UKC

I wanted to have a look at a rather aged socket and being sensible I plugged in a lamp and flicked the breakers one at a time till it went off. After 15 minutes of scratching my head and a lot of wandering between the consumer unit and the socket it appears I have a problem:
If any one of three breakers labelled "sockets" are on then the socket is live. Firstly are the rcds likely to work if they are bridged and secondly is this legal? (This is a rented property if that makes a difference)

In reply to Reach>Talent:

Im wondering if its because the light is picking up a voltage via the neutral conductors, they will all be connected in the neutral bar together. Just a hunch!

Try using a socket tester (cheap) or a hairdryer. Something which will not have a chance of operating under a small current.

In reply to Oogachooga:

Ah, interesting. I tried a hairdryer which also had the benefit of being audible from the consumer unit and only one switch worked this time. I hadn't come across that before.

In reply to Reach>Talent:

What kind of light? LED/Halogen/CFL?

I've had odd effects with CFL life that.

In reply to Reach>Talent:

It's surprising and bamboozled me a few times with lighting. You could have no neutral and some lights will still work.

In reply to Philip:

It was a halogen lamp.

In reply to Reach>Talent:

I think this could be what's known as a 'borrowed neutral'. Not a good situation to have.

I would be surprised if it's due to the neutrals at the CU.

In reply to Oogachooga:

I just took a multimeter to the socket in question:

No voltage between Neutral and Earth
230VAC between Live and Neutral
230VAC between Live and Earth

I spoke to a tame(ish) facilities engineer who had suggested it could be that the generator* wasn't properly staked and things were floating but I think that the above readings would contradict that theory? The worrying thing is that the bulb should be drawing about 100mA which is at least 3x the trip current for the RCD.


*Forgot to mention that I'm currently on a generator as SSE have got some sort of local screw-up.

In reply to Reach>Talent:

Ok, lets see what voltages you are getting when the other breakers are on, that circuit off (originally the lamp would work with any socket circuit on).

In reply to Oogachooga:
Ok, I've just tested it and now I can't get it to work. I tested it 3 times earlier and it worked each time, now it isn't working. That is to say the circuit seems to behave normally and under the control of a single breaker.

So I'm really blooming confused:

- Could a borrowed neutral be caused by an appliance with multiple supplies (If the labelling of the CU is accurate then my boiler has rather confusing wiring), which would only manifest when that appliance is live?

- Could local supply issues be altering the behaviour of the circuit (I think the generator is currently supplying 12 homes)? This generator has faulted and cut out a couple of times in the last week.

- I'm pretty sure this isn't a hallucination as I was talking someone through it while I was testing it :-S

I've got an SSE engineer turning up shortly for another issue so I'll ask him and see what he thinks.

In reply to Reach>Talent:

Yeah could be either of those issues, best thing to do is get the circuit tested. Especially the rcd if anything else to ensure it trips within the rated timings for 30ma.

Generator power is the elephant in the room here, I'd put my money on that creating the issue. I was thinking of scenarios of when the rcd may not trip as it works off an imbalance in current but I'm at work and it was making my head spin, maybe the lamp being on didn't create enough of an imbalance to trip the rcd? Who knows.

In reply to Reach>Talent:

I wouldn't think it would be a borrowed neutral because that would trip the RCD. It would pick up the in-balance between what current is going in on the "Live" and what's coming out on the Neutral. If it's not balanced (Which a borrowed Neutral wouldn't be) the RCD will never stay in.

It may be the Genny, but then if it was, everyone else would have the same problem. Some of the Genny's we fit will supply whole villages, so that shouldn't be the problem. It shouldn't cut out though.

As with regards boiler wiring. Forget it. Cables only come in certain colours, and as long as the sparky has got the right colour match on both ends and it works, leave it. With the CH systems around nowadays, sometimes lots of wires are required. The motorised valve in our house has an Orange coloured wire, its connected to a Blue wire from the Boiler control. That's what was available at the time.

