db1 to db2 to shed

[tuckermot]

Hi al

question !!!!

DB1 in house ( dual rcd ) feeding small ccu in garage with swa !!! this in turn will feed a swa to a garden shed!!!

Question !!!

i fully under stand 30ma rcd,s in line is a no no !!! so can i install the garage unit with just a main swith and no rcd/main switch and in turn feed the shed via swa from ccu in garage !!!

Question !!!

because of feed from db1 being on a rcd i can not tt the garage ccu so will have to export the earth from db1 . iunderstand you can do this as long as there is no extranios conductive parts in out buildings also i am going to rely on the rcd in db1 ( house ) to protect all the out side installation!!!! i know that if a neutral earth fault occures it will knock half the board out in db1 but can not see any other way round it !!!

Question !!!!

would you guys and girls approach it in the same manner

thanks
Richard

 

[JUD]

You could feed the garage from an unprotected way in DB1. As you are using SWA there's no need for RCD protection. Just earth the armour of the SWA at supply end only.

You could then fit RCD to garage CU and TT it. Then feed your shed from garage CU.

This would save you knocking half the house circuits out in the event of a fault.

 

[Deleted member 9648]

Hi al

question !!!!

DB1 in house ( dual rcd ) feeding small ccu in garage with swa !!! this in turn will feed a swa to a garden shed!!!

Question !!!

i fully under stand 30ma rcd,s in line is a no no !!! so can i install the garage unit with just a main swith and no rcd/main switch and in turn feed the shed via swa from ccu in garage !!! Yes,as long as there is RCD protection it can be at either end.

Question !!!

because of feed from db1 being on a rcd i can not tt the garage ccu so will have to export the earth from db1 . iunderstand you can do this as long as there is no extranios conductive parts in out buildings also i am going to rely on the rcd in db1 ( house ) to protect all the out side installation!!!! i know that if a neutral earth fault occures it will knock half the board out in db1 but can not see any other way round it !!! It will not knock out the entire installation therefore it will comply with the 17th.

Question !!!!

would you guys and girls approach it in the same mannerI would prefer to feed of an unprotected way and RCD at the garage,but if this is not practical your plan sounds fine.One advantage with feeding from the house RCD is that it will be immediately apparent if it trips.If there are freezers in the shed/garage and the rcd is at that end it may not be noticed until it is too late.

thanks
Richard

Hope this helps

 

[Risteard]

It will not knock out the entire installation therefore it will comply with the 17th.

The 17th simply requires that inconvenience is minimised in the event of a fault. It doesn't say that front-end RCD protection is never acceptable or that simply not taking absolutely everything out will necessarily comply.

I think a risk-based approach is really what it is calling for (i.e. a risk assessment) before putting a front-end RCD on a distribution board or whatever is intended.

That said, I don't think an RCD-protected distribution circuit in the instance mentioned would be a particularly wonderful design.

 

[Deleted member 9648]

True,but in practice it is generally accepted that a fault on one circuit should not take out the entire installation to meet the requirements of the 17th.....that said you are technically correct.