The perenial garage question...

[cybdarren]

Hi all,
I know that questions similar to this have been asked before but I could not find a relevant answer so here goes...

I have an existing CU (early 90s with cartridge fuses) that is being replaced with a 15way unit as part of a house extension. The existing system was 'expanded' some time ago for a garage/office feed. The supply for the garage is fed through a 20A fuse and then into a ~20A 30ma RCD which is fitted to the side of the CU, it then goes via SWA to the garage where there is a DP isolator.
[there is a host of other switched stuff around the CU such as bell transformers and immersion heaters which will also be cleaned up in the process]

Given that we don't want to touch the garage side of things if at all possible is how to adequately protect the outbuilding and 'incorporate' the old RCD into the new CU?
I cannot seem to find a suitable DP RCCB (Wylex brand) so how best to configure the system.
My current thought would be to put a 20A DP MCB in the CU with the live feed then fed through an adjacent RCCB/RCBO?

 

[plugwash]

Another possibility would be to replace either the isolator in the garage CU or the whole garage CU with one that has a RCD. SWA runs do not need to be RCD protected.

What size and how long is the cable to the garage CU?

 

[cybdarren]

Hmm... I'm trying to avoid hacking around the garage too much (plasterboard walls and the like). The garage just has an isolator feeding the sockets and an a fused spur for the lights... which is presumably why they put all the protection at the house end at the time. I'm happy for it to stay at the house end, just a matter of 'neatly' configuring the CU.

The SWA looked to be 4mm2 per core (from memory, since I'm not at home at the moment) and the total run is probably about 15-20m.

 

[RF Lighting]

Just use a normal SP RCBO in the new fuse box.

 

[Electrifying]

Someone will appear shortly and ask - 'What does your electrician say??'

But instead, I'll ask the other relevant questions:

What is the installation earthing system?

What size is the SWA that runs to the garage?

Are there any extraneous conductive parts in the garage?

What is the garage constructed of....walls/roof/floor?

You might think these are strange questions considering you don't intend to alter the garage electrics - but as you are swapping the CU, and the garage sub-main is one of the circuits fed from that CU, then the questions are relevant.


EDIT: Just seen the 4mm in your previous post.....need the other answers, though.

 

[cybdarren]

Hi, thanks for the folow ups... to answer your direct questions...

Earthing is TN-C-S
The SWA is actually 2.5mm2 per core (16mm overall). my mistake..

No there are not any extraneous metal conductive parts in the garage/office,
It is brick built with solid floor and tiled roof, dry-lined. The wiring arrangment in there is pretty clean in my opinion (although was done by the previous owner).

As I said before I guess my real question is what would be the neatest configuration for a single CU to provide comparable (or better) protection to what is currently installed (whilst minimising any changes to the garage/office)

thanks for reeading