I have a question regarding the use of dual RCD consumer units as appose to RCBO's on individual outgoing circuits.

I had always assumed that the wiring regulations stated that the use of an RCD shall only affect the operation of the individual circuit and that all other circuits shall remain unaffected by the tripping of the RCD. With this in mind how does a dual RCD board comply with the regulations. My understanding is that a dual RCD board would trip 'x' number of circuits upon a fault of an individual circuit.

Can someone please point me in the right direction as to where the regulations state this or where the use of dual RCD consumer units is detailed.

Thanks

Graham

The regs (specifically 314) aren't as black-and-white as that. It's qualified with words such as 'due account shall be taken of the consequences of the operation of any single protective device'. Things like 'minimise inconvenience' are minimise, rather than eliminate.

It's not helped by the fact that circuits are defined by overcurrent protective devices - not RCDs - so dividing into circuits (by MCBs or fuses) isn't necessarily undermined by a common supply side RCD as far as the wording of the regulations is concerned.

Obviously, an all RCBO solution (ideally using double-pole switching RCBOs), is technically superior. It's really just cost (and possibly size of the CU) that's the stumbling block.

- Andy.

Graham, are you saying that you're reading it as all circuits require their own rcd? if so that would be very impraticable, could you imagine how big the consumer unit would have to be! One way to achieve the same result would be rcbo's for each circuit but as Andy mentioned cost would be a problem there. I do sometimes use that method in small flat's and maisonettes where there are only 3-4 circuits, it does have advantages when a fault develops in as much as only one circuit goes off but it would be impracticable on the usual jobs where there are 8-10 circuits,

Dave.

Andy, thanks for the advise. I was generally referring to 314 and nuisance tripping.

Dave,

i was referring to all applicable circuits to have RCBOs rather than their own rcd.

I was merely asking the advise to see what the regulations actually state. We are seeing more and more on design and build projects that contractors are proposing the use of Dual RCD consumer units which goes against what we as the consultant have wrote into our Performance Specifications.

Thanks for your help

Graham

Unfortunately, that has become "standard practice" under the 17th.

That said, getting full discrimination can be pretty tricky. Feeding submains from an MCB (e.g. to a remote CU) is unlikely to discriminate properly on faults, regardless of the relative ratings of the devices; most British CUs are "conditionally rated" - i.e. over a certain fault level they rely on the supply fuse to open before the MCB, so still de-engergising the entire installation.

Real "nuisance tripping" is pretty rare - if a (domestic) installation is loosing >15mA, there's usually something pretty wrong somewhere (Just ask those with TT installations with a 30mA RCD main switch) - i.e. usually it's a proper fault, even if the customer regards it as "unwanted"!

- Andy.

I`ve always thought of minimise to take it to its lowest rather than just low.
In other words strictly speaking we should put each point on its own circuit . Of course in real life that would be OTT in almost all installations .
If minimize does indeed mean to the lowest then none of us comply.

Although Historically I am a front ender I do tend to use either Dual RCDs and some RCBOs or plain switch and all RCBOs, Ie cooker, shower, boiler not affecting or being affected by others

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

The use of 17th edition (sic) dual RCD boards, offers if at all, the minimum compliance with the Regulations.
As for using individual RCBOs, the board would not have to be any larger. In fact, it could be smaller as ther would be no need for space for the second RCD.

One of the problems with the 17th edition dual RCD boards, is that their use falls foul of 610.4 "For an addition or alteration to an existing installation, it shall be verified that the addion or alteration complies with the Regulations and does not impair the safety of the existing installation."
Fine to use the 17th edition dual RCD boards on new installations or a complete re-wire.
However if the work involves replacing an existing CU, where existing circuits are provided with individual CPDs. Altering the installation so that CPDs protect multiple circuits could be considered as impairing the safety of the installation.

Spin what board would you use on a TT system as a replacement board?

I think we also have to bear in mind that in 99pc of domestic situations, the loss of a few ccts due to a neutral earth fault taking out and preventing the resetting of an RCD wouldnt matter.

I would love to fit boards full of RCBO's to domestic premises but we have to face the facts that the customer will not pay an extra £200 for such an 'upgrade', and also in my mind no domestic friendly boards are fit for purpose when stuffed full of RCBO's.

When MK start making their enclosures from the 1990's that were double the height then i might consider it.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry

Hi , peteTLM I totally agree.

Last week I wired a kitchen and bathroom and changed a consumer unit in a small flat in Harlow.

The consumer unit had to be left where it was in the corner in the kitchen and down low next to the main fuse and meter.

It is going to be enclosed within the new kitchen units ( just about accessible ) .

For it to be able to fit in the cupboard with the space that I had left It had to use a Wylex 8 way board.

I had already bought the 8 rcbos to install so I had to use them.

By the time it was all connected and I must say I took my time and done it as neatly as poss it still looked a right pigs ear.

There was also a 10.0mm circuit and a 6.0mm circuit.

I must say I was quite upset with the way I had to cram it all in and if someone takes the lid off they will probably think it was a right bodge.

Lesson learnt.

Regards

typoid,
I don't see that a system being TT makes much difference to board choice.
peteTLM,
There are lots of instances where 99% of the time something doesn't matter.
Based on that assumption, do we actually require RCD protection in the first place?
The problem here, is that in 99% of cases, we will be stating that the work has been carried out in accordance with BS7671.
Part of that statement will be that the safety of the installation has not been impaired.
We have been required for many many years, to divide installations into circuits to avoid danger and minimise inconvenience.
Why should we ignore this long standing requirement, in order to comply with what is a relativly new requirement, one that in 99% of the time won't matter.

