A curious question. I'm wondering how are heated metal surfaces grounded, such as those electric kettles which have metal shell, or heat/hot plates where the surface have to be heated over an extended period of time, etc. I'm thinking if the earth wire is simply attached to a part of the heated metal surface, wouldn't it cause the wires to heat up especially when copper is a good conductor of heat, thus affecting the insulation, and also transferring the heat to devices which are connected to the same earth wire on other sockets.
Wires that are exposed to high temperatures use special heat resistant insulation.
This could be silicone, PTFE, ceramic, and/or glass fiber depending on the temperatures.
Heat along a wire leaks out to the sides eventually as electrical insulation isn't that great a thermal insulator. An interesting question though - there will be a formula to calculate how far the high-temperature insulation has to go along the wire no doubt.
The "trick" is that very little of the kettle is constructed of a good heat conducting metal such as copper.
In fact, you definitely do not want a kettle to be constructed of a good heat conductor - most are now made of plastic which keeps the heat in. The only part that needs to be a good conductor is the element (casing) as it contacts the water.
This also applies to the connections to the heating element. You use a not-so-good heat conductor for a distance from the element so that the insulated wires remain relatively cool. Of course eh actual heater elements - and any other parts that will be hot - are insulated with ceramic beads.
Your kettle element is not grounded. In most cases the ends go into a ceramic connector with wires insulated with fiberglass woven material.
I recall that heatsink fins loose a lot of cooling capacity once they extend more than 5 material thicknesses. I would expect a few inches of copper wire going through the fiber sleeving will not be able to conduct much heat.