DIY Doctor

Lighting fault

Postby PVS » Sun Sep 07, 2008 8:46 pm

I have just had a new Chint NX2 consumer unit installed and now the upstairs landing light trips one of the two RDB's and one of the lighting circuit breakers. This happens as soon as the light is switched on either at the bottom of the stairs or the switch at the top.

The electrician has checked both the switches and the light fitting and bulb, but seems unable to resolve the problem. I noticed that he has wired the upstairs lights on a different CB and RCB to the downstairs (2 RCB's in the consumer unit). As the problem light is the only one that can be switched on from either upstairs or downstairs, is it possible that it has been wired incorrectly in the consumer unit? if not, any ideas on what the problem may be?
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Postby jaffa19 » Sun Sep 07, 2008 10:19 pm

hi PVS ...

sounds like you've got a two-way light switch, from the upstairs lighting circuit. What interests me the most is that the MCB and the RCD is tripped at the same time.. The MCB give overload protection and on a 6amp circuit this is 30 amps and the RCD gives earth fault protection. the MCB is tripping because of an overload and the RCD because of an earth fault. I'm wondering if you have got a LV light because cheap transformers can give out and when they do, the effects your saying do happen need more info really..
1) 230v or 12v
2) how old is the wiring (is it run in steel conduit)?

NB new fuse boxes can often pick up latent problems that existed prior to having the fuse box replaced. A Sparkies nightmare.
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Postby ericmark » Sun Sep 07, 2008 10:29 pm

It is a common problem and is normally due to different neutrals being used by up stairs and down stairs lights.
Easy way out is put both up stairs and down stairs lights on same breaker.

Postby TOPSPARK » Mon Sep 08, 2008 7:37 pm

Under 17th edition regulations lughting circuits have to be on separate rcd's. The problem could be as this is a split board that both upstairs and downstairs neutrals have been connected into same neutral busbar.These need to be separated and then put into their correct neutral bus bar and this should solve the problem
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Postby ericmark » Tue Sep 09, 2008 12:05 am

If we look at 314.1 to 314.4 in 17th Edition and 314-01-01 to 314-01-04 in 16th Edition there is very little change. “(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit” would maybe prevent the lighting being for being bunched with power on same RCD but splitting upper and lower floors is hardly going to help only the use of emergency lighting could really help or side to side lighting split and the latter would require quite a lot of rearranging in most cases.
”(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation” this would may be mean spitting the power into a number of circuits and of course keeping it away from any lighting circuits.
The rest with exception of EMC is the same. My house built 1980 has a single lighting circuit I have fitted two emergency lights one at top of stairs and one in garage but this is unusual for the estate and I am sure most are as original with a single lighting circuit. Where the fuse is replaced with a MCB then consideration should be made as to using type C or D to avoid nuisance tripping. This would depend on the earth loop impedance reading of course.
I have heard say many times how floors should have independent supplies but unable to find anything in either old or new regulations to say this must happen. Desirable as it may be. Maybe it is a building regulation but I have not found it and I have attributed it to Chinese whispers.
Although I have seen great lengths gone to in order to split the neutrals between the upper and lower floors where stairs lighting has used both neutral supplies on both Line supplies due to the pour wiring of the two way lighting personally I don’t see it is really worth the effort.
After all a caravan if wired with two 30ma RCD in series which breaks both the discrimination rules 314.1 and 531.2.9 and I think some common sense must prevail.
In the same way as if we followed 522.6.5 and 522.6.6 we can’t bury the wires feeding door bells, computer LAN, and telephone. As neither earthing or RCD’s can be used to protect these cables.
One would have to be rather pedantic to insist these are wired surface.
If anyone can quote the regulation I will be interested of course. But in the same way as lack of earths in lighting circuits are dealt with using a risk assessment and if the owner with now budge you can use class II fitting I would say the same goes for when major wiring is required to split the floors and even though it may not be 100% correct one does need to bend.
Having said that since type C or D RBCO or MCB are to be considered and earth loop impedance will require measuring to ensure they can be safely fitted and only some makes do C and D RBCO’s and there could be compatibility issues it is not a DIY job. And all departures would need entering on the installation certificate this may mean authorisation from overseeing bodies would be required either LABC or self cert system and one would hope the governing body would not be pedantic but one does come across the “jobs worth” from time to time.

Postby PVS » Tue Sep 09, 2008 7:25 am

Many thanks to all of you who have taken the time to reply to my lighting problem. I will get your suggestions checked out and with this additional information hopefully resolve the problem. Last resort will be to ask a different electrician to investigate!

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