On Wed, 24 Jul 2019 23:45:14 -0700 (PDT), wrote:

You're presumably losing the whole CU, so it sounds like it has an RCD
that's tripping. If that's the case, there are 2 tests to do.
Insulation test each final circuit, then PAT test or insulation test all
your appliances. Usually that will pinpoint the culprit.

Requires a PAT or insulation tester. A multimeter is only going to
find a gross short not the leakage that may be causing an RCD to trip
assuming that is what is tripping, more info needed.

Assuming it is an RCD were any water heating appliances (kettle,
washing machince, dishwasher, WHY) in use when the afternoon trip
occured? If the the chances are the weater heating element of that
appliance has failed.

An insulation test might not find that as the heating element won't
be switched on. Does an insulation test do L-N, L-E and N-E? Only the
last will find a leaky element and only then if the heater isn't
double pole switched (unlikely).

--
Cheers
Dave.