Garage Db2 To Db3

[gazdkw82]

So my neighbour caught me as I was bringing the bins in. "your a training electrician aint ya" oh feck ere we go haha

anyway, story goes...

He has a 10mm swa from his house cu to a garage in his back garden. Its terminated into a 2 way Volex consumer unit that has a socket radial circuit and a lighting circuit. This 2 way board has both circuits protected by an RCD however The SWA from the house cu is on the non rcd side of his cu in the house. I told him I think it should be RCD protected.


So his question was,


“we are thinking of having a little summer house at the far end of the garden. Could I buy a larger board for my garage to replace the 2 way board and then take a swa feed from that board and put this little 2 way board in the summer house to supply the sockets and lights in there”


I was honest with him and said I wasn’t sure. I did stress that the swa feed from the house should be rcd protected even though the 2 circuits in his garage are protected by an rcd on the garage board.


Can you just keep adding CU to CU to CU if you worked out the CCC of all cables/total demands and ze/zs was satisfactory? It just got me thinking

 

[hightower]

Why do you think the SWA should be RCD protected?

 

[gazdkw82]

I thought all outbuildings should be rcd protected?

if only rcd on the board in the garage then doesn't that leave the SWA unprotected?

 

[hightower]

Just trying to get you to think about it bud, show me a reg that says the SWA should be RCD protected.

 

[gazdkw82]

I haven't got any books with me at the moment as im at work but some google searching has come up with

reg 522.6.6

cables that Incorporate an earthed metallic covering do not require RCD protection

so it seems your right

Also doing a little research it seems having RCD protection in the house for a garage SWA supply can be somewhat inconvenient compared to rcd protection in the garage itself. For example, if you have a spinning blade or saw running in the evening and RCD trips in the house you will be plunged into darkness and have to make your way to the house to reset the rcd, or worst someone turns the rcd back on in the house and your saw restarts itself with no warning.

So it seems RCD protection in the garage itself is safer and also provides greater discrimination.

 

[hightower]

Bingo - don't take everything you hear/see at face value. There's a lot of myths out there for one reason or another, BS7671 has the answers we need. (P.S. I haven't got the regs with me either so not sure if that is the right reg number).

As for your original question, it is fine to have distribution circuit added to distribution circuit added to distribution circuit so long as everything checks out. The thing that will probably catch you out first is voltage drop - that will be a problem usually long before your Zs values are an issue.

 

[gazdkw82]

thanks hightower! appreciate the advice!

out of interest i'm going to see if he wants me to use his set up as a way of practicing and getting used to my meter. I can get all circuit information and try and work out voltage drops/zs etc.

iv not long completed my initial verification (2392) so a testing sequence is still fresh in my mind!

 

[telectrix]

you're correct about the SWA. it would only require RCD protection on a TT system to comply with ADS times.

 

[hightower]

you're correct about the SWA. it would only require RCD protection on a TT system to comply with ADS times.

Which could be 100mA S-type as it's not for additional protection.

 

[happysteve]

As a general comment to all the trainees here... it's ace to see you learning knowledge and applying it... love it!

 

[gazdkw82]

I should have said in OP, its a TN-C-S arrangement

 

[hightower]

I should have said in OP, its a TN-C-S arrangement

Haha, that's gonna open up a whole new "Exporting the earth" debate now

 

[gazdkw82]

Haha, that's gonna open up a whole new "Exporting the earth" debate now

whats exporting the earth

 

[hightower]

http://www.electriciansforums.co.uk/search/34266/?q=exporting+earth&o=date&c[title_only]=1