Big transformers at switch on can have similar effects.

Automated systems like heating and cooling devices would obviously be
prime candidates.

I've pretty much convinced myself it's something electrical blowing
itself apart in a self resettable way. Maybe water gets into a joint
and is instantly and explosively superheated, Do you think that could
happen on a circuit protected by a 30 amp fuse, again and again
without it blowing? I think the answer is yes if the loop impendence
is high and the events are of short enough duration.

I'm just afraid that it might be a cable protected by a fuse 10 times
that value in a substation!


Personally I would isolate *every* non used appliance for a few hours,
fridge, freezer, any heating etc and then reduce the fuse capacity to 15a
and switch a circuit on one by one individually to see if it's house side of
the CU.

If the incoming main is damaged there is an immediate need to get the board
in to investigate as shared runs for water, leccy and gas often result it a
far larger problem when any two coincide during a failure :-(

A scan with a CAT device might be used to determine where any incoming
supply issues might be.

A local "engineer" for the high power supply bods round here who is one of
my customers used to check for blown cables underground at streetlamp
circuit problems by shoving a 6" nail in the last known live feed at a pole
and watching where the ground erupted nearby so he knew where to dig and
repair .... dodgy but it worked.
Can't imagine where he got the nickname dodgy digger from......