Re 30ma rcd main switch

[ianphilip007]

hi guys first post here just wondered if any advice was available.

my consumer unit has a 63 amp 30ma RCD mainswitch fed from a 60amp TNCS supply.i intend to run a supply from one of the spare ways in the consumer unit via 20a mcb to the garage which will supply 2 lights ,one outside socket and one internal socket via 2.5mm T+E clipped through loft and then out to garage via conduit (2m) horizontal and (2.5m) in height (bungalow) to a 5way 63amp main switch garage cu and then via (16amcb) and (6amcb) to power and lighting. 2.5mm and 1.5mm singles mounted in conduit.

i have two main questions

1/ im not happy with the fact that 30ma front end rcd is covering the whole installation( garage and bungalow), but apart from either changing the consumer unit to allow for dual rcd board, or the splitting of tails and seperate garage supply i can not see any other way of providing discrimination to satisfy the regulations.
as it stands any fault currently trips all circuits within the installation

2/ taking account of the issue of discrimination would this be noted as a limitation and a recomendation for a dual rcd board be made.


many thanks ian hope someone can advise

 

[Octopus]

Maybe a henley block and a 2nd CU for the garage?

 

[Mark.W]

1/ split tails into a switch fuse and then out to the garage cu with 30mA RCD.

2/ there is not reason to change your main consumer unit.

 

[SirKit Breaker]

Maybe a henley block and a 2nd CU for the garage?

Thats the way i would go as well.

Cheers.......Howard

 

[ianphilip007]

thanks for your reply Murdoch, thats what i was considering, henlyblock too switch fuse then swa to garage rcd protection(at garage end) just seems a little over the top for two sockets and a light, but with the board limitations i cant find any other way of providing discrimination to the garage circuit. if discrimination can not be obtained then i was thinking that due to the fact that at present the whole installation is covered by the 30ma rcd main switch and the fact that it is a bungalow (no stairs,one lighting circuit,one power circuit) it may not add to the dangers or risks if the garage was to be wired from a spare way in the existing CU.

 

[ianphilip007]

but in general i agree, garage supply via henly would remove problem.

 

[ianphilip007]

just to float this if it were to be wired from the spare way in consumer unit via the front end rcd and the risks associated with rcd tripping were minimal is there anything in the regs ( i cant find anything) that would dictate this shouldnt be done? i agree with the points made here just for future refeference really ?

 

[Mark.W]

No as long as it is RCD protected!

 

[ianphilip007]

thanks mark thats what i kinda thought. not best practice and not ideal to have one rcd covering all circuits due to loosing all circuits but retrospectively rcd main switches have been fitted so i guess its pretty common.

 

[Octopus]

I'd "sell" the benefits of the henley block/2nd CU to the customer, but its your customer!

Just imagine next winter and little fury creatures make a home in the garage and start eating the cable, bang out goes the house - not an ideal situation!

 

[ianphilip007]

i agree murdoch, just wanted to clarify that if needed could be done the other way and stay within regulations.

 

[i=p/u]

if not going to be much power , why couldnt you just spur of ring via fcu and spur of that for light.... providing not going to be much power used

 

[keniff]

If there's not many ways, change to a main switch and use rcbo's, although dual rcd board would prob be cheaper, I'd personally go down the splitting the tails route as the others have said, if it's for urself and you are doing the work, a dual board fully loaded only 70-100 quid

 

[ianphilip007]

thanks for the replies much appreciated.
on a similar note if you find this arrangement ie front end rcd mainswitch at consumer unit and customer requests a new circuit be added on a spare way ie shower or cooker radial. without replacing the board or totally changing its configuration, i see no way of complying with regulation 314 ( segregation of circuits) is it therefore a case of the consumer unit beeing treated as a retrospective installation and therefore does not need to comply, or by adding a new circuit to the exisiting CU configuration mean that reg 314 has to be met. any advice would be gladly appreciated.

 

[Johnny-G]

Split the tails & supply the new circuit via a new cu on an rcbo.
Then move any subsequently altered circuits to it.

 

[Mark.W]

There is something in the regs about circuits in a cu protected purely by an RCD needing earthed mechanical protection (steel conduit). It's in the OSG as well! Will look it up tomorrow

Mark

 

[keniff]

Think you may be thinking of if the ze is to high and have the rcd protecting the installation, think the rcd is like 2997 ohms or around that to operate

 

[keniff]

1667 ohms sorry

 

[IQ Electrical]

There is something in the regs about circuits in a cu protected purely by an RCD needing earthed mechanical protection (steel conduit). It's in the OSG as well! Will look it up tomorrow

Mark

Not in mine.

 

[ianphilip007]

can anyone advise as i cannot find a definitaive answer in the regulations. is it permissable to add a new circuit via a spare way to a cu with a single front end rcd? or is the only option to change board? is it not possible to just note as a deviation and advise customer that a single rcd is not ideal and that the board be updated.

 

[malcolmsanford]

You have answered your own question many times here and really know that the best solution is to split the tails and fit a switch fuse to feed the garage. That is the best design criteria but if the customer does not want this then you have to go down the route of the next best way.

Your the designer of the new distribution circuit and if you feel that by fitting it to an existing spare way in a CU that is controlled by a single 30mA RCD contravenes regulation 314.1 then you should inform the client and not do the job.

Personally as the garage is just a couple of meters from the house I would install it as the customer wants, I would not mark anything on the EIC as a deviation either, I would just inform the client on your quote that you feel the installation would be better by splitting the tails and fitting a seperate CU in the garage, and that by fitting the garage via the existing board it could lead to the problems you have highlighted in previous posts. You could also advise him that it could be a case that he fitted an emergency light say near the CU.

There are several ways you can get around things, but the bottom line is as the designer you have to make the descision.