BAS dear, thank you for your comment, tongue-in-cheek though it might have been. I don't recall counting or discounting the possibility of a larger fuse element being fitted - simply because the O/P had already stated that the fuse was "still intact". Had that fuse element been oversized or 'doubled-up'. then surely, the O/P would have said so. But he didn't say so. He merely said that the fuse was still intact, from which I inferred that it must have been the correct 0.85mm diameter wire - otherwise, he would have said so, wouldn't he?
So this 2.5mm cable, spurred from a standard 30A ring-circuit is supposed to have fed the following load:
A 3kW inverter.
A 3kW extractor fan........ (some extractor that must be!).
Sockets and lights, various.....
Washing machine.
50 Amp water heater...... (some water heater that must be!).
40 Amp shower.
Sockets and lights, various......
Electric heating for thirty kennels.
Sockets and lights, various......
Perhaps all that stated load is true, perhaps the O/P hasn't exaggerated it for effect..... But the fact remains that he has said that the fuse element was still intact. This is why I have suggested that that damage was caused by a combination of a heavy load and a loose connnection.
Look at the photos again. That damage was caused by a loose connection or an ill-fitting fuseholder. This is simply a case of neglect of the condition of the fusebox over a number of years. I've seen such damage - and worse, from a load of less than 3kW due to the localised heat damage that a loose connection can generate.
I'm elderly enough to have fitted those Wylex units when they were brand-spanking-new. The next chance that you have of inspecting such a unit, take a look at the reverse side of the fuseholders, where you'll see the potential for insecure connection due to the amount of screw-fixings.
There, you will find the Achilles' Heel.....
Lucia.