I was asked by a friend to investigate a seemingly faulty 20A DP
switch for a washing machine today.

In hindsight slightly unwisely, I unscrewed the switch and pulled it
forward to be rewarded with a flash, bang and the appropriate cloud of
noxious smoke ! Hey isn't it great when you get the pyrotecniks
rather than just something going dead.

Anyway my guess is that the line terminal had not been tightened
properly originally, had over heated, the plastic broken down and my
approach had resulted in the line terminal breaking free and coming in
contact with the earth lead.

The fuse in the CU did not rupture !!

The switch was replaced.

The question arises now is should I strongly advise my friend to have
his fuses CU replaced with a modern one; the emphasis is on the
'strongly' as my feeling is that despite a large flash/short as this
was, the fuse did not protect the system as an RCD or MCB would have
done. In fact an RCD would probably have tripped out long before I
was asked to look and would have given the protection that the house
should have (said having just has my own RCD trip and finding that it
was due to the oven fan becoming leaky).

Rob