Another electrical question

[craigb]

My 15 amp extension cord has a safety card on it that says "Caution: Comletely uncoil before using".

So what's the big deal? As far as I can see, the worst that can happen if I use it coiled is I'll induce a magnetic field around the coiled cable .

So what?:confused:

Is there more to it than this?

I've pondered this for a while and the time has come for the definitive answer. :D

So...?

[echnidna]

The electrons are much happier running in a straight line, they get dizzy if'n they have to run around in circles:D :D :D :D

[Wood Butcher]

In inducing the magnetic coil, I think that you also generate a certain amount of heat. If there is enough "coil" generating heat you could (and it is a very loosely defined could) cause the plastic to melt and the wires to short. You would need a decent size coil of lead and to have sufficient amount of continous current passing through the lead for this all to happen.

[Clinton1]

Yep, heat.

[Sturdee]

In my younger days I used a 30 metres extension cord without fully unwinding it. After a few hours the wound up part of the cord became quite warm. Now I always uncoil a cord before use.


Peter.

[outback]

I have seen the result of a really long cord left mostly wound on the hose reel on which it was stored. After a few hours of use it let out all the smoke. :eek:

[craigb]

Well I've got a 15m cord and the coils are about 450mm in diameter.
Even drawing max current all day (which I don't) is that sort of coil really going to generate a problem amount of heat?

I kind of think that Boban's explanation is the most likely.

Ie cover your asre with both hands.

[boban]

I deleted my post because even though I think its unlikely, I still, out of habit or overcaution, undo my cords completely.

Im no scientist so I'd probably better keep my trap shut.

All said with both hands on my bum

[journeyman Mick]

Years ago my brother's mate came over to my brother's place to do some welding. Used a tradesman's single phase arc welder for a good part of the day, running off an extension cord that was in a pile on the ground. My brother went to use the welder the next day and it wouldn't go. He went to uncoil the lead to see what was wrong and discovered that the lead had melted together in several places and shorted out. :eek: I don't think it's the magnetic field being induced that causes the heat, just the current draw.

Mick

[Harry72]

It makes a big inductor!

[Bodgy]

Covering one's ass against heat.

If you really ran the 15 amps to its max, probably 25amps, then, if coiled, it would overheat.

Another example of Seppo and lawyer inspired bs.

[Schtoo]

The reason they out that on the lead is because it's a plug in thingy, and as such the regs regarding it are open to interpretation.

Short version is there are no enforcable regs to speak of. ;)

If it was fixed wiring, there are rules and regulations governing how many bits of wire are allowed to fit into a certain space in a certain place.

As 'they' can't know how or where the lead will be used, they try to minimize the worst case scenario by taking one factor out of the equation, that being several bits of wire all heating each other up.

'They' figure that taking out a big bit of the problem makes the problem (the problem being turning customers into crispy critters) unlikely to occur, even when you do lots of other bad things to the lead.

And it's only resitance.

If the cable was, say 15mm in diameter per wire, then inductance does come into it and you have to fix the cable in a proper manner to reduce the effects of inductance.

And there you go.

Schtoo, card carrying dumb ex-sparky.

[knucklehead]

Craig most 15 amp cables are over designed. Nearly all 15 amp extension leads are used by tradesmen or in industry. Therefore the cables are heaver duty (both wire and insulation). If you are getting a 15 amp cabel hot there maybe something wrong with it.
With standard 10 amp extension leads the quality of construction varies widely. Running max current through a cheap (thin) lead while it is coiled up will produce a lot of heat. The same load run through a better quality (less current loss and thicker) will have no effect.

I only by heavy duty extension leads, which are really 15 amp cables with 10 amp plugs on the end. This gives a lot more protection against physical damage but also lessens the current loss and heat build up.

[Clinton1]

For all the different views on the subject, using it when coiled has led to enough accidents to warrant the warning being used.
Do it and cause a fire or accident and it'll probably void the insurance. Doesn't take much to uncoil it and stay safe. ;)

[Iain]

The Medical profession are major shareholders in cable companies, by uncoiling the leads gives one something to trip over thereby generating dual income (dividends and business).......honest:rolleyes:
If the cable is wound on a metal spool it will generate more heat than on a non conductive spool......fire risk is the key word here, regardless of how slight.

[craigb]

Well I'm not advocating that anybody should use their cables coiled. I was merely curious as to why the warning label.

It still seems to me that it's mainly asre covering.

Anyway, thanks for the info. :)

Cheers
Craig

[Barry_White]

Running an electric drill wouldn't be a problem. Run a welder a whole different kettle of fish. I have run a welder high up on the duty cycle with the lead uncoiled and it has got warm.

[doug the slug]

Well I'm not advocating that anybody should use their cables coiled. I was merely curious as to why the warning label.

It still seems to me that it's mainly asre covering.

Anyway, thanks for the info. :)

Cheers
Craig

yes it is,to an extent, but with high-end usage there is a genuine danger, aned what the sparky's wont tell you that falls into the realm of us amateur radio operators is that it can cause TV and radio interference too. Even if they are coiled up and not even plugged in, a strong radio signal can induce sufficient current into the loop to emiit a signal, albiet a weak onehttp://www.woodworkforums.ubeaut.com.au/images/icons/icon7.gif

[q9]

Actually is the magnetic field that adds to the heat produced...Electric motors with lots of tight coils do get quite hot after all - but that is designed into them.

