Lighting wiring problem - sanity check

Just set off to do my good deed of the day. Replacing the PIR outside light by the front door which had given up the ghost. New light half price from Homebase was the incentive :-)

Skip to the bottom if you wish to avoid the detailed route to disaster.

All going swimmingly at first - took the old fitting off the wall and noted that the screw holes looked to be the same spacing as the new back plate, so no drilling needed.

Noted that there were two cables coming in, with the red wires joined. I knew where one cable went - switch by the door. I assumed normal lighting circuit - one cable is power in, other cable is to and from switch. In this case the two reds commoned, so the two blacks are power when the switch is closed. Checked with my trusty analog multimeter and this seemd to be the case. Confirmed on the downstairs lighting circuit by turning off circuit breaker at the fuse box and finding no current afterwards.

Unwired and rewired and then tested, but the circuit breaker for the downstairs lighting wouldn't reset. Bugger - what did I do wrong? Took the fitting off the wires - no change. Took the back plate off the wall - no change. Uncoupled the two red wires and the two earth wires and pushed the two sets of wires apart. This seemed to fix it. ?????? Double checked - one set of wires are power, others are to the switch.

O.K. - start again but slowly. Wound the two earth wires together and slid the sleeving over. Checked breaker - wouldn't reset. Took off sleeving and seperated the two cables and spread the three wires in each cable at 120 degrees to each other. Breaker wouldn't reset. Checked for obvious wire damage on live cable - no obvious sign. Joggled cables. Breaker wouldn't reset.

[Noted that whilst mucking about with cables and lighting breaker out, I did get a belt off something.]

Back to the trusty multi-meter.

Checked all the wires for 240V a/c - nothing. Checked switch cable with the 1ohm setting - switch open nothing, switched closed open circuit between red and black. O.K. - seems right.

Check between wires in power cable. There seems to be open circuit between live and earth, neutral and earth, and live and neutral.

My conclusion is that somehow in moving the wires around I have induced a short between live, neutral and earth somewhere in the power wire which disappears into the cavity wall to gawd knows where. Have I missed anything which might be good news?

I assume the best result I can get is to lift the bedroom floor above the cable exit in the wall and find a cable coming out of the inner wall and going across to the light fitting under the centre of the floor. Of course, it could go to the outside light or the hall light. Or the lighting power may not be wired directly into the light fittings.

Whichever way it goes, I forsee a world of hurt. To get at the light fittings from below, I have to deal with thoughfully applied Artex which probably means breaking the light fittings to get at the internals because the cover will not unscrew. To get at the light fittings from above involves lifting carpets and floorboards, and in the case of the front bedroom draining and disassembling a super king size water bed.

So please tell me there is something simple and obvious I have overlooked which is easy to fix.

Cheers

Dave R

Reply to
David WE Roberts
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Hi Have you checked the switch? Your shorting of the various cables may have welded the contacts.

Reply to
chudford

*Hi *Have you checked the switch? *Your shorting of the various cables may have welded the contacts.

Not sure which switch you are talking about. As noted in the original post the switch for the light has been checked and the fault shows when the switch is isolated from the circuit. So the fault must be in the power circuit or elsewhere in the downstairs lighting. Personally I've never seen contacts in a switch welded - circuit breaker should prevent this.

Reply to
David WE Roberts

st set off to do my good deed of the day.

I speak from experience when sorting out a problem at my daughter's house. My son in law had connected up a light fitting incorrectly and had wired the light switch between the live and neutral at the light fitting. This had welded the switch contacts. Took me ages to find the fault as I had wrongly assumed that the switch must be OK. This was on a RCD protected circuit.

Reply to
chudford

That's where you went wrong........

normal lighting circuit - one cable is power in, other cable is to

Reply to
Ian

Trust me, the circuit breaker will not prevent this.

The bit about getting a belt whilst the breaker is removed is scary. Is there just one light?

I would suggest trying to connect the light straight into the "power" supply to see what happens and then try swapping the new light for a pendant for testing purposes (it could be a faulty PIR light).

