Lighting circuit short

[Chris Green]

Huh? That won't necessarily work unless the RCBO has a significantly lower leakage trip current than the RCD.

[John Rumm]

If they are cascaded, then even that will not ensure discrimination if the leakage is enough to trip both devices.

Some split load CUs have some non RCD protected ways off the main switch to let you use RCBOs as well as the more traditional two RCD split.

[nothanks]

I'd hope that an electrician would put the RCBO on the non-RCD part of the board, or run a wire from the switched live bus (he'd have to rewire the neutral anyway so this should be a no-brainer)

[Jeff Layman]

Indeed. I must have misheard - there is no RCBO. I had assumed that RCBOs have a lower tripping current than RCDs, but it seems they are both 30mA.

I've just checked the CU and he has simply removed the MCB and replaced it with a blanking plate, so it is not possible to power the lighting circuit by mistake. The RCD is still there for all the other circuits on that side of the CU.

[Jeff Layman]

The electrician reappeared this morning. Of course, as soon as he reconnected up the MCB the circuit worked without problem! He disconnected an old, unused PIR floodlight at the top of the gable which I had been unable to get to, just in case it was that which caused the original fault. After a time looking round the loft for anything obvious he said there was no point in wasting money trying to find a now non-existent fault (don't you just love intermittents!). So he left and I said I'd contact him again if the lights failed. So far he's been gone an hour and amazingly everything is still ok.

[Jeff Layman]

Tried to turn the lights on an hour ago, and found the MCB had tripped! I reset it and it lasted less than a minute. This is going to be a pig of a fault to sort out.

[ARW]

Jeff it's amazing how many intermittent faults disappear as I reverse up the driveway in the van...

It's not my magic or my God like appearance and behaviour that causes this - it's because intermittent faults are bastards.

I hate them as much as I hate some apprentices.

[The Natural Philosopher]

MCB? That used to go every time a candle bulb blew.

You have some critter attack there. Sounds like a fresh mouse across live and neutral.

Wait till it rots and you will be fine

[Jeff Layman]

Never tripped before in the 10+ years it's been there since we had a new CU fitted. And that includes the times I've put in a total of 6 new wall light fittings in the lounge and main bedroom.

We have had (still have?) mice in the loft, and it could be the reason. I've called the electricians back to start the elimination process by disconnecting circuits in turn.

[Jeff Layman]

Yep, Sod's Law applies just about every time.

Pretty high on the list then! ;-)

A thought occurred to me that nearly all the discussions we see here are about "13A" ring, spur, and occasional radial circuits, together with the odd one about high-power cabling for ovens and showers. Lighting circuits issues are pretty rare. Out of interest, are lighting circuits always radial, or are there rings and spurs for them too? Or can it be just about anything? I assume there's something in Part P, but I haven't got one handy to look at.

[The Natural Philosopher]

If you have a solid trip its much easier to find...

[The Natural Philosopher]

Not for me. As I said small candle bulbs would trip mine EVERY time one blew. Now all LED thank god.

And I had a leaking roof plus a light socket screw up the RCD on the old house.

Rings are where you plug in stuff that is prone to failure, that's all.

I think that is as true as it gets. My sparkies wired em up any old how. Radial, daisy chained. Ringed.

Me neither, and I bet there isn't. IIRC there is a limit on how many bulbs you can have and that's it.

[charles]

and, when we bought this house, I found a junction box where the wires went in Red/Black and came out Black/Red !

[nothanks]

Unless there is more then one wire going into the MCB (in which case remove all but one, in turn, to see when the fault disappears) I think you're back to replacing the MCB as a first step to rule out the easiest thing to fix.

[Jeff Layman]

The electrician came round after lunch and after a couple of hours located the fault. It was in the garage lighting somewhere (he's just disconnected it for the time being, as I don't need lighting in the garage). I had been told when we moved in 10+ years ago that the garage was on a different circuit to the rest of the house. Well, that was true but only as far as the 13A ring mains supply. The lighting was off the house lighting circuit.

He said the wiring in the loft was awful. There were junction boxes with some wires inside "joined" with insulating tape, and plenty of joins in mid-air also made with insulating tape. I've seen some of those on previous occasions. We're going to leave things for a couple of weeks to make sure no other faults appear, and then he's going to send me a quote for replacing the wiring. As some of the switch back boxes are wood, and the wiring to them is embedded in plaster, it would require channelling out the walls for the new wiring to the switches. So I'll be looking at replacing all the switches with Quinetic ones. They'll just fit over the hole left when the switches there at present are removed, and no redecorating will be involved.

It's only money...

[Bob Henson]

My first time in the loft in my last house led me to a main junction of about eight or nine heavy cables (don't ask me what rating, I'm no expert) nailed very neatly in parallel fashion to a ship's beam rafter (the house/mill dated to about 1650) which connected to their opposite numbers by a strip of plastic chocolate block connector with the ends more or less open to any passing person to touch. I nearly found it the hard way.

I recall it cost me quite a bit to get things made safe too!

[Jeff Layman]

I will need to have a think about what to do with the wall lighting in a couple of rooms. The cables for those will be embedded in plaster so are not "pullable" to be replaced. Once the cost of redecorating is added, it all adds up to rather a lot.

[nothanks]

It's a pretty straightforward DIY job to: cut a cable chase, sink a back box, drop the cable, PVA the chase and fill it (slightly low) with bonding plaster, then finish with something that can be easily sanded flush.

Why does the section of the cable that's in the wall need replacing?

[Jeff Layman]

Oh, I'm sure that it will be no problem when I have a spare 5 minutes. You seem to have forgotten the redecorating - I don't think that it's "on trend" to have vertical white channels down the walls. All the walls would have to be repainted. But first the furniture will have to be emptied and moved, then the dustsheet will have to go down to protect the carpet. Only then will painting be possible (I nearly forgot - I detest anything to do with paint).

That's a good question. I don't know that it does, but the wiring here is very strange. I mentioned it 4 years ago:

There was quite a bit of follow-up discussion.

If the electrician is happy to leave all the wiring for the wall lights in place, remove the supply to the switches (by changing to Quinetic switches), replace the crappy wiring in the loft, and certify the work, that would be ok by me. We have 8 wall lights in 4 different rooms, and the thought of chasing out all those and redecorating doesn't bear thinking about.

[Andrew]

If these walls are just painted then take a fragment of the plaster with paint attached to any decent paint supplier and let them use their optical equipment to work out a match.