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masterplug cable reel 45m £26.99 @ screwfix - Free c&c
masterplug cable reel 45m  £26.99 @ screwfix - Free c&c

masterplug cable reel 45m £26.99 @ screwfix - Free c&c

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good price for 45m cable reel

11 Comments

just remember to unwind all the lead when in use or it will keep tripping as the cable gets hot.

good find op, heat added

chelseablue

just remember to unwind all the lead when in use or it will keep tripping … just remember to unwind all the lead when in use or it will keep tripping as the cable gets hot.



​It will act as a big inductor if not unwind

Thanks

sam_of_london

​It will act as a big inductor if not unwind


Not only that Sam, when wound, there is less surface area for heat dissipation which will cause the conductor's temp to rise and so increase resistance, which increases the temp rise further ..... possibly resulting in a degradation of insulation and the possibility of short circuits and even fire.

Original Poster

Rhead

Not only that Sam, when wound, there is less surface area for heat … Not only that Sam, when wound, there is less surface area for heat dissipation which will cause the conductor's temp to rise and so increase resistance, which increases the temp rise further ..... possibly resulting in a degradation of insulation and the possibility of short circuits and even fire.



The reel has the thermal cut-out switch, which is capable of cutting power if the reel is getting close to a temperature level whereby the PVC outer of the cable might be at risk of melting....

This is good if you need a extension with 45 meters or you don't mind winding it all out and back in again. I got about 15mins use before the thermal cut out operated, that was half unwound and using a medium size lawn mower. Then I let the missus loose who unwound the whole cable and it didn't cut out till she run over the cable.

sam_of_london

​It will act as a big inductor if not unwind


... not that it particularly matters on a hot deals site, but the coil will not act as an inductor because there is no secondary coil to induce a current into (unless you haplessly leave another conductor right next to it and connected to a load, which I think one would have to admit would be an unlikely circumstance).
There will be a significant heating effect due to the current in the cable (I^2.R), so if a fairly large mower with a 2kW motor was drawing 8A, and the live + neutral flexes had a combined resistance of 1 ohm (a bit higher than one might expect for 90m of flex, but not inconceivable) then there would be 64W trying to escape from a tightly wound cable drum - or about the same as an incandescent light bulb inside the wound cable drum, which would get pretty hot if not unwound to allow that 64W to dissipate.
It is good that the cable drum has a thermal cut out device, but if that does not operate for some reason than it is unlikely that the overcurrent device in the consumer unit (probably a 32A MCB, or even worse a fuse) would operate before the fire starts in the cable drum - so always fully unwind the cable, even for fairly small electrical loads.

Edited by: "othen" 6th Oct 2016

othen

... not that it particularly matters on a hot deals site, but the coil … ... not that it particularly matters on a hot deals site, but the coil will not act as an inductor because there is no secondary coil to induce a current into (unless you haplessly leave another conductor right next to it and connected to a load, which I think one would have to admit would be an unlikely circumstance). There will be a significant heating effect due to the current in the cable (I^2.R), so if a fairly large mower with a 2kW motor was drawing 8A, and the live + neutral flexes had a combined resistance of 1 ohm (a bit higher than one might expect for 90m of flex, but not inconceivable) then there would be 64W trying to escape from a tightly wound cable drum - or about the same as an incandescent light bulb inside the wound cable drum, which would get pretty hot if not unwound to allow that 64W to dissipate.It is good that the cable drum has a thermal cut out device, but if that does not operate for some reason than it is unlikely that the overcurrent device in the consumer unit (probably a 32A MCB, or even worse a fuse) would operate before the fire starts in the cable drum - so always fully unwind the cable, even for fairly small electrical loads.



Thanks for the clear explanation. I can relate some of this stuff to the some of the school Physics I did and can remember many moons ago. It's always helpful when someone explains why something needs to be done a certain way.

Good practice but really depends what you have running off the end of the cable reel (power/current draw) as to whether it all needs to be uncoiled or not. Thermal cut out is handy as this will decide for you when you start to overheat the environment but better to err on the safe side.

othen

... not that it particularly matters on a hot deals site, but the coil … ... not that it particularly matters on a hot deals site, but the coil will not act as an inductor because there is no secondary coil to induce a current into (unless you haplessly leave another conductor right next to it and connected to a load, which I think one would have to admit would be an unlikely circumstance).


... not that it particularly matters on a hot deals site, but whilst I think you've overlooked that the coiled wire is effectively a secondary wire to itself - i.e. each loop induces a current in every other. This is "self-induction" and is the fundamental property of any coil. What you are talking about is "secondary" induction. All basic "inductors" that you buy or make only have two connections but are definitely still inductors and possess inductance.

That said I'm unconvinced that the inductance of a cable drum that is wound up will have any effect on how hot it gets? This is because any impedance it adds is reactive (rather than resistive) but also because the wiring is "balanced" (e.g. like a "twisted pair"). The current fluctuations in the live wire are matched with an equal and opposite fluctuation in the neutral wire for the return path and the wiring cross-section and spacing is constant - the net magnetic field is (to a first approximation) zero and any change to the impedance through winding the cable up will be minimal.

I think you are spot on with your explanation for the source of the heat though - i.e. the cable resistance! The wire is possibly dissipating around 2W/m and that isn't heat that you want to concentrate in one small area!

Edited by: "qyestionmark" 9th Dec 2016
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