adding a shed submain

[braineebird]

Hi.

My apologies because i'm having one of those mental block moments.

I have been asked to provide a supply to a shed for a few sockets and lighting. The main consumer unit is a 17th ed split RCD unit so there are no 'non rcd' spares.

I want to use a garage unit with its own RCD and two mcb's to keep the shed ccts seperate but am unsure of the best / easiest way of doing this.

I have thought of taking a feed from the main switch on the house consumer unit into a seperate enclosure with a main isolator. Then run my supply from this to the shed unit.

Does this seem ok? I am going to run swa to the shed but this will not have any rcd protection if i do what i have described above. The only way i can see to do this would be to take my feed from the main switch on the house consumer unit into a separate enclosure with a time delayed rcd as a main switch then off to my shed as before. Is this overkill? Do i need to rcd protect the cable run to the shed?

Any advice on this is very welcome

 

[Spark123]

You could get an electrician to henley in a separate switch fuse for a supply to the garage. (Worryingly you fail to mention any overcurrent or fault current protection for the SWA).
What is the supply type?
If it is TN then there is no need to RCD protect the SWA submain.
This work is notifiable to your LABC under part p of the building regs.

 

[braineebird]

Thanks for the swift reply. The supply is TN-CS.

Yes I understand it is notifiable to my council.

Thought about the Henley block. Would it fail under part p if i take a feed to a separate fuse switch from the main switch on the consumer unit (where the tails go in). There is already an isolator to enable me to cut the power to this. Or is this really bad practice?

 

[ColJack]

it's really bad practice..

and falls under part P anyway since you're installing new circuits..

Isolators are not designed to take 2 25mm cables..

Henleys are the way to add an extra CU

are there no spare ways in the CU at all?

you say "no non RCD ways" but what about RCD ways?

nothing wrong with RCD protecting the SWA even if you don't have to..

 

[braineebird]

Hi ColJack

It is a 13way dual RCD crabtree starbreaker board with 3 spares on RCD1 and 2 spare on RCD2 ...all RCD protected.

If the SWA is RCD protected and this then goes into a garage unit with a 40A 30mA RCCB as a main switch will it cause problems? I didn't think you could have 2 RCD's in series unless the first was time delayed as in TT. Or would i simply have to remove the RCCB from the garage unit and replace it with a standard isolator?

 

[Taylortwocities]

As you have spare ways on the board. I would do this:

Re arrange the board so you have a spare way between the main switch and RCD 1.
Put in a contact breaker to protect your SWA - with the neutral on the supply busbar.
The size of CB will be depending on the cable calculation that you will have done/will be doing.

EDIT: I see you have a Starbreaker. You may need to make some buss modifications to create the arrangement I've outlined. It depends if your board is high-integrity, or not........

Or, run the feed from a CB on one of the RCD circuits with a standard (non-RCD) board in the outbuilding.

Like the above posts, and with respect, I feel that you shouldn't be doing this yourself. The quuestions you are asking leads one to question your competence in this....

 

[Spark123]

It wont matter electrically - it is considered bad practice as it is just a guessing game as to which one will trip in the event of a fault, if not both.