Discrimination for RCD protection on garage CU

[SpannerSpark]

Hi all,

This is my first post, hopefully someone will be able to confirm my suspicions. I am newly qualified so need a bit of hand holding sometimes. I am most probably going to use this piece for elecsa part P registration as its notifiable, so better get it right!
I am fitting a garage with a small CU (16A radial sockets, 6A radial lights), 30mA RCD protecting the two circuits. The garage will be fed from a CU in another larger building which has a 10-way CU but only has 2 of the ways being used so there is plenty of space.
The 10-way CU incoming supply is TT and the 10-Way CU already has 30mA RCD on it. Therefore i want to find the best way to ensure some discrimination between the 2 CUs (i.e a fault in the garage doesnt trip both garage 30mA RCD & larger building 30mA RCD). I was going to to add a non-RCD protected neutral bar to the larger building CU and take the L & N from the input of the 10-way CU RCD, feed it to a 30A Type B in the 10-way CU and then on to the garage. hence the only RCD protection for the garage is in the garage CU. Is the best solution?
Also I am exporting the earth from the larger building CU to the garage (i.e. the garage will not have its own ground spike) is there a preference on this? I think either exporting it or adding a garage spike should both be fine. FYI The garage will be fed by fed by 4mm 3-core XLPE SWA. The distance between the buildings is @10Metres
Any help welcome. Tks

 

[ukpablo]

Firstly why not 20A radial sockets?
thats your first problem, page 363 of regs will show you this, unless u are running ccts with a dedicated specific load?

As with the main board, you could split it and add another neutral bar and use an rcbo to feed garage. (32A)

see link for additional neutral bar. or buy one specific to board?!

WAGO Kontakttechnik | PUSH WIRE® Connector for junction boxes for 6 mm²

or change board for new split board as it is only running two ccts so would not be too much hassle to change and test?

With regards to earthing, use a wiska box for armoured, use three core, earth gland nearest to main CU and connect to earth with fly lead to earth armoured sheath, this will comply and will be less hassle.

Just a suggestion really!

 

[AlanWirral]

How come the CU in the larger building only has 2 circuits being used? Sounds weird. As said though i'd use a 2 way board in the garage with a non RCD main switch and use an RCBO in the house CU. In regards to the earthing I wondered myself until it was answered by Pablo. I suppose as long as the ZE is low enough then it should be fine?

 

[SpannerSpark]

Thanks for replies. The small garage CU came fitted with 30mA RCD (essentially a 40A main switch/RCD), a 16A B MCB and a 6A B MCB. I dont know why is has a 16A, I know 20A is normal for radial 2.5/1.5 i will change to a 20A.
Do I take it from your replies that my suggestion to split the 10-way board to make it one side RCD and one side non-RCD via 30A type B MCB and then use the garage CU RCD to cover the 2 off final circuits is not ok?
I agree that the RCBO suggestion means everything downstream from 10-way CU to the garage is now RCD protected, but it does also means that any fault in the garage that brings out the RCBO in the other building can only be reset by going to the other building. the garage feed doesnt come from a house, it comes from a large warehouse/outbuilding which is not used at night and normally in darkness, therefore any RCBO trip at night would mean a difficult/dangerous walk between buildings in the dark. There is definitely only 2 ways used in the 10-way CU. lights and sockets. i think they must have just had a 10-way CU convieniently to hand and probably thought of future expansion which was never needed.
If the RCBO answer is the only way (I dont think the householder will go for the expense of a new CU to replace the 10-way one) then i will do it this way, but i do want to be sure before buying the RCBO and buying a main CU switch for the garage CU.
I think i am fine with the SWA stuff to the garage CU, although using the third core for earth I know the SWA must connected to earth too. but i will look up a wiska box for interest.
tks

 

[AlanWirral]

Could you not replace one of the main RCD's in the 10 way board with a normal switch, then use a normal MCB in that side of the board. That way you can keep the garage unit as is and have the RCD protection in there? No need for 2 RCD's in the outbuilding if ones not doing anything?

 

[SpannerSpark]

Ok bit confused now. bear with me. The 10-way CU only has one 30mA RCD covering the whole CU. My original suggestion (in original post) was to make 10-way CU it a split load board with the one RCD covering the 2 circuits that are used on the 10-way CU. then on the other non-RCD side of my newly split 10-way CU i fit a 32A MCB to feed the garage. and then use the RCD in the garage CU to protect the final circuits. i think that your post above agrees with this? If so then i am home and dry. best regards

 

[AlanWirral]

Ohhhhh sorry mate I read that you already had a split board in the outbuilding! My bad.

 

[Deleted member 9648]

Ok bit confused now. bear with me. The 10-way CU only has one 30mA RCD covering the whole CU. My original suggestion (in original post) was to make 10-way CU it a split load board with the one RCD covering the 2 circuits that are used on the 10-way CU. then on the other non-RCD side of my newly split 10-way CU i fit a 32A MCB to feed the garage. and then use the RCD in the garage CU to protect the final circuits. i think that your post above agrees with this? If so then i am home and dry. best regards

If you do this and the earthing is a TT the Zs will almost certainly be too high to provide earth fault protection to the SWA distribution circuit.....RCD protection,100ma or 30ma will have to be provided for the SWA in some form.

 

[SpannerSpark]

Right, totally with you. To recap...
As its a TT, in order to ensure disconnection requirements are met on the SWA bit between boards there must be some RCD protection, and this effectively "trumps" my arguement of trying not to have the RCD in the remote building.
I will explain to the customer and look into 100mA type S to provide protection at the large building, + 30mA RCD in the garage which should satisfy both the RCD protection for all parts of the install and means that only the garage trips when a fault is in the garage final circuits. Or if he prefers not to spend £££ money on 100mA type S, then just the 30mA RCD (or RBCO) in the large building protecting the small CU in the garage (main switch and MCBs only), and accept the inconvienience of a trudge between buildings if the 30mA RCD trips. You have all been a massive help thanks.