Reply to
ARWadsworth

A RCD trips on earth faults. You had a LN short.

Reply to
ARWadsworth

Interesting. How did you measure the 240v - between what and what?

Try measuring again between each wire in turn and a known good earth, both with the CB engaged and tripped.

crb

Reply to
crb

Interesting thought - however I can't test with the CB engaged because it always trips. For those reading this, please note that the fault manifests with no light fitting attached and no switch attached. Just bare wires out of the wall, and the two cables (power and switch) clearly seperated. So it cannot be a fault in the light fitting or switch. The fault is inwards of the power cable coming from the rest of the downstairs lighting circuit.

Reply to
David WE Roberts

Yes, the belt worried me too. As stated elsewhere the fault is present with no switch or light attached which rules out the switch and the light fitting as causes.

Reply to
David WE Roberts

Identify and isolate the cable at the consumer unit end, then neatly chuck a supply through an upstairs window and plug it into a socket outlet. Make note to rip up floorboards next time place is being redecorated and replace with fixed wiring.

I have sympathy. I got a belt from an upstairs landing light that I was *sure* I'd isolated as I'd pulled the fuse, only to discover that it had been wired to both the "upstairs" fuse and the "downstairs" fuse. The previous owner had obviously thought: the stairs have an upstairs light and a downstairs light, so of course it has to be wired to both fuses, doesn't it?

JGH

Reply to
jgharston

A twin and earth cable that has been cut using a stanley knife!

Reply to
ARWadsworth

However since the fault is inbound of anything I've touched, it wasn't me, guvnor. Added probem is that the central heating seems to be powered from the downstairs lighting, whic I will describe in another post.

Reply to
David WE Roberts

Not sure this is going to help me - the entire downstairs lighting circuit is off and the cable where I believe the fault has manifested disappears into the cavity wall.

If normal lighting practice has been followed then this cable will go to a junction either in a junction box or in a ceiling rose where it will join the main lighting power circuit.

I am assuming that if I can find where the power feed joins the rest of the circuit I can isolate it and everything else will then work.

Finding the other end is the issue.

The challenge is not to get the outside light to work. It is to get the circuit breaker to close and the rest of the downstairs lighting to work.

Reply to
David WE Roberts

set off to do my good deed of the day.

I have done the same.

Reply to
The Medway Handyman

I was double checking that the central heating was off the lighting circuit by taking the CB up half way (this gives power - full up trips) whilst someone watched the programmer for signs of life.

I had just used this trick to locate any lights still turned on and turn them off. To my surprise, after I heard the boiler fire up the breaker stayed set.

So I am now wondering if the CH was just putting too much load on the CB when first reset - the initial suge knocking it over again.

So the thing I do need to confirm - in a power cable from a lighting circuit where some lights may be turned on should I see an open circuit between live and neutral? Doesn't seem right to me. Seems more likely that a small contact has burned itself out during the repeated short power ups.

I am now not going to touch anything until it is daylight and the house is toasty warm.

Then presumably I play hunt the phantom fault.

Reply to
David WE Roberts

Could it be your fixing screws were longer than the previous ones? Perhaps the cable is routed in such a way as a screw has damaged it. If that were the case, at least the fault would be very close to the light.

Another possibility is the MCB is faulty. What happens if you disconnect the circuit wires from the top of the MCB - will it reset then?

Reply to
John Rumm

Go on Ebay and buy a Fluke Voltalert.

Reply to
js.b1

snipped-for-privacy@mid.individual.net...> Just set off to do my good deed of the day.

The whole account of full of question marks. I'd disconnect the CU end of teh cable, and use a multimeter to check what the circuit is doing, plus establish what all the wires are at the light end. Ultimately if it cant be fixed, run new cable, but hopefully you can find & disconnect the faulty part of the circuit. We cant, its jsut a case of test, divide & conquer.

NT

Reply to
NT

Floating neutral - fecking things. Had a mild jolt or two from those.

Reply to
Grimly Curmudgeon

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