[Schtoo]

Actually is the magnetic field that adds to the heat produced...Electric motors with lots of tight coils do get quite hot after all - but that is designed into them.

Huh?

I don't buy that, not in a little lead. :rolleyes:

In standard wiring, the magnetism concern is more about destructive problems than heat. About the only devices that use electromagnetic effect to do anything in regard to heating are induction heaters, and they don't work too well unless you crank up the frequency of the current.

In a motor, the reason it gets warm is mostly because of the resitance in the windings, not because it's a spinning magnet. If the core was a solid chunk of iron, the thing would melt down due to inductive heating, but the cores are laminated to avoid eddy currents building up and melting the motor into slag.

Anyways, a motor might heat up due to magnetism, but it's a miniscule percentage when compared to the resistive heating of the windings and other losses.

Heck, wind resistance is prolly a bigger loss... :eek:

[Cliff Rogers]

I'm with Schtoo on this one, it the resistance & the heat that is the main problem.

A short, small, low freq AC coil that is counterwound (IE 2 conductors side by side with current flowing in oppisite direction cancel each other's magnetic field) has very little inductance.

The problem is about 99.99% resistance.

If I run my big compressor on a straight 5m 10Amp ext cord, the lead is warm by the time the compressor is up to pressure.
If the lead is lying straight out across the concrete floor, it cools down fairly quickly.
If it was wrapped up in a heap, it wouldn't cool so quickly.

Doug, the stuff you are talking about means nothing to sparkys 'cos it's high freq & that is a whole different bag of worms.

[bennylaird]

Remember we are talking air core and high cross sectional area of the coil as well as low frequency. While a small amount of inductance will occur I doubt if you would either generate much in the way of rf or dissipate to much power as heat. Remembering the coil will produce apparent power not true power. True power is purely Resistive as Cliff so rightly put it.

Now if you were to wind the lead tightly around an iron bar then you may have problems. Hence the reel being a problem.

However it pays to keep your nether regions covered which is what the warmning is.

[Andy Mac]

It is something drummed into most tradesman, but I tend to go with it being a normal heat build up that can't dissipate due to the thermal mass. I just put it to a couple of guys here who disagree, one the OH&S officer and the other an ex-RAAF elec. fitter. Both of them are of the opinion its a back EMF buildup that increases the current through the coiled wire, and that exceeds the wire rating. I've also put it to a tradey electrician who spun me some yarn about the reel inside starting to spin due to induced current and friction builds up from all that spinning!!:D LOL
Whatever the cause, unroll the lead and loosely spread it across the floor.

Cheers,

[Ivan in Oz]

Huh?

I don't buy that, not in a little lead. :rolleyes:
Heck, wind resistance is prolly a bigger loss... :eek:

Schtoo,
You mightn't Buy it;

BUT
You WILL pay for it, sooner or later,
normally later, Believe me:)

[craigb]

O.K. so it's the heat either from resistance or induced magnetic field.

Actually, I generally only use my 15 amp extension when I have to move my TS further than the cord on the saw will allow.

Like I said, it's usually loosely coiled on the floor in around 450 - 500mm diameter loops.

I've never noticed the cord become warm but then I'm generally only making a few cuts before moving the saw back to its normal position.

Every time I do it though, I see that little safety label and feel guilty about the looped cord. :o

If I ever get a welder, I'll definitely fully uncoil my extension lead. :p :D

[bennylaird]

The only way you can get the heat is from resistance, ie True Power. Reactive power is stored and returned and produces no heat. Back EMF if anything will oppose the voltage producing it and therefore reduce the current. Back emf will give you a good kick but you would have to be touching the bare cable which with 240V live I think is beyond the stupidity of most humans, animals as well, fear of pain does it.

Basically a coiled cable can't dissipate the heat, gets hot resistance goes up, current goes down. If it gets too hot it melts the insulation, short occurs, fire starts if breacker doesn't cut out in time . nuff said?

[Cliff Rogers]

Yeap, back EMF is horses hit.


In the end, as Craig says, it is heat we are trying to avoid.

[DavidG]

Resistive heating.
Solid mass unable to dissipate heat.

Don't make me dig out the electronics books and actually calculate the inductance of a 30 meter coil of three core wire, but it will be fairly low.;)

[bennylaird]

Want the formula?

[Cliff Rogers]

No, the dept to the bottom of the pile of books to where the electronics ones are now. :D

[Schtoo]

Schtoo,
You mightn't Buy it;

BUT
You WILL pay for it, sooner or later,
normally later, Believe me:)

You want to explain what you mean there because you are not making very much sense to me. :rolleyes:

[RETIRED]

You want to explain what you mean there because you are not making very much sense to me. :rolleyes:
Same here.:D

[boban]

Schtoo,
You mightn't Buy it;

BUT
You WILL pay for it, sooner or later,
normally later, Believe me:)

Probably getting confused and talking about sex in the wrong thread :D:D:D:D:D:D

[Schtoo]

I think I kinda worked it out an hour after I asked for a Pauline.


Getting slow in my old age... :p