RCD keeps tripping on reconnected lighting circuit in new CU

[robinw01]

Confused by this one, and any explanations & advice gratefully received. NB Although I'm a DIYer, I will be getting a professional to do the part P and a PIR when I am done.

I have an old house that was last rewired more than 30 years ago (no earthed lighting circuits, etc).

I have replaced the CU and am going through the circuits replacing them, one by one.

I do have one lighting circuit that is about 5 years old, is earthed, and because of the decorative work involved in replacing the circuit, I would like to try and reuse it.

When I disconnected it from the old CU, it was in a 6a MCB. I have inserted the cables into a 6a MCB in the new CU.

When I flick the MCB on, the RCD protecting the socket circuits in the CU blows (but not the MCB for this lighting circuit), and the negative bar in the CU lights up a neon screwdriver suggesting it is live. If all the switches on the circuit are off, this doesn't happen.

If I wire the circuit into a 32a MCB covered by the RCD, I get no fault, and the lights work and switch fine.

The circuit has 2 sets of 4 GU10s (single-way switching) and 2 60w bulbs on 2way switching.

I don't think I've done anything to the circuit in between disconnecting from one CU and connecting to the other. I originally suspected a short, but that wouldn't explain it working on a 32a MCB would it?

Advise gratefully received, or suggestions of what to stick my multimeter in (polite ones only please!)

Thanks for your help.

Robin

 

[robinw01]

Update: continuity testing suggests it is a short. Why didn't it trip the RCD though when plugged in to higher rated MCB?

Any suggestions as to how to track down the short? Have multimeter - will probe...

 

[Spark123]

I think you need to read up on Part P, you should have notified your LABC before you started the work and paid their fee. You should also check all your circuits are sound as the new CU is commisioned, filling in an EIC.
A couple of causes I can think of immediately - the neutral for the lighting circuit may not be in the correct neutral rail in the consumer unit or there is a shared neutral. Connecting lighting circuits to a larger MCB can be dangerous.
I think others will agree, it is advisable to get an electrician in who is a member of a competent person scheme and get them to do the work for you - they should have the correct tools and experience for the job.

 

[robinw01]

Thanks Spark123.

Advice gratefully received. Will re-read up on part P - I know I'm supposed to notify BCO first...

I'm just trying to prove that there is still a place for careful DIYers, who do as much as they can before bringing in the professionals. I'm slowly restoring a 16c farmhouse - and if I didn't take that approach I couldn't afford to get the job done to the high standard I would like.

Incidentally, is it possible to hire the some of the more sophisticated testers required for PP/16th Edition testing regimes?

BTW - was my mistake, I was connecting the neutral to the wrong neutral bar. I'm generally slow and proud of the standard of my work, so am embarrassed at such a simple oversight...

 

[Spark123]

You can hire the testers, but (not getting at you) if you don't understand what you are doing with them and how to interperet the results then it is a complete waste of your time and money. You'll also need to inspect the condition of the wiring, accesories, bonding etc etc etc before you start testing, a good understanding of house wiring and the regs is required.
On another note though, if you go the LABC route it is up to them to ensure it complies with the building regs, this will mean it will still need to be inspected and tested but is up to them.
Have a read here : //www.diynot.com/wiki/electrics:diy-electrical-work-and-the-law

 

[saxondale]

and throw your "neon driver ...." away too, theres some good books talked about on the forum that`ll explain the basics to you but I`m afraid your going about it the wrong way with Part P etc

 

[ColJack]

nevermind, didn't see you found the fault..