Un-coiled power cable

wanabechippie · 2 May 2007

Heard it being mentioned a few times that you should always fully uncoil power extension cables before using them.

Thought this was some sort of 'old sparks tale' until I saw a notice printed on a new, 50m reel from B&Q telling me to do just this!

Is it true? If so, why and what's supposed to happens if you dont? I've never uncoiled cables fully before useing them to no ill effects....

.... unless.... I'm actually already dead..... WOOOOO! *makes ghosty noise and runs off to womens shower room*

JohnD · 2 May 2007

the real reason is that the coiled cable will overheat in use. This is because the heat loss is prevented. Think of it as 20 or 30 cables bunched tightly together (or however many turns there are on the reel) and each carrying whatever the current is (bunched cables have to be de-rated for current carrying capacity as they accumulate heat and prevent its escape).

Sometimes people think it is because of induction currents, but since the cable contains both phase conductors tight together within the same sheath, these will cancel each other out.

BTW, in the Electricity Distribution trade, you are not allowed to leave those big wooden reels of supply cable under or near transmission lines... apparently they really can pick up quite big induced voltages.

Steve · 2 May 2007

Running at full load or overload, the cables are tightly bunched in a coil, and the heat created by full load or overload has nowhere to go, thus melting the cables. Or the cable could be damaged internally, but no visible signs, even more dangerous IMO.

I'm sure Rob has a photo.

JohnD · 2 May 2007

hee hee!

RF Lighting · 2 May 2007

Crafty said:
I'm sure Rob has a photo.

I dont actually, but the builder I was working with last week melted his extention by running a tumble dryer off his still coiled extention. it all melted into one solid mess

Steve · 2 May 2007

I'm sure i have seen a photo of a melted extension coil, i just assumed it was you Rob!

RF Lighting · 2 May 2007

Do you want me to go and melt one just for the picture?

(I am not going to)

ricicle · 2 May 2007

Wasname will do it then - he likes melting things

ricicle · 2 May 2007

JohnD said:
Sometimes people think it is because of induction currents, but since the cable contains both phase conductors tight together within the same sheath, these will cancel each other out.

Both live conductors John

JohnD · 2 May 2007

I was absent-mindedly thinking of steel enclosures for 3-ph where you have to lead them all through the same hole or duct

plugwash · 2 May 2007

some manufacturers give you a seperate maximum current for thier reels when fully wound. IIRC its usually about half the unwound max current. Strangely i've seen some which give a figure for fully wound and then tell you to unwind fully before use -- weired.

most of those reels can take 13A when fully unwound (and many of them are 1.5mm rather than 1.25mm making the rating of the actual flex 15A) and most handheld tools are only a few amps and then only intermittantly. so its rarely a problem in practice unless you start using stuff like heaters, big kitchen appliances, welders etc but with the long reels of 1.25mm or 1.5mm the diy sheds sell you are likely to run into volt drop issues anyway if you draw the full 13A.

ColJack · 2 May 2007

BTW, in the Electricity Distribution trade, you are not allowed to leave those big wooden reels of supply cable under or near transmission lines... apparently they really can pick up quite big induced voltages.

proven as a falasy in an episode of mythbusters..

they were trying to get free electricity by making a large coil of wire on plastic pipes as a former and then hoisting it up under some HV power lines..

they didn't register anything more than you get from radio signals...

plugwash · 2 May 2007

mythbusters is way too unsientific to prove anything.

what current was flowing in the line? its mainly current that matters when doing this not voltage, just because a line is high voltage doesn't nessacerally mean its also carrying a high current.

How close were the conductors on different phases to each other and to the coil?

how close was the coil to the line?

how was the coil orientated? the magnetic flux goes in a circle purpendicular to the lines. Therefore a coil that is also perpendicular to the lines will link almost no flux.

how large was the hole in the coil?

how many turns did the coil have?

If they didn't give all of this information then thier test is pretty meaningless.

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Un-coiled power cable

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