Outbuilding - CU Needed?

[imroberts]

My house has (all installed prior to my ownership) a brick outbuilding.

There is an SWA cable feeding the outbuilding via a 16A MCB in the main consumer unit within the house protected by a 30mA RCD.

Within the outbuilding there is a small consumer unit with a 30mA RCD, a 6A MCB feeding some lights and a couple of 16A MCBs feeding some 13A sockets and also a heater in a greenhouse which is attached. All the wiring is a bit of a mess and I'm planning to refit it all at some point. I don't need anything particularly elaborate, probably a couple of twin 13A sockets, an external 13A socket, and a light.

What I'm pondering is; is there any need for the CU in the outbuilding? Given the feed is protected by both a 16A MCB and a 30mA RCD anyway, does the second CU add anything? I'll probably keep it anyway given it's already there and it provides a good enclosure to house all the cabling rather than cramming it all into socket backboxes, but it strikes me the whole lot could simply be wired as a radial circuit, subject to using an FCU instead of a light switch to provide appropriate protection for the lighting?

Alternatively, would it be a better idea to wire the SWA from a non-RCD protected MCB in the house consumer unit, thus removing the possibility of nuisance tripping the house circuits as a result of issues outside, and instead rely on the external RCD?

What would be the normal / recommended way of connecting such an arrangement?

 

[chivers67]

I'd place the SWA on it's own MCB without RCD protection. Is the Armour Earthed correctly?

What Cross sectional area is the SWA ?

 

[imroberts]

I'd place the SWA on it's own MCB without RCD protection.

That makes the most sense to me - that way any nuisance or accidental tripping of the RCD from outside won't impact the house. In which case I do need a CU in the outbuilding - it still feels a little redundant right now given the whole circuit is protected by a 16A MCB and RCD within the house

Is the Armour Earthed correctly?

I hadn't looked until today but the SWA is terminated into a metal adaptable box at both ends, and the earth gland is then connected to the earth in both consumer units alongside the earth core within the SWA. Is that how it's meant to be?

What Cross sectional area is the SWA ?

I haven't checked the internal cores but based on the external diameter it's only 1.5mm so 16A is probably about as high as I can go at the house end?

 

[ericmark]

A 13A fuse will not likely open before the MCB in the house, a 3A may, so lights protected by a switched FCU with 3 amp fuse makes some sense, but for sockets to avoid walking to house to reset would need 10 amp MCB. Personally can't see much point in 16A or 13A in outbuilding if 16 amp in house.

Since you have a CU a 10 amp MCB likely best option so over load not walking to house, but not really required.

 

[SUNRAY]

As others have said 16A in the house is adequate to protect the sockets,
My view is no RCD in the house and retain the CU as a convenient housing for the RCD and effectively a junction box for the rest.
In the event of an overload it will be a toss up which MCB (or both) goes but that is also true with the 16 & 10A option from earlier.

 

[imroberts]

My needs for power in the outbuilding are minimal - I'm intend to fit a couple of LED battens and my needs for power are unlikely to extend beyond the use of garden equipment or power tools. As such I'm not concerned about overload and in this event if I have to walk to the house I can live with that.

My primary objective is to avoid interruption to the house in the event of tripping the RCD so it sounds like I'm best moving the circuit off the RCD in the house and instead relying on the RCD In the shed - and use the MCBs for handling the lighting etc. rather than FCUs.

Thanks all for the thoughts.