Extension leads - why you shouldn't use them when they are coiled

Electrical cable gets warm when it is used. The copper core has resistance and the Current * this resistance is power dissipated (as heat) in the cable. Normally this is fine, provided the cable is not pushed beyond its rating and it is in free air to allow it to dissipate this heat. Here is a story of what happens when it is not in free air ... 

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.

What's even scarier is that the person who did this to the extension lead either didn't realise how close they had come to electrocution or starting a fire, or worse yet they did realise and returned this death trap to the shelf anyway!

The moral of the story is never use an extension lead coiled unless the load is below about 25% of the rating of the cable. Or if you are not sure just never use extension leads when they are coiled.