I have a friend that works with automotive mechanical sorts of things, and he told me that he had a pair of jumper cables that was ruined by too much current; it wasn't that the insulation melted: he said that the wire didn't carry hardly any charge after that.
I can't think of any reason why this would be. If a wire carried too much current it would obviously get hot, and that heat would eventually melt insulation, but even then, the copper (or whatever it is) should be able to carry charge just as well as before, shouldn't it? When it actively was hotter, the resistance would increase and this would impede the flow of charge, but once it cooled down, it should work like before, as I understand things. Does something occur when copper gets to a certain temperature that could permanently impair the electrical conductivity?
I'm not an electrical engineer or even an electrician. I just do some mechanical and automotive work and I have a little bit of experience with physics.
Best Answer
Some newer jumper cables have fuse type elements included, (either a very large value fuse or just a thinner [sacrificial] wire that burns out before the main cable begins to super heat). This is done to protect those people who have trouble following simple instructions. Another possibility is that the wire to clamp connection point had a weak spot and that small area burned out during the current overload.
The small increase in resistance (with high temperature) of a thick copper wire is insignificant in this situation. Even molten copper would pass a lot of current.