RCDs in sheds

Hi,
I'm looking into running power to a new shed. I intend to add a new "way" in the house consumer unit (with MCB), go with T+E to the back wall, then SWA down the garden and a small consumer unit in the shed. I want to fit a few sockets and a light or two; since I might be using power tools in there it's been pointed out to me that I shouldn't have both sockets and light on the same RCD or I might end up in the dark with a still-spinning tool.
Most of the small CU kits I've seen (eg http://cgi.ebay.co.uk/Weatherproof-Garage-Shed-Consumer-Unit-c-w-RCCB-2-MCB_W0QQitemZ250179324902QQihZ015QQcategoryZ26215QQcmdZViewItem ) appear to have a single RCD covering the whole thing, then a pair of MCBs for lights and sockets. This seems backwards to me; I really only want the RCD on the sockets, with a MCB for the lights and a "main" MCB as master switch and to provide overcurrent protection for the sockets.
Opinions?
Also, I'm pretty sure that my house CU is not a split one. I haven't looked at it since reading up on this stuff, so I don't know if the "master switch" is an MCB or RCD. If it's an RCD, should I be concerned about connecting the shed through it? My Dad's (rather gash) installation 'trips out' the house when you press the test button on the single RCD-built-in socket he's got down there; I don't want my shed to be able to trip the house either via the test button or via a (non-catastrophic) fault.
Is my main switch likely to be an RCD in a 1990s house? (The CU is in a 2000 extension so might be newer.) Presumably it's /possible/ to wire from *before* the switch to the "in" side of the shed MCB, but I assume this is very dodgy (I mention it just in case it is actually allowed; it would solve the problem). Could I replace it with a suitable MCB, and add RCDs to the two house socket rings? (I believe non-RCD is preferred for lighting circuits to avoid trips when bulbs blow.) Is the whole question purely academic and I shouldn't be worrying about it?
Also note that I'm not soliciting Part P advice :-)
Pete
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Pete Verdon wrote:

I can give a fuller answer later, but in the mean time in case you have not seen it, have a look at:
http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside
--
Cheers,

John.

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Pete Verdon wrote:

http://cgi.ebay.co.uk/Weatherproof-Garage-Shed-Consumer-Unit-c-w-RCCB-2-MCB_W0QQitemZ250179324902QQihZ015QQcategoryZ26215QQcmdZViewItem

Yup, that would be the ideal...

Yes very much so. Outdoor electrics are more likely to cause nuisance tripping of RCDs in the first place. Adding such a circuit to a house protected with a "whole house" RCD (something already undesirable[1])

That does not sound right either!

Hard to tell - use of a single RCD was already deprecated by then. A photo of the CU and surrounding gubbins would help.

It would depend on where you went with it next.... too another main switch would be ok.
Your ideal scenario in the shed would be as you described above - RCD protected sockets, and not lights. Obviously if the feed to the shed is RCD protected then this is a non starter. One option may be installing some emergency lighting instead - so if you do trip everything you are not left in the dark.
You could use a split load CU in the shed (but these don't tend to be available in a small number of ways). You could install two small CUs, one switched, and one RCD protected. You could use a simple main switch, with say 20A MCB, feeding a RCD spur that in turn feeds a radial to the sockets. Plus a separate non RCD feed to 3A fused spur for the lights.

No not academic. There is a significant safety advantage to have RCD protection on general purpose socket outlets. Especially if they will ever feed extension leads that you might use outside etc,
[1] http://www.wiki.diyfaq.org.uk/index.php?title=RCD#Whole_House_RCD
--
Cheers,

John.

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I was about to add some comments on earth impedance on runs to distant sheds when I realised that the 30mA required to make the RCD trip would require an awful lot of ohms to make the earth voltage rise to any level.
But it is still worth asking the question whether a local earth via a rod is worth while installing for 'remote' sheds, regardless of the type of house earthing ?
My solution to the garage, workshop and gardening sheds was to buy small split CU's off Ebay; none cost me very much and have given me peace of mind !
Rob
Rob
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robgraham wrote:

This is an issue for systems with no RCD at the head end (i.e. TN-S and TN-C-S systems). There is a section in the wiki on it, but it basically comes down to adequate cable selection.

The more remote the building the more preferable that can become. Even on close buildings it can reduce the complexity of needing to maintain the houses equipotential zone as would be required if exporting a a TN-C-S earth.
Some more here:
http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside#Exporting_an_earth
--
Cheers,

John.

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John Rumm wrote:

I think I may have found it:
http://www.screwfix.com/prods/61591/Electrical/Consumer-Units/Wylex-Consumer-Units/Wylex-Metal-2W-63A-DP-Isolator-Sw-Consumer-Unit #
This has a simple switch in the "master" position, and two ways for the circuits. This doesn't give overcurrent for the sockets as-is, but presumably using an RCBO instead of an RCD on that way would solve that. Or I suppose I could replace the switch (it looks removable) with an MCB?
Being a metal box, it also seems better for the more mechanically firm connection involved with SWA.

