his life in the house fire.

Putting an extension cord underneath a carpet or rug or a sofa cushion is a MISUSE of the extension cord. It can and does lead to fires. Absolutely.

Remind people to use the objects properly.

But use the proper terms and reasons for why and how.

There is nothing wrong with using an extension cord. There is plenty wrong with misusing an extension cord, such as by concealing it under a rug or coiling it up and sticking the coil under a rug or cushion. But the reason is because the cord normally warms up when used because of resistive heating in the conductors and the cord needs to be spread out and in the open to cool off, not because of inductive heating.

In normal use I've felt an extension cord or tool cord and felt how warm it has and is getting. Take that heat and concentrate it by coiling the cord and then insulate that heat underneath a carpet or foam cushion and there very likely could be a melt-down and/or fire. So the safety reminder is to not coil cords up when they are in use and to not 'hide' cords underneath a carpet or cushion or whatever.

Another reason it is a misuse of an extension cord to hide the cord underneath a rug or carpet is because then the cord is 'forgotten' and may get stepped on or otherwise physically damaged, Which can cause a fire for various reasons, like increased resistance in the wire because of broken conductors which means increased heat and because the cord is underneath something and hidden and/or forgotten then the fire starts even easier.

Scare-mongering is using a tidbit of fact and extending the scare factor. Like mentioning the (intentional) misuse of some BX in the burn exercise and saying that BX is no good. BX is fine. Misused BX can cause a fire.

Romex (NM-B actually, Romex is a brand of NM-B just like Kleenex is a brand of facial tissue and Xerox was once the pre-eminant photocopier) can cause a fire if misused as well.

Properly used, electricity is useful and generally safe. Misused or abused, electricity can cause fires and/or kill. Heck, we -use- electricity in arc welding to melt STEEL or other metals. That's a goodly amount of power there to melt steel and an arc welder can do that pretty easily.

The built-in cord of the vacuum cleaner in my house gets dang warm in use. Leave that coiled up and running for a longer period of time and a melt-down or fire could occur. But for the typical duty-cycle of a household vacuum cleaner and if the cord is out in the open, everything is fine.

A neighbor had a typical 14 gauge orange outdoor extension cord, maybe a 50 or 100 footer, mostly coiled up on the cord reel and was using some outdoor tool. He melted and charred the cord insulation and then it melted/arced the copper wire.

[Tailshaft56]

Non of this would be any comfort when a house burns down, and in my opinion (Journeyman electrician with an Electrical Engineering degree from Purdue) extension cords are downright dangerous for more reasons than we have talked about here.

Extension cords are for temporary use only. As I said earlier, anywhere an extension cord is being used on a long term or permanent basis, is a location that needs to have a proper electrical outlet installed.[/SIZE]

That is the one of the reasons the newer codes require more outlets. There was a day when rooms usually had only one or two outlets. The code now requires an outlet every 6 to 8 feet. I don't have a code book or I would be more exact. Many a house has burned do to an extension cord in continuous use. It's very easy to over load a "lamp wire" type cord. It's also possible towear out those light duty cords over time. One is less likely to notice a problem on a continuous use cord than one that is being switched between being stored and being used and back. (Hope you follow this last remark)

[MoonRise]

For residential electrical convenience outlets, the NEC spacing requirement is no space along a wall can be more than 6 feet from an outlet. So that is actually an outlet every 12 feet along the perimeter of a room. There are different spacing requirements in bathrooms and along kitchen countertops, as well as different circuit requirements in kitchens and bathrooms. I think hallways had a possibly different spacing formula as well. And there are different 'reset-the-distance-and-measure-again' rules/formulas/exemptions for doorways and windows.

Different spacing and circuit rules apply to commercial and industrial buildings.

Electricity is mighty powerful and useful. Respect that power and use it properly and it is useful and generally safe.

Misuse it and you can die. Either the electricity can kill you, or it can start a fire which can kill you.

[Brad Blazer]

My interesting little book, "High-Frequency Induction Heating," by Frank Curtis, 1944 (another find in my local independent used book store, for $3) confirms that the heating effect is primarily the result of eddy-currents in a conductive workpiece, not the magnetic heating I erroneously referred to in my original post. Additionally, a less used but important field is dielectric heating applied to such things as curing glue lines, curing plastics, etc. This is the molecular excitation I was thinking of, but is still not dependent upon magnetic properties, so I was doubly wrong.

awright

Dielectric heating may have been "less used" in 1944 but today most of us use a microwave oven every day. :)

[awright]

Yes, I thought of that, but was not sure microwave heating came under the description of dielectric heating, so tried (in vain, it seems) to dodge the issue by not mentioning it. The implication of the statement in the book I cited was that the dielectric heating worked by electrostatic forces on the molecules of an insulator, including those of uncured plastic, while the heating of a microwave works by oscillating the water molecules in the food. Since I really have no detailed knowledge of the electromagnetic or molecular level process in either and didn't have the motivation to research it, I tried to bail but my shirt caught on the door latch.

For one thing, the frequency of a microwave is (I think) in the 2.4 GHz region and is driven by a magnetron launching electromagnetic waves into a closed "reverberation room" via a waveguide, whereas the frequency being discussed in the book for dielectric heating was in the low MHz region and is applied with a pair of plates on each side of the work piece. This may be a distinction without a difference, but since I didn't know, I decided not to mention it. I don't doubt your statement. Happy to receive further elucidation.

awright

[deadman11699]

That's good to know in my shed I have power cords wraped up and running everything. Looks like Im going to have to put some money into it and run it with proper outlets.