My shed extension lead was stored in a neat, wind-away cassette, holding about 15 feet of cable. My son was doing something with his motor bike involving an electric drill but its lead was a little too short, so he unwound about 2 feet of the lead and went to work on it. The cassette caught fire and burned a hole in the bench.
Has anyone else experienced this? I think there should have been a warning of it, embossed on the casing.
I don't understand how heat was generated in the coil, not having done physics at school. Acting like an induction coil perhaps?
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Caught fire after an interval, that is.
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>> I don't understand how heat was generated in the coil, not having done physics at
>> school. Acting like an induction coil perhaps?
It is an induction effect. If you visit a carvanning forum you'll not look far to find horror stories of mains hook up cables left coiled and causing damage to the cable and reel and in some cases the 'van too.
The cassette should have a warning to uncoil completely before use, certainly so if being used for high load applications. I think one of mine suggested uncoiling fully if appliance was over 500 watts. Is it possible a label had dropped off?
Had a close call with one at work while sat the Quango. Somebody setting up a meeting plugged two catering size coffee filter machines into one without uncoiling it. Fortunately a participant in the meeting spotted the risk and pulled rest of cable out.
Last edited by: Bromptonaut on Sun 19 May 19 at 11:26
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The ones we have at work won't work until the cable is completely unwound. I'm surprised this safety switch isn't mandatory.
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It's a known and well publicised risk. Some loon on the web even connected 6 in series to see which one would bring up first.
Not only is it an induction effect it's a heat soak effect
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>> It's a known and well publicised risk. Some loon on the web even connected 6
>> in series to see which one would bring up first.
>>
got a link?
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>> It's a known and well publicised risk.
Absolutely it is. As I said earlier there are any number of pictures on, for example, Caravan and Motorhome Club forum and Facebook page of EHU cables that have self immolated after being left on the reel.
Doesn't stop the odd Muppet from claiming it's OK really because he's left his coiled for years and it hasn't burst into flame.
Yet.......
Last edited by: Bromptonaut on Sun 19 May 19 at 13:54
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I know and alwats managed that risk. Currently ( :-) ) have a 30m cable for the lawn mower, always unwound when in use. I thought it was to do with a build up of heat from increased resistance - anyway, always unwind it !
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As Zero says it is a combo of induction and heat soak.
All units will state an unwound (typically 3000W) and wound max load (typically 720W).
Your warning probably melted...
When I charge my car at my cottage I use a 3kW rated cable (charging at around 2kW) and the plug and lead warm up but don't become hot- first couple of times I used it I checked every 30 mins for a few hours to ensure it wasn't developing any hotspots.
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>> All units will state an unwound (typically 3000W) and wound max load (typically 720W).
Just checked mine while lending it to next door who've lost there own (or rather it's buried somewhere in their garage). Her lad who's about 18 needed it to mow the grass. It says max load 750 fully would and 3000 and a bit if unwound. Don't know what a medium size Flymo uses but I reminded him to unwind it fully.
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i thought the heating was was due to the electrical resistance of the cable when a current is passed through it. Is that the same as induction? I though that was something to do with magnetism It's a long time since "O" level physics.
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>> i thought the heating was was due to the electrical resistance of the cable when
>> a current is passed through it.
It can be, and normally the heat dissipates, but not when it curled tightly. So we get small scale heat soak if its coiled up tightly.
>>Is that the same as induction?
When you have the cable coiled up, you generate an EMF, the EMF screws with the resistance/inductance of the cable (more so with impure copper cables) increasing the heat generated
Add that to the heat soak and ...........
Last edited by: Zero on Sun 19 May 19 at 19:51
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>>When you have the cable coiled up, you generate an EMF, the EMF screws with the resistance/inductance of the cable (more so with impure copper cables) increasing the heat generated
Wonder what would happen if the coil was wound round a metal bar...
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>> Wonder what would happen if the coil was wound round a metal bar...
Magnetic north would be disrupted in your garden,
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I am no expert, and perhaps quite wrong, but I believe that it is nothing to do with EMF and all those other clever things. It is simply that wire has a natural resistance which results in heat generation. If it is coiled up, then it cannot dissipate that heat.
