[Taf]

I've seen it often with high-current devices. A touch of tarnish or muck on any contact point is resistive, That generates heat, which worsens the contact so it becomes more resistive, so it generates even more heat, so even more tarnish, etc., etc.

The worst I have seen, in several cases, was the insulation of the wires to the device, and to the socket, overheated so much that it had melted.

Contact points are where the house wiring is screwed to the socket connection, the socket pins, the screwed connection to the plug pins, the plug pins themselves, and both ends of the fuse.

It's most common with devices that are almost never unplugged. So next time you use your kettle, toaster or whatever, give the plug pins a gander. If they are not bright brass, give them a clean with something abrasive.