ToughDiamond wrote:
Why do you think it's salt?
It is certainly salt - I tasted it. Most likely table salt, which was available in the kitchen beside this room. The effect of salt has been to corrode the copper and brass of the internal wiring and connections (which are green and pitted), and salt also adsorbs humidity from the air, so the internal wiring was damp. I have been unable to think of any accidental means by which the salt could have fallen in, and it does seem most likely that a human deliberately poured it in through the cable access hole at the top - there is some salt on the top of the box, but none anywhere on the skirting, floor, shelves or anywhere else nearby. No animals - except my cat, who lacks opposable thumbs.
Sometimes I find human behaviour utterly bizarre - I can not imagine doing something like this myself. I assume that it was intended to cause my central heating to fail, or the RCD to trip, so that I would call in the electrician again.