If I may add a little to what Slide had to say. Heat cord, metal hotplates, bar radiators are all common examples of non-conductive heating elements. When you put a metal pot on a metal hotplate, you do not get electrical shorts. The core of traditional heating elements contains a metal wire or rod, which has a high electrical resistance. When an electric current is passed through this it generates heat. The greater the current the more heat produced. You can think of high resistance as being a lot of disorganised movement of the metal atoms, resulting in a lot of friction as the electrons try and pass along it. And as we know, friction produces heat.
Surrounding the high resistance metal is an electrical insulator which will not conduct electricity. It does, however, readily conduct heat. So the heat comes out but no current. The outside layer just depends on where and how it is going to be used. The outside layer will also be a good conductor of heat to allow the heat out. Whether it conducts electricity or not does not matter - there is no exposure to electrical current.
In wiring up heating globes and lights, you need to strip the electrical insulation from the ends of the wires. If excess insulation is stripped off this leaves bare wire exposed beyond the fitting. If the screw used to hold the wire in place is not tight, the lead can slip out enough to again expose bare wiring. Even if the screws are secure and holding the wires in place, many insulating layers on leads are soft enough to be compressed. So pulling on the lead can result in the insulation being pulled backwards, once again exposing live electrical wiring.
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