Half of old house suddenly without power. Causes?

[MM]

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

[tabbypurr]

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

[MM]

Thanks. I'll take a photo tomorrow. Maybe the electrician will be able to squeeze him in again today, although yesterday the news was that he's fully booked up today.

My own house is only 15 years old and, of course, has up-to-date electrical equipment that one should expect in a 2004 build. My CU, in the garage, has about 6 smaller MCBs and one larger MCB in the middle. I rarely get any outage, although the MCBs are all super-sensitive. To the point where when I was still using filament lamps, just the bulb blowing would invariably trip the MCB for the relevant lighting circut.

I gather from what I've been told that the electrical system in the old house is quite ancient, as the electrican stated that after the immediate fault has been rectified, at least temporarily, the whole CU urgently requires replacing. Maybe the wiring, too, which would be an enormous (and costly) job. It's a large house!

MM

[Jeff Layman]

If an RCD is tripping then the simplest thing to do is switch off all the MCBs, reset the RCD, then switch on each MCB in turn until the RCD trips again. Then you know which circuit is faulty. It can be turned off and left off until the electrician appears. The other MCBs can be left on so at least some of the house had power of some sort.

However, if it's an intermittent fault, and the faulty MCB is already known ("But then the "fuse" (i.e. switch) popped out again later in the afternoon."), why wasn't that just left off and the RCD reset until the electrician was able to do further testing?

[Brian Gaff]

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

[Dave Liquorice]

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).

[Harry Bloomfield]

Jeff Layman formulated on Thursday :

50/50 chance at best - it might be an N to E fault.

[Robin]

That leaves a lot of room for speculation untrammelled by facts :)

As already noted, pictures might help - ideally of both fuse boxes and of where the cable comes into the house. It might also help to know if the supply to the house is from overhead cables.

[newshound]

I was going to say both those things too!

[Andrew]

On 25/07/2019 08:43, MM wrote: <snip>

The only thing to look out for (initially) is the presence of rubber coated wiring or those sockets with round pins (except for diddy little lighting ones).

If the cables going into the sockets and lighting roses looks like pvc cable then an urgent isolate and rewire is not needed, IMHO. Sparkys OTOH just love to frighten and bullshit clueless owners into expensive rewiring.

[Andrew]

Or even it it used to two houses, now combined but still with two incomers, main fuses and CU's *(like my bro & SIL's house).

[MM]

Yes, I did gather from speaking with my relative that the side of the house that is unaffected has a separate consumer unit.

MM

[ARW]

Bugger. I was hoping that the OP had passed away.

[tabbypurr]

I suspect the larger one says RCD on it, and is therefore not an MCB. Photo should tell.

Funny how peple always seem to recommend you give them money. We await the pics.

NT

[Brian Gaff]

Yes when something triggered similar problems here back in the 60s, it was merely old perished wiring over the ceiling of a bedroom. the rubber had fallen off and there was a short. We had the house rewired. Brian

[Andrew]

If there are two separate incomers (?originally 2 houses) then if they are on separate phases, be careful if they resort to running long extension leads from the unaffected half into the 'dead' half, because of the voltage difference.

[jrwalliker]

Some houses use three-phase power. Nobody seems to worry about that. Does anyone take special precautions when using a UPS whose output phase may be random relative to other unprotected mains circuits nearby?

John

[Tufnell Park]

There is a well established procedure for detecting which circuit is causing an RCD to trip.

Viz with the RCD off switch off all mcb's. Reset the RCD Turn on each mcb in sequence to determine which circuit is causing the RCD to trip. Once identified investigate the 'faulty' circuit for line and neutral to earth continuity and if necessary check insulation resistance. Visual checks at accessories may then be required to establish wiring condition.

[Andy Burns]

Or if you're unlucky you find it's several of them together, or it's intermittent.

[tabbypurr]

Yes. That may catch it though, and it takes no real work, learning or equipment.

NT