Joining two Lighting circuits


Postby Julian Lovegrove » Wed Nov 18, 2009 12:37 pm

Hi all,
I'm working on an Edwardian house which used to comprise a house and flat, I have re-converted it back to just a house, and am tidying up the electrics.
As the Flat's CU was located in what is now an en-suite bathroom I am removing its circuits, and joining two lighting 'rings' on the same floor, to run off the House CU in the cellar. I have already completed the 30 amp rings successfully.
When switching the lighting back on, several lights of each former ring failed to work, yet others on each former ring work fine.

As I didn't fully map and test each ring beforehand, I can't 'prove' that the House ring was unbroken, and I may have inadvertantly created a new ring by discarding sections of old ring or spurs etc.

Q What would happen if I had connected two rings with one cable ie: like a pair of spectacles? Would that work? or be dangerous?

I am sure the Flat ring was correct at the start, as it was only a few years old, and of good standard.

PS I connected by choosing one adjacent light rose in each circuit, and diverted one cable from each into its own JBox and another from each JBox to the other light rose. I didn't affect either switch cables.

Q Any ideas & suggestions please?

Thank you
J
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Postby ericmark » Wed Nov 18, 2009 6:06 pm

The smallest cable permitted in permanent wiring is 1mm and this even even with installation method 103# is rated 8 amp but ceiling roses are only rated 5/6A so normally 5/6A fuse or MCB is largest that can be used so for current carrying capacity no need for ring.

With volt drop of 3% maximum there may be need for ring on large house but unlikely the prospective short circuit current at furthest fitting should be higher than 200A. Really one should be working in ohms but since the meters read in amps I have given rough minimum as shown on meter.

Forming a loop will not stop anything from working but it will mean that the testing will become a problem it is not permitted on ring circuits for sockets as it can mean unequal power sharing but since your not over cable limits anyway I can't think of a reason why it would need correcting.

The lighting radial often spurs off in many directions as unlike the power you are not taking the cable to the limits.

However with power the double ring is a big problem and must be avoided mainly as it is impossible to test to ensure the ring is unbroken.

Because under the old regulations you were allowed 4% volt drop and that has now been reduced to 3% for lighting plus now lights in bathroom need RCD protection one does have to be careful when doing alterations as what would have passed years ago will now fail.

I am sure many electricians only worry about earth loop impedance and never bother with the prospective short circuit current and don't worry about volt drop. But may DIY people have the wiring tested by a third party like the LABC under Part P. So they may find after all their hard work it fails so feel it is important to say what should be done.

I noted with a little surprise the new forms do not require the prospective short circuit current to be entered at other than the consumer unit but the ELI is and where that shows over 1.5 ohms then I would suspect the volt drop would be exceeded however unless over 7.8 ohms many inspectors would let it go.

The problem is discharge lighting which is what most so called low energy light fittings are is very dependent on correct voltage hence why the rules have changed. However since they use less power when all fittings use low energy luminaires there is rarely a problem.

The problems are more to do in reality with pedantic testers.
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Postby moggy1968 » Wed Nov 18, 2009 7:07 pm

having a ring on a ring is dangerous.
You may also have overloaded your current lighting circuits
If you have fitted junction boxes under the floor unless they are of the non maintainable type (i.e. without screws) you've breached the regs.
Are the new cables you have laid protected by RCDs, if not you've breached regs
have you checked that your bonding is in place, if not you've breached the regs
Are any new circuits in the bathroom RCD protected, if not you've breached the regs
Are your sockets RCD protected, if not you've breached the regs
Are you able to issue the appropriate certification for this work?

It can be tested using the appropriate tester. with the power off you cross connect the incoming phase to the outgoing earth and vice versa with the other leg. you then test R1 R2 at each socket outlet and your readings should be substantially the same. If they aren't then a comparrisson of the results and a map of the circuit can be made to try and identify what are spurs and whats on what ring.
Your into heavy and complex stuff here (unless your a spark in which case I appologise for all the above and below because you know it all ready!), and assuming you aren't part P registered or notified building control already your already in the poo because the work you've done is notifiable.

If I were you I'd stop digging and get someone in.
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Postby Julian Lovegrove » Thu Nov 19, 2009 9:40 am

Hello and Thanks guys for your advice.

Just to clarify - All my circuits are RCD protected, and all bathroom, shower and gas meter fittings are 10mm earth protected. No I don't put junction boxes under floors, but I do in the loft space for lighting, where they are accessible.

I will of course have the whole house checked and certified when completed.
J
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Postby moggy1968 » Fri Nov 20, 2009 9:55 am

Good to here your response, it suggests you have a greater knowledge than most.
I think you will struggle to find a spark to certify this though as they are certifying to say the whole installation conforms, which they don't know unlesss they have been informed all along. the best they would do is a periodic inspection. to register with building control yourself they need to know before work commmces, I would get in touch with them now unless you already have.
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Postby mickel » Wed Apr 20, 2011 1:39 pm

Thats what used to be done, before we started splitting the circuits. Providing you don't exceed the loading(unlikely)

http://www.lightingforthehome.com/Lamps-s/20.htm
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