Don't forget, there are two current carrying wire and essentially it's going in opposite directions. Surely they would disrupt/cancel each others' fields?
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>> I am no expert, and perhaps quite wrong, but I believe that it is nothing
>> to do with EMF and all those other clever things. It is simply that wire
>> has a natural resistance which results in heat generation. If it is coiled up, then
>> it cannot dissipate that heat.
>>
>> Don't forget, there are two current carrying wire and essentially it's going in opposite directions.
>> Surely they would disrupt/cancel each others' fields?
Technically, the resistance of the wire itself doesn't change due to bends in a wire. But the wire is much more than just a resistive component. With enough turns/loops you can make the wire exhibit inductive properties.
Every current carrying conductor produces a magnetic field in the plane perpendicular to the flow of current, and revolving around the conductor.
Each loop of the inductor exhibits a capacitance effect with the next loop. That is, a looped wire will exhibit all three properties: inductance, resistance and capacitance.
So, explicitly, the resistance of the wire doesn't change when you bend it. However, the potential drop across it can change enormously by the way you bend the wire and make it interact with itself.
All valid for AC
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"It can be, and normally the heat dissipates, but not when it curled tightly. So we get small scale heat soak if its coiled up tightly."
Yes I get that bit
"When you have the cable coiled up, you generate an EMF, the EMF screws with the resistance/inductance of the cable (more so with impure copper cables) increasing the heat generated"
So you are saying that when you pass a current through a coiled up 10 metre cable say it will actually generate more heat than if the same cable were to be laid out straight? In effect the resistance of the cable is increased by it being coiled up?
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Cables get slightly warm uncoiled - I've noticed caravan mains hook ups melt snow or frost.
Pretty sure there's more than just that level of heat in a coil.
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>> So you are saying that when you pass a current through a coiled up 10
>> metre cable say it will actually generate more heat than if the same cable were
>> to be laid out straight?
Yes
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>> Yes
I asked Mrs B to explain the physics but in spite of teaching subject to GCSE she's decided she's only a chemist.
Can you fill in the blanks?
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>> Can you fill in the blanks?
Ahh, we cross posted, you already have.
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Like the comedy game show?
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Ok then. That's something I've learned
So presumably since the energy needed to heat the coiled cable is greater than the straight cable there will be a measurable loss of current in the coiled cable compared with the straight?
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>> Ok then. That's something I've learned
>>
>> So presumably since the energy needed to heat the coiled cable is greater than the
>> straight cable there will be a measurable loss of current in the coiled cable compared
>> with the straight?
Err now you are getting a bit clouded. There wont be a loss of current in the coiled cable there will be more current which the cable will dissipate as heat. I guess what you meant was would there be less voltage available at the appliance on the end, and the answer is that if there is more resistance/impedance/capacitive effect in the supply lead, then the answer could be yes.
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I suspect that somewhere in this thread the issues have been over complicated, by the introduction of factors which have very little effect in the real world.
The inductance of a single wire coiled in 25cm loop in air with 50 turns is less than 10mH - and if the load is purely resistive the return current in the cable will effectively cancel out that inductance.
A solid iron core would increase that very significantly, but you would not be able to lift the reel!
The effective capacitance between the turns will create a complex model which in association with the inductance becomes a tuned circuit. But not of any consequence at 50Hz.
If you want to get detailed why not introduce eddy currents in the core and skin effects of the wire.
I would stick to the explanation of local heating of the cable and the inability to dissipate the heat in the enclosed space of a coiled up cable drum. You only have to look at the necessary derating of conventional domestic wiring when enclosed (eg conduit) vs clipped to joist to get some ideas.
I may try some back envelope calculations later to see the amount of power dissipated in a typical cable reel.
I am happy to be proved wrong - but back it up with some decent theoretical calaculations or a source document with some credibility.
Last edited by: sherlock47 on Mon 20 May 19 at 08:36
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So in summary. "It gets hot when coiled - Bad"
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>> I suspect that somewhere in this thread the issues have been over complicated, by the
>> introduction of factors which have very little effect in the real world.
Lets overcomplicate it even more then. Lets add the resistive effect at higher temperatures of poor quality conductor material to the melting pot.
