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

 

[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.