Here we have a graphic demonstration from the U.K. of the damage which can result from running a heavy load on a coiled extension cord.

Quote
I came across this extension lead in work, I was borrowing it from another department. They kept it coiled on a cable reel. When I collected the extension cable it looked fine, when I began to uncoil it; it still looked fine, until I had removed the first set of turns, the whole outer coil, then I noticed a problem. The inner coils wouldn't unwind, they had melted into an amorphous blob of plastic and copper. Clearly someone had run this extension lead at close to full load when it was coiled. The cable heated up but as the cable was coiled the heat couldn't dissipate fast enough and the cable just kept getting hotter, until it melted.

I forced off some of the second layer of coils so that I could see exactly the extent of the damage.


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