Old consumer unit/ extending circuits

Postby Nicktoxic » Tue Nov 18, 2008 11:50 am

I have a friend who has an old style consumer unit, he wants to extend a lighting and socket circuit to make another room, can you extend circuits from old consumer units? Or does the 17th edition mean if the work is done then things will have to change in which case what would probably be done?

Also what is the correct way of configuring a new consumer unit if everything has to be on RCD as there seems to be no guidlines!!! and what are the common problems once everything is on RCD's

Many Thanks

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Simply Build It

Postby ericmark » Wed Nov 19, 2008 5:44 am

First be aware consumer unit changes come under Part P that said I’ll try to answer questions.

To extend sockets all new sockets under 20 amp i.e. all 13 amp sockets need RCD protection except where they supply a designated piece of equipment like an oven or freezer. You could fit all RCD sockets to comply with this. However also all cable buried in walls, ceilings under floors etc at less than 50mm also needs protection this can be by using a RCD or by using one of the special cables etc. So if all extension wiring is in Al-tube cable and RCD sockets are used it would comply.

Lighting with the exception of the bathroom does not need RCD protection however as with sockets the cable may again using Al-tube cable to BS 8436 would comply there seems to be four major manufactures of Al-tube cable GuardianTM, EarthshieldTM, FlexishieldTM, AfumexTM assuming no outside, kitchen, or bathroom work is required then this may be best option to avoid Part P.

As to replacing the consumer unit if all RCBO’s are used there is no problem but where one wants to use just Twin RCD’s you need to consider the risks and results if one RCD should trip.

Regulation 314 does not say you have to fit two RCD’s but refers to dividing into circuits to avoid hazards and inconvenience with special note on danger that may arise from failure of a single circuit such as a lighting circuit. This has been the case for many years but it now includes reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced. This points out that the dividing into circuits also includes RCD as well as MCB and maybe we should have been considering twin RCD’s for many years? But the way it is worded using emergency lighting would alter what is required. As would battery backed smoke alarms etc.

Hagar do an example of wiring a twin RCD consumer unit where upstairs lights are combined with downstairs sockets and downstairs lights with upstairs sockets and most people seem to think this complies.
However one may consider having any sockets combined with lights is wrong or one could consider in small houses what is used is no different to a caravan which has to supply all power through two RCD,s in series rather than is parallel?

Here we need to consider who is signing the paperwork and if the LABC say you need to use Two RCD’s plus a RCBO for smoke alarm that’s what you need. And I would suggest you check with who ever is signing paperwork as to how they want it configured.

As to common problems well most are to do with RCD’s and I will try to list a few.
1) Cooker or other heating elements which may be hydroscopic having absorbed water causing the RCD to trip.
2) Stairs lighting having been wired with only twin core strappers on two way lighting causing borrowed neutrals scenario if lights switch on with one combination of switches RCD trips but may work OK with another combination.
3) Damp and dust causing leakage and so RCD to trip.
4) Combination of numerous leakage paths including computer surge protection building up and causing RCD to trip.
Except for No 2 the more RCD devices the less likely it is they will trip.

RCD’s should not trip at half their rating so with 8 RCBO’s you could have 8 x 15ma or 120ma leakage without it tripping assuming all circuits with same leakage but with twin RCD’s only 2 x 15ma or 30ma before you get problems. Also of course where one can’t cure fault for example faulty cooker one could replace the RCBO with a MCB as a temporary measure until the cooker is changed or repaired but one could not really justify removing the protection for whole house as a temporary measure.

It is normal to do a full PIR before starting on a consumer unit change as very hard once started to convince a customer why it costs more. The main problem is lights the Electrical Safety Council to which most the regularly bodies subscribe say if there are no earths to lights certain measures must be taken and if not then the electrician must refuse the work. A little late if he has already started hence always full PIR before starting then another once complete. It is of course all this inspection and testing which raises the price of the job and one is sometimes temped when everything looks good to cut corners and only test on completion.
Does that answer your questions?

Postby Nicktoxic » Wed Nov 19, 2008 2:07 pm


Many thanks, I have part P but will get someone to show me how to test and inspect etc as I am new to electrical work and am learning slowly!! Its finding someone good to learn from these days thats hard!!!

So the sockets will be ok if I install rcbo, or replace consumer unit with new 17th edition consumer unit all rcds?

One last thing you fountain of knowledge!!, if its a new consumer unit all rcds how do you wire a freezer/ oven and not have it on rcds?

Many many thanks

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Postby ericmark » Wed Nov 19, 2008 4:19 pm

Consumer units can have one to three neutral bars and Line bars with one you would use all RCBO units and to supply anything without RCD protection you simply change a RCBO to a MCB.
With two bars there are two types those with two RCD's and those with single RCD and the other section without a RCD would normally have RCBO's fitted again simple change a RCBO for a MCB. Those fitted with two RCD's are not really designed to have any non protected circuits and you would have to change a RCD for an isolator or RCD of higher than 30ma at 40ms design and fit RCBO's in those requiring earth leakage protection and MCB's where it is not required.
Then we get to the three bar type where there are two blocks of RCD protected MCB's and also a couple of positions designed to have RCBO's fitted.
It seems very few people are at the moment going on to the Ali-tube route and most freezers and cookers are supplied with RCD protection.
Freezers are unlikely to have any problems with RCBO supplies and I can't see any real reason to provide them with non protected supplies.

Cookers and other items with mineral insulated elements which are by nature hygroscopic may need a supply with more than 30ma earth leakage or being real 15ma as this is what the units will hold in at.

Where the problem then arises is 543.7.1.2 comes into play as over 10ma leakage so 10mm or twin 4mm earths are required. And Ali-tube does not have that so then you need to supply the cooker with steel wire armoured cable or steel conduit and very few electricians would want to go down that path.

So for any normal home I would RCD protect every circuit. There will form time to time be specials for example Radio Amateurs have some special requirements but in the main if a customers cooker trips out the RCD you dry it out once and if it happens again you tell them they need a new one.

I am not willing to tell you on here how to dry it out as it breaks the rules and if someone is not careful it can be dangerous if you email me then person to person I would tell you how but people read these posts and try doing things without the skill required.


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