Mike wrote:

B****y hell. Assuming it is coming in through the supply to the house, I'd
get that investigated by your electricity supplier. Something is switching
large loads either from or next to your supply.

Fast transients on the mains of up to 2kV or so as described are
perfectly normal.

I believe there are filters they can put in, sort of mega-baluns, or in the worst case a private 100A
1:1 transformer.

Equipment on the market these days should have adequate immunity - BS EN
IEC 61000-4-4 refers.

--
Andy