Keep us posted.

In reply to Reach>Talent:

That bit at least should not be too much of a worry.

The trip current for the RCD is any discrepancy between live and neutral at that point. (ie: current that is going to earth instead, whether that's via a bit of faulty wiring, a dodgy appliance or your fingers in a light fitting). If you have a single RCD that supplies all three circuits, you wouldn't expect it to be bothered by any weird inter-connection between them such as current going out on the live of one and coming back on the neutral of another.

RCD's have a 'test' button btw, have you tried that? If the power is on and it fails to trip when you press the test button the RCD is goosed, otherwise it's almost certainly fine.

In reply to Oogachooga:


Most likely scenario is that the OP is slightly confusing RCDs with MCBs: consumer unit has a single RCD which supplies three MCBs (the breakers that the OP is talking about) and maybe a couple more besides.

An imbalance in current between the live and neutral of any one circuit won't affect the RCD, as long as the sum of the currents in all three lives balances the sum of the currents in all three neutrals.

In reply to deepsoup:

There was a slight confusing of terminology, I did in substitute RCD for MCB at least once, although I did know the difference This board has 2 RCDs supplying 8ish MCBs. When the problem was manifesting itself earlier the circuits that were causing confusion were split between both RCDs.

In reply to Reach>Talent:

That's me out of ideas then.
(Have you tried the 'test' buttons on the RCDs though?)

Hope I didn't inadvertently create the erroneous impression that I know what I'm talking about earlier btw. ;O)

In reply to deepsoup:

Test buttons work

In reply to Reach>Talent:

So here is my very unlikely stab in the dark:

Maybe some appliance on some other circuit was drawing significant amperage during the earlier tests, and that inductively coupled enough power into your circuit to light the halogen. You'd presumably need a break in a ring somewhere to create a local current imbalance in the transmitting ring to be able to send any power, and another break in the receiving ring. You'd also need a really, really, small bulb, like an electricians screwdriver bulb.

That or someone was flipping breakers behind your back...

In reply to Oogachooga:

the reason the rcd doesn't trip is because the neutrals are shared between circuits on the same rcd. Not a great situation but the RCD will work as it should in a fault situation. The problem is more for whoever works on any of these circuits as they risk getting a shock off the neutral if they turn off only the circuit they're working on, best turn off the whole rcd.
a sparky

In reply to admackie:

After an enlightening chat with the bloke from the electrickery company I am none the wiser, however I am now aware that they have known that the rotten pole in the garden has needed replacement for the last 4 years and done sod all about it. At least if it falls over and crushes the garage there is a good insurance claim to be had!

I am hopefully borrowing a Martindale EZ150 for the weekend and can spend some quality time testing all the sockets in the house.

In reply to Reach>Talent:
Circuit breakers don't isolate sockets

They simply stop too much current traveling down the live side of cable. They do not isolate you from neutral.
As neutrals are all connected in common you can still get a shock of them if other circuits are drawing current.

To work on any circuit turn off the supply with the double pole isolator in your consumer unit.

In reply to rogersavery:

I always stick my left hand in my trouser pocket and stand on my right leg: The sparky who told me that was horrendously accident prone, short sighted and old enough to have gone to school with Newton so it must work

In all seriousness I got my first mains shock before starting school and I've been fairly careful since, I do occasionally work live but only on stuff wired by myself or a couple of other people and I'm normally the irritating person adding extra mitigations to peoples risk assessments.

In reply to Reach>Talent:

As a careful person you may find a volt stick useful when trying to trace what is going where.

In reply to Reach>Talent:

It is essential that SSE take down the earth at their supply as well as L and N. When I used to supply them with generators they could not be persuaded to do this and the customers suffered as a result. Leaving the earth (PME) meant the little elcb on a 100kVA genny was try to deal with every earth fault on the national grid! Not good.

There are some genny setups that could give problems depending on the phase balance. That and leaving the earth up can easily make N and E dirtier than normal.