Losing half a flat or house is minimising inconvinience while keeping it compliant. Were it to be something a bit more critical, then a different approach is always required. I always put stair lifts for instance on a separate non rcd way because the surface wiring in this instance allows me to do so.

99pc of the time the loss of a circuit or 2 isnt critical. I dont see how we can compare that to not using RCD protection when we are required to do so. That remaining 1 pc might mean life or death for a customer. Just because it sits around for years waiting to be put to use is no excuse.

A domestic install is always a compromise on price. By this i mean we dont do them in micc or galv conduit do we.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry

With the exception of off peak i have used rcbos totally since the 17
edition came into being.Crabtree starbreaker are around £18.50 in cornwall!
Just wish crabtree would hurry up and increase the size of their units-
will catch the rep at eddies redruth on wednesday .........

To reiterate, it's not just about minimzing inconvenience. It's also about avoiding danger.
Just think of all the things someone could be doing which would become dangerous if the lights suddenly went out.
If you alter an installation so that the lighting can now be affected by a fault on another circuit, or even just through an accumulation of normal usage earth leakage, then your alteration has impaired the safety of the installation.
As such your alteration would not comply with either BS7671 or the Building Regulations.

Although you could argue that such a situation could just as easily arise due non-final-circuit related faults - supply network failure for instance (more common in some areas than others), or a fault on submain, or just lamp failure - thus if there is a danger from loss of lighting, it should be addressed in a comprehensive manner - e.g. multiple self-contained emergency lighting, rather than laying all the responsibility on final circuit protective devices alone.

- Andy.

" which goes against what we as the consultant have wrote into our Performance Specifications."

Graham

Performance specification: "Comply with BS 7671"

Design: "Each individual circuit shall be protected by a RCBO" and accompanied by the designer's sinnature (sic) on the EIC

Artist's impression (sic & (c) BOD) : What most contractors receive from the builder's architect/consultant and still end up signing the EIC themselves.

Regards

BOD

I would agree that most architects and interior designers drawings and specifications are "artists Impressions (c the BOD). Merely positioning sockets and switches on a drawing along with the positions of the sofas does not amount to an electrical design although it would appear they can charge a fee that would give a sensible human a nose bleed for the "design". Of course the acid test is to ask them to sign one of the design boxes on the EIC!

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John Peckham

http://www.astutetechnicalservices.co.uk/

Is it not design John ? - clearly selecting the outlets and the locations to meet the clients brief is design - it's just a partial design though - it requires the contractor to develop the design, add the confirmation of circuit selection (and possibly calculation) in accordance with a defined set of criteria (usually BS 7671, BS 5839-6 etc for a domestic).

If pre designed circuits are available for that industry sector then it's as much a design as a contractor undertakes when selecting circuits from say OSG.

Best not to mix up design with calculation or selection - the latter two aspects being only a minor part of design - design is all encompassing and often requires input from several parties to the project.

Right, on to my next blob diagram at a healthy fee -

Regards

OMS

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Failure is always an option

Graham, that is a great question. I think the manufacturers have made good use of the grey areas and have subliminally branded the dual RCD board to us as a 17th Edition board.

An all RCBO is good for the client because of the loss of only one circuit in the event of a fault, but not quite as helpful for us on a call out. Just the RCDtripping, the RCD and the 'MCB'( naughty of me to use that term but suitable here I reckon), or the MCB only? Makes it easier to know where to begin on an RCD/MCB board with a fault.

I tend to use all RCBO on older wiring/ DB changes because an RCBO will act as a precursor to major issues ahead and tell us which circuit is priority. On my own new wiring I tend to go for RCDs x 4, I work in big places but Zs towers is RCD x 2.

I remember looking at the '17th Ed' boards and back at the regs when they changed and seeing grey. I wondered if we were being used by the marketing machines. Not forgetiing of course that The manufacturers are represented on important panels. I supressed my suspicion at the time for fear of being negative but it still nags me.

Glad you asked that out loud.

Talking of manufacturers; have any of you had a letter from Trading standards recently on the subject of the wylex etc. recall? Start a new thread if you have one too....I've got one which seems like a standard letter but I'm not sure.

Zs

Oh, I lied...I've just remembered that Zs towers is actually RCD x 3 because of a little sub DB resulting from my being too lazy to run a cable all the way across to the main DB and wanting something separate for the dream tubed amp which I didn't have at the time but do now (You'd love it R&R). God Bless the Sunday afternoon Henley block and the tiny DB eh? It's a wylex.....Recalled!

Edited: 14 August 2012 at 09:05 PM by Zs

To much speculation and not enough facts here, without actually being told what the consultants have written in the performance specification we cannot really comment.

Personally on the job in progress I have opted for a duel RCD consumer unit with one RCBO.

The smokes and hall (exit) lights are on the RCBO, then there are at least two lighting circuits and several socket circuits split between the two RCDs, so no RCD trip will take out all the lights, the smokes aren't with sockets and if you try turning them off at the RCBO you lose the hall lights making it a pain in the butt to discourage anyone from doing so, plus the smokes all have battery back up anyway. Also if a RCD trips you only lose half the circuits, but not the hall lights or smokes.

What more do you want? Why make it £400 in materials instead of around £90 per consumer unit?

On the five flats I have in progress you would have a increase in cost of over £1500 to achieve what? My customer would not agree to it!

Andy