This explanation is not entirely correct, electrically. It does explain it and gets the message accross to most people, though in electrical law it is incorrect.
Voltage does not, will not, and never will be the cause of heat in wiring, or anything else for that matter.
It is Current (Amps) that creates the heat, the more current, the more heat, and a bi-metal strip is constructed of 2 dis-similar metals which have different characteristics at diferent temps. The expansion of those 2 metals (caused by the flow of current) increase at different rates, meaning one will expand more than another. Once the pre-set temp cut'-out (max current) has been reached, the metal of the bi-metal strip which expands more will spring the opposite direction to where it was. This opens the contacts in the circuit and disconnection occurs. Current is now broken, the metal cools and resets over time and the cable/elec equip has been protected from over heating or worse, fire.