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>> >> I suspect that somewhere in this thread the issues have been over complicated, by
>> the
>> >> introduction of factors which have very little effect in the real world.
>>
>> Lets overcomplicate it even more then. Lets add the resistive effect at higher temperatures of
>> poor quality conductor material to the melting pot.
Ah I forgot to throw into the pot, stress gauge cracking of the conductor material (caused by coiling it)
Last edited by: Zero on Mon 20 May 19 at 09:15
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Let's keep to a perfect world and not muddy the waters.
Is there any significant measurable heating effect in a coiled electrical cable other than that caused by the resistance of the wire and the inability of the heat to be dissipated.?
You have told me there is and it is caused by induction.
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>> Let's keep to a perfect world and not muddy the waters.
>>
>> Is there any significant measurable heating effect in a coiled electrical cable other than that
>> caused by the resistance of the wire and the inability of the heat to be
>> dissipated.?
The waters are muddy. Heat soak, inductance, physical properties of the cable when bent even the type* of load on the end are all factors. You can pick any one of them and say it has an effect, but they can all add up.
*there are many types of loads, all may have similar quoted power consumption in watts, but cause different behaviours and types of load.
you need to get hold of a HOPI (energy meter) and test it coiled and uncoiled I think.
Last edited by: Zero on Mon 20 May 19 at 09:43
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The question arises is, how can young people be expected to know about such hazards? Mine had an earlier brush with electricity and I may well have saved his life. He sat in a filled bath and turned on the mains radio he had taken into the bathroom, via an extension lead in our bedroom. By pure chance, I went upstairs to get something from the bedroom and switched off the power. He hadn't seen the Bond film in which a villain throws a live electric fire an occupied bath.
My wife's dippy aunt lived in old cottage with the bath in the kitchen. With in an arm's length of the bath was an electric kettle, on top of a counter which was on he same level as the bath rim. She told us how convenient it was to make a cup of tear without having to get out of he water. I hoped she remembered my warning and explanation but she died later in a road accident..
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>> The question arises is, how can young people be expected to know about such hazards?
>> Mine had an earlier brush with electricity and I may well have saved his life.
Parents teaching them I guess :-)
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>> The question arises is, how can young people be expected to know about such hazards?
Shock tactics ;)
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Getting a shock does tend to make you a bit more cautious although since I’ve had a number I guess I’m a slow learner.
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I have had more than my fair share, both professionally and domestically. Couple of scars on hands as lasting testimony. Pretty blase about domestic mains, but was considerably less so when dealing with higher voltages via multiple phases.
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Holding a scrambler's sparkplug while turning over the motor "to see it it was making a spark" learned me real good.
(I learned it was fine as long as the electrode/threads of the plug were touching the crankcase, but not if I was an intervening conduit!)
Last edited by: Lygonos on Mon 20 May 19 at 13:27
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Oh, and also that a central pendant and wall lights may have totally different fuses despite sharing the same double light switch.
Ooooyah.
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When I was a kid (maybe 7 or 8) I had a light switch for the centre light dangling over my bed. One night I unscrewed to bottom to look inside, and turned it on by sliding the brass bar across. Certainly got a shock but didn't dare tell my parents!
That was the first, but not the last...
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All part of learning.
I have seen a pair of scissors with a hole blown through one of the blades.
I was told that " if you want to check if it is live mains you can use the back of your hand"
If you see a "sparks " up a ladder with his hand in his pocket , he is not a lazy ***, it is so he cannot grab an earth if he finds the circuit is live.
A good idea in the domestic scene is when working on a circuit, if possible, put the fuse in your pocket to avoid anyone possibly reactivating what you have just disabled.
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>>
>> If you see a "sparks " up a ladder with his hand in his
>> pocket , he is not a lazy ***, it is so he cannot grab an
>> earth if he finds the circuit is live.
>>
And isn't it true that if you do get a shock from mains AC you can pull away the part of your body that's touching either supply or earth? (if you're still conscious). I heard that if it's DC, you can't pull away until the current is switched off.
Robert Shaw in one of the final scenes of 'The Taking of Pelham 123' cooking himself on the 3rd rail.
