New circuits on existing 17th ed. RCD / RCBO Consumer Unit

[GoodDIYjob]

We had our very old cartridge fuse board replaced with a 17th ed consumer unit about 2 years ago (2014).

The new unit (2+ years old) is a Wylex (I know there is mixed product opinions, so please keep that to one side for the moment).

It is configured with a few MCBs on the RCD protected ways and the remainder are RCBOs. There are spare ways presently on both the RCD side and on the unprotected side for RCBOs.

There are no problems with this replacement unit to date. It has only tripped once (RCD trip) since the electrician installed it - due to a faulty oven element. He provided test certificates and self cert to the council at the time of installation.

We are close to getting quotes for a rear extension to the kitchen / Diner approx 24m2.

I would like to spec what expectations are for electrical a little more detailed than the notes the designer has, which says something like electrical work to be carried out in accordance with BSxxxxx.

My question is, for new circuits say at a minimum 1no new lighting circuit and 1 new socket ring for this extension area - will a new consumer unit need to be installed or can MCB or RCBOs be added to the existing consumer unit whilst complying with relevant regulations?

Hope I have made the question clear enough. Many thanks

 

[Taylortwocities]

Like most installation queries, It depends on what is planned and what is already there.
Even a large addition to an existing house can be catered for by simply extending the existing light and socket circuits. New circuits may only be needed if special requirements are involved - eg high current items like a cooker/shower, or a new kitchen/utility area which may need a new circuit of its own.

Even if a new circuit is needed, all that is required is an additional MCB or RCBO, and a registered electrician to do this and notify the works, also that what he has done complies with BS7671. Building Control will want all of this to complete the buildiong notice completion..

A new consumer unit is NOT required. Ignore anybody who says that is must now be changed to a metal one, or other such rubbish.

 

[GoodDIYjob]

Even if a new circuit is needed, all that is required is an additional MCB or RCBO, and a registered electrician to do this and notify the works, also that what he has done complies with BS7671. Building Control will want all of this to complete the buildiong notice completion..

A new consumer unit is NOT required. Ignore anybody who says that is must now be changed to a metal one, or other such rubbish.

Excellent. Very clear thanks.

I appreciate the existing circuits could be extended but with potentially quite a bit of lighting and power demands, I was minded to think new circuits could be useful. The current arrangement is a little unusual as well, in so far as the ring from upstairs also feeds the kitchen sockets at the back of the house on ground floor. The kitchen being an extension from the Mid 1980s. The electrician tested it all and said it was ok but also it is not how it would be run now. The extension would be an opportunity to tidy that up too.

Like you say, it probably doesn't have to be on new circuits but it might have some advantages. I'll discuss options with an electrician nearer to works. Good news I don't have to pay for another new consumer unit though after paying for this one fairly recently and the idea was to allow sufficient ways for future works.

Thanks

 

[ericmark]

With lighting today using less and less power it is unlikely you will need another lighting circuit. My house with three bedrooms has a single lighting circuit and years ago it was getting near to the 6A maximum where ceiling roses are being used as junction boxes, as they are rated 6A.

However today LED lighting has resulted in the lighting load getting less and less, and the quartz bulbs I used years ago will shortly be discontinued so I could not return to them even if I wanted to.

Unless electric heating is used, then most houses are supplied from a 60/80/100 amp fuse, and it does not matter how big you make the house, as long as only one kitchen and laundry area i.e. not turned into flats, then power used does not vary much. So really there should not be a problem.

 

[JohnW2]

With lighting today using less and less power it is unlikely you will need another lighting circuit.

Whilst that is certainly true in terms of power requirements, there is a lot to be said (some would probably say 'required by regs') to have lighting spread across two or more circuits (with different RCDs or RCBOs), in case of failure of one of the circuits.

Kind Regards, John

 

[securespark]

My point. I would fit a new circuit for lighting that satisfies the regulation John is referring to. At the same time, I would disconnect the existing sockets from the first floor ring final, complete the ring if necessary with an MF JB or accessible connections and install a new circuit or circuits for the extension power, be they a ring final or radials.

 

[JohnW2]

My point. I would fit a new circuit for lighting that satisfies the regulation John is referring to.

As I said, I would say that it makes good sense to divide lighting circuits. As for what satisfies the regulation I was referring to (314.1), that regulation is worded so vaguely that there will be a lot of variation in judgement and interpretation as to what is actually required.

314.1(i) requires that the installation be divided into circuits such as to "... minimize inconvenience in the event of a fault". If one takes the word "minimise" literally (OED definition = "reduce to the smallest possible amount or degree"), then every single light, socket or connected load/appliance would have to have its own circuit, with its own RCBO. Since that would be unrealistic ('ridiculous'??), it is down to each designer to decide how far they have to go in "reducing inconvenience in the event of a fault" in order to reasonably satisfy the regulation. Opinions are likely to vary considerably.

Kind Regards, John

 

[securespark]

Yes, of course, the book is open to a wide variation of interpretation.

I even rewired the house of a retired engineer such that the 3 lighting circuits were arranged that if any one failed, there would still be light in an adjacent room or area.

 

[ericmark]

I have a battery backed light top of stairs for stairs and landing and a single lighting circuit. Also a battery backed light in garage where the old fuse box (now with MCB's) supplied by one of the RCD's, housed an a separate box, are located. I have never had a problem resetting the MCB or RCD when they trip nor have I had any danger when they trip.

In daughters house even with two RCD's and two lighting circuits failure of a RCD or MCB can cause danger, so it really does not comply!

It is nothing to do with how many lighting circuits are used, it's how the house is designed. With a stair case with windows allowing street lights to light the stairs then any circuit failing in the house is not a problem. Where as with my house the stair case is central so no light from street lights then it needs some method to ensure one is not plunged into darkness. Although using two circuits may reduce the chance, using a battery backed light will not only provide light with a trip opening in the house, but also with a general power failure.

In general putting the lights on a 6A RCBO means even with a single circuit it is rare now tungsten lighting is no longer used, for a bulb failure to cause lights to trip. Using a RCD which also supplies other devices specially a socket circuit does mean that the lights are likely to fail more often. I have seen where both lighting circuits are on the same RCD, which if the RCD feeds other items as well means it is very likely you will still loose all lights.

Use of table or standard lamps means as long as sockets are not on the same RCD and lights two RCD's need to fail to plunge one into darkness. It really does not matter if one or two lighting circuits, as long as not on same RCD as sockets.

We have to sign that we have designed the system, and as part of that design we should do a risk assessment. As long as we have done that then we should not incur danger in normal use of the premises. Designing does not mean simply saying we always do it that way, one should take into account street lamps. So a house mid way between two street lamps may need slight wiring changes to a house with a street lamp right outside. OK street lamps can change, but so can everything else, so we wire with consideration of how it is at the time of wiring.

 

[securespark]

I don't agree that you should rely on street lighting if a houses lights go off...

 

[bernardgreen]

Area power cuts put houses and streets into darkness. Hence battery backed emergency lights on the stairs is a very good idea.

Off thread With new LED street lights the option of battery back up becomes a possibilty. ( unlikely to happen though ) Battery back traffic lights would reduce accidents in blackouts.

 

[AndyPRK]

If the extension is single story you may find the lighting is already split suitably.

One of are local traffic light junctions has backup due to frequent power cuts.