Selectivity to an outbuilding

[cabbilero]

Hi guys. I'm new to the forum. I've just completed an electrical course beacuse of lockdown and am keen to build on the knowledge I have gained. My question is If I have a supply from a consumer unit to an outbuilding which is RCD protected (NOT an S type) would the final circuits (lighting for the outbuilding plus some garden lights and a couple of sockets), be RCD protected by the one in the house and would just need mcb for each circuit. Indeed, would there be any need for a garage unit at all. Could the circuits in the outbuilding be protected by a switched FCU for each circuit? The system is a TN-S. Probably a simply question but would appreciate any help you could give.

Thanks

 

[Strima]

It's not uncommon to run a small supply, 20 amp for example, to run some sockets in an outbuilding then fuse down using an SFCU for the lighting circuit.

And yes, they would be covered by the existing RCD. Not an ideal set up as a fault on one circuit would take out several more.

 

[cabbilero]

It's not uncommon to run a small supply, 20 amp for example, to run some sockets in an outbuilding then fuse down using an SFCU for the lighting circuit.

And yes, they would be covered by the existing RCD. Not an ideal set up as a fault on one circuit would take out several more.

Thanks for the reply Strima. I said RCD, to be honest I think it is an early type RCBO (32A) which is for the outbuilding only, sorry I didn't make it clear. So to clarify there would be no need for a seperate garage CU which is what I was thinking with a 6A breaker for lighting and 20A breaker for the sockets. (To be honest as I type this I think I'm answering my own question as they are all protected at the house, right?)

 

[pc1966]

If the supply cable and feed to the sockets is rated for 32A, say 4mm (maybe more for feed if long due to VD), then you don't need a 20A MCB for that. Adding the 20A MCB would provide adequate overload protection for a radial socket circuit wired in 2.5mm but won't be selective in any meaningful way with a 32A MCB or RCBO due to the 'instant' magnetic trip going on both with a hard fault.

For the lights they need lower OCPD so either a 6A MCB or a FCU with 3A or 5A fuse (depending on how much lights, etc. But with LED I really doubt you will go above 3A even allowing margin for simultaneous switching of several fittings.

As the supply is a RCBO you don't need any more RCD protection, but if a loss of power causing the lights to fail is a risk (e.g. it is a workshop or similar) then you might want to consider dome emergency lighting (often available built in to some LED strip lights)