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>> And isn't it true that if you do get a shock from mains AC you
>> can pull away the part of your body that's touching either supply or earth? (if
>> you're still conscious). I heard that if it's DC, you can't pull away until the
>> current is switched off.
Its not at all straightforward when it comes to electric shocks, there are generalities, but so many variables that hard and fast rules about shocks, ability to ungrip, whats fatal, etc from dc or ac can not be relied upon.
Here is a guy who shocked himself for our amusement and education.
www.youtube.com/watch?v=-5R-KBa18ME
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>>Here is a guy who shocked himself for our amusement and education.
Big Clive is epic - his breakdowns of shonky adaptors etc is quite sobering.
240v live 0.5mm from the charge lead supply on one cheap ebay adaptor...
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>> >>
>> >> If you see a "sparks " up a ladder with his hand in his
>> >> pocket , he is not a lazy ***, it is so he cannot grab
>> an
>> >> earth if he finds the circuit is live.
Trouble is he should also be adhering to the "three points of contact" rule. Unless he is an octopod he wont have too many tentacles to do any actual work.
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>> Oh, and also that a central pendant and wall lights may have totally different fuses
>> despite sharing the same double light switch.
>>
>> Ooooyah.
Or that neutral can bite!
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I used to test dry cell batteries by wetting finger and holding it to the base whilst putting the terminal on my tongue. You get a mild tingle if there any power left
I did the same one day with one of those PP9 batteries. Boy does that make you jump
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One of the first fuzes I built was a PP9, switch, and wire wool. it had sufficient current and voltage to work over a long wire.
Yes I was a bit of a pyromaniac as a child. Managed to "blow up" a dumped car once.
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The Ambos must be hazard-prone family. When I was sixteen, and both of my parents were away for the day, I decided to string a long cable (left by the previous owner) overhead, to the former stable of the house. I fitted a plug and plugged it in to the empty and unused light socket of the unused coalhole and began stringing. I got it inside, down to the place where I hoped to build a bench. The cable was far too long so I took a pair of uinsulated pliers and squeezed tight. You guessed it...
Education has been defined as a change of behaviour resulting from experience. Dead right!
But there's more. When I was very young, in a former house, we had a twin-bar electric radiator on the floor of the bathroom (you know how cold it can get on the North Sea coast). My father designed the house and supervised it's building, and I can't believe he specified a bathroom socket, or that an electrician would comply. Our (now) old friend, the extension cable, must have been pressed into service.
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>> (you know how cold it can
>> get on the North Sea coast).
Oh Yes - The Hoar frost aided by the east wind was the worse, you would get to school with the front of your duffel coat and face covered in white frost.
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Strangely enough power sockets in bathrooms are the norm in Europe and Canada. I think it's a purely UK regulation
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>> Strangely enough power sockets in bathrooms are the norm in Europe and Canada. I think
>> it's a purely UK regulation
And in the countries of South America, and in my house in the States.
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>> And in the countries of South America, and in my house in the States.
One place we stayed in the States the coffee maker was in the bathroom.
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in Las vegas we had a phone, TV, and a minibar in the bathroom.
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The phone and TV is quite normal. The mini bar less so.
Where did you stay? I always stay at the MGM whenever I can. I love the place though I don't gamble. Not with money, anyway.
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The venetian, a mid priced split level suite
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Been once, stayed at the Excalibur.
Impressed by the monstrous gastropods using moving walkways to travel from one hotel to the next.
No wonder Alabama passed their recent laws on abortion - Evolution is in reverse.
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>> No wonder Alabama passed their recent laws on abortion -
Bubba was getting bored with the lack of sisterly choice and the source of mute banjo players was declining.
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Wandering round the net to see if somebody else had done the hard work on heat loss calculations I found this,
physics.stackexchange.com/questions/282465/what-happens-in-a-tightly-wound-up-cable-reel-under-ac-power
Whilst it is not totally definitive, it supports the view that heat issues are the primary problem.
Maybe the answer to improving safety amongst a non technically literate population is the introduction of a Wimshurst machine at Key Stage 1. That could teach some respect :)
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Ignitable Extension Lead - Ambo
