RCDs on outbuilding circuits

Yes, that again. ;) I've done lots of Googling and read the Which? guide, but I still need some clarification, as the Regs (or at least the way they've been summarised in the sources I've looked at) seem to be ambiguous.
On the one hand, they say outbuilding circuits should have RCD protection.
On the other hand, they say lighting circuits shouldn't have RCD protection.
So what do you do about lighting circuits in an outbuilding???
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Cheap option: put them on the RCD
Expensive option: have a 100mA time delay "whole shed" RCD. Lights go onto this. Sockets go through this and an additional 30mA RCD (either split load or RCBO). Not really necessary unless the shed is really a workshop with dangerous power tools.
Christian.
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applied to "risk assessment"). Two extremes to illustrate. One: you have a garage used pretty intensively as a workshop, with lots of spinning sharp things. It's physically very close to the house, and the earthing arrangements bring it within the same equipotential zone (i.e. you've exported the house earth). (Oh, and the lighting switchgear happens to be all-insulated, i.e. plastic, rather than metalclad). With these circs, it's a no-brainer to have no RCD protection on the lights: the risk of having the lights go out (making the spinning machinery more dangerous in the dark) far outweighs the risks of anyone getting a fatal shock from any fault in the lighting socket.
Other end of the spectrum: shed/outbuilding at the far end of nowhere, TT earthing of uncertain earth resistance, outbuilding used for storage and a socket for connecting the occasional strimmer or similar. Now it makes no sense not to have all of the shed electrics on a single RCD.
Thoughtful design to balance risks in intermediate situations/extra complications might involve additionally using: a whole-system time-delay 100mA RCD for the whole installation, with a 30mA RCD protecting the sockets alone; separate RCBOs for lighting and for power; some light fittings with battery backup so they stay alight when a whole-outbuilding RCD trips out. Good judgment based on the specifics of an installation always trumps slavish adherence to generic advice conservatively based on regs...
HTH, Stefek
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On 28 Jan 2004 03:11:27 -0800, snipped-for-privacy@4-lom.com (4-LOM) wrote:

All depends on what earthing system you have. (Cue long thread about how to earth an outbuilding!). Generally it easier to fit a 'garage unit' with a main rcd covering the two circuits for lighting and sockets. The other way is to run a non rcd supply to the shed, fuse it down for the lights then fit rcd sockets. As I said though, it all depends what earthing arrangements you have.
SJW A.C.S. Ltd.
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What we have atm is a spur off a house ring circuit which goes to a fused spur box in the shed feeding one socket and one light. At present the shed seems to be included in the house earthing. The distance of cable from house to shed is about 6 metres.
I'm planning to build a new circuit as we're in the process of renovating and extending the shed to include a music room at the back. Fortunately, there is a spare way in the house CU, so that won't need upgrading. My plan is to build the new circuits in isolation, then get a qualified electrician to check them, connect them to the mains and sort out the earthing.
My favoured option on grounds of cost and convenience, is to get a 2 way garage CU in the shed. Although the lights going out will be irritating I can't see it being a huge safety risk. The existing shed is used as a "workshop" but we don't have (or intend to get) any really dangerous tools, and we rarely do anything with power tools after dark.
If both the lighting and socket circuits are protected by a 30mA RCD in the CU, will this be OK whatever the earthing arrangements? As my plan was to get the electrician in at the end of the job, I won't know for sure what the earthing arrangements should be when I start and I'd prefer to avoid paying for an extra visit. As far as equipotential zones go, there won't be any exposed metalwork apart from a woodburner. Would that need to be bonded or not?
Another thing to think about is how the music room affects things. After one or two bad experiences with electric guitars I would definitely like some protection there, but otoh I'll be using a PC for recording, and I've read that they increase the chances of nuisance trips.
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Good idea.

Yes. Basically it's exceeding the requirements for that installation on any type of earthing system so whatever earthing arrangemants you have will be compatible.

Not if it is purely isolated, i.e. just a metallic object that sits in the corner burning wood. You wouldn't bond a metal dining table would you.

1 PC wont cause nuisance tripping, it's when you get several or more connected onto one circuit you can get problems. I wouldn't worry about that if I were you.
SJW A.C.S. Ltd.
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I know one hall of residance where in every room the metal wash stand is bonded to the pipes!
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On Fri, 30 Jan 2004 23:21:45 -0000, "James Salisbury"

bonded, whether it moves or not. If it's not nailed down, you just have to leave a lot of slack on the earth cable!
SJW A.C.S. Ltd.
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