The thing is I'd have thought the first thing he'd do is unplug or unwire or
switch off everything on the circuit. Then see what happens. If lucky it
will work, if not then you have probably cleared most of the devices. Then
the slog starts. If its been hot where the property is that might have a
role to play if its an appliance like a fridge or are conditioning unit.
I hate those things. Shortly after moving here we had something similar and
that turned out to be somewhere in the loft between the main wiring and a
ceiling light and its switch. It had perished, very old rubber covered wire.
Brian

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On Thursday, 25 July 2019 07:29:51 UTC+1, MM wrote:

A relative lives in a 300-year-old house. Yesterday he called to warn
that when I visit tomorrow (Friday) it's likely that roughly half the
house will still be without power. The other, smaller, half is
unaffected.

The electrician he called has so far not been able to diagnose the
cause of the problem. The half that is out is on a separate "fuse"
box. The other half of the house is unaffected. I write "fuse" in
quotes, but I understand that it does at least have the switches, not
old-fashioned fuse-wire fuses, although the equipment seems to be very
old.

The electrician, who is fully booked up today, tried for several hours
yesterday to pinpoint the cause and thought he had found it. But then
the "fuse" (i.e. switch) popped out again later in the afternoon.

Does anyone have any comments as to the cause? And how would an
electrician go about narrowing down the problem area? According to my
relative, the electrician knows what he's doing, i.e. isn't an
amateur.

How does one trace through all the wiring in the affected side,
especially given the age of the property?

Thanks.

MM

Half the house without power means a whole CU dead, not one final circuit.
They're called MCBs btw. If any one MCB tripped you'd only lose one circuit.
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.

Sometimes you can find the bad appliance by just insulation testing with a
multimeter, one probe on L&N & the other on E plug pin - with the appliance
unplugged of course. There should be no conduction there. So you may solve
it with a £10 multimeter and an hour. Sometimes the fault is only picked up
with high voltage testing.

A clear pic of the 'fusebox' would help


NT