I've now checked. I'm pretty sure the master switch in my CU is an RCD. There's a picture here:
http://www.verdonet.org.uk/stuff/cu.jpg

No. His cable run is spurred off the back of a socket in an extension to the house (which I think might be a spur itself). I assume it's a question of discrimination - the leakage that the test button applies just happens to trip the RCD in the house rather than the one in the shed socket.
I'm planning to make sure my installation is a bit more professional.

I'm not sure what you mean here. I was referring to (within the CU) adding an MCB for the head end of the shed feed, but powering it by connecting to the *input* side of the whole-house RCD instead of the output, to avoid having the shed RCDed. As I say, though, taking power back out of the input terminals of the main switch seems a bit wrong - and having a way in the CU that isn't actually under the control of that switch is bound to be a bad idea.
Might it make more sense to remove the whole-house RCD main switch? If so, what would be best to replace it with? A simple switch? A large MCB? Presumably in either of those cases I should be fitting RCBOs for the two socket rings to keep them RCDed?
While I'm at it, I'd replace the broken MCB on the downstairs lights (see picture).

I think the box linked above would do it. It has a main switch in it already, and two ways. Add an MCB to one (lights) and an RCBO to the other (sockets) and I'm sorted.

The "question" I was referring to was the idea of avoiding having the shed lights behind an RCD. I don't have an objection to RCDs in general. I probably wasn't very clear.
Cheers,
Pete
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Pete Verdon wrote:

You'd need (preferably) two RCBOs - one for the lights and one for the sockets. The lights could be on an MCB, assuming the earthing is TN.

No, a main switch-disconnecter (isolator) is required and there's no need for it to be external; the 4-module housing has enough room for the isolator and two outgoing circuit protective devices.

Yes, except that if you opt to go down the TT earthing route it is much simpler to use a plastic housing.

Clearly a 30 mA whole-house RCD.

An RCD's test button does not "apply leakage" and another RCD upstream should not see any residual current when the button is pressed - so there's something very screwy going on there.

In which case the first step is /design/. You could post your completed design with justifications and calculations for review here...

This would be highly dangerous and is *absolutely* out of the question. For one thing it would not be obvious that the main switch/RCD does not isolate one particular circuit.

The simplest option would be to install a new switch-fuse (e.g. 1-way Wylex type) with a 30 A HBC cartridge fuse (BS 1361) to feed your sub-main. Feed this by teeing into the meter tails using a Henley block. That way the house installation can be left alone and a possible hornets' nest avoided. Earthing and main bonding will still need to be checked though, and upgraded if necessary.
--
Andy

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Andy Wade wrote:

Yup - as I said immediately afterwards: "having a way in the CU that isn't actually under the control of that switch is bound to be a bad idea."
(I don't blame you for snipping it - I'm just rather embarassed about even coming up with the idea in the first place and wanted to make clear that I *had* rejected it on my own :-) )
Pete
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Andy Wade wrote:

Hmm. This could well be the way to go - in fact (since my meter and consumer unit are not in the same place) it would also result in an easier and slightly shorter cable run.
I haven't yet measured the "as the cable runs" distance in order to do any voltage-drop etc calculations. I don't even have the shed itself yet, so there's no hurry to size and order cable.
Pete
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Pete Verdon wrote:

http://www.screwfix.com/prods/61591/Electrical/Consumer-Units/Wylex-Consumer-Units/Wylex-Metal-2W-63A-DP-Isolator-Sw-Consumer-Unit#
That is just a basic 2 way CU....

Yup, you populate the MCB positions with whatever you like. It is worth noting that many RCBOs are as wide as two modules - so a 4 way CU may be a better bet. You can get single module RCBOs but these are taller instead of wider - again you need to make sure there is clearance space for them in the chosen CU.

Depends on if you want an electrical connection to the armour of the SWA or not. In cases where you have a separate TT install in the outbuilding you typically don't want an earth connection between house and outbuilding earth.

Yup, whole house RCD. A classic case of "you don't want to start from here".
To keep things simple for the outbuilding feed, you could do with a new dedicated switchfuse or mini CU teed into the meter tails with a service connector block:
http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside#Using_an_independent_supply

The test button should not introduce any leakage into the supply circuit though - it should be an entirely internal test of the RCD in question.

Sorry, I did not spell that out very well. I was suggesting that you may under certain circumstances connect to the incoming terminals on the supply at the incomer, when you need to feed a second incomer. For example if you want to feed two CUs from a single submain, and there is space in the input terminals of the first one, you may be able to run a second SWA to the next CU from there, without needing a discrete junction box. (this only tends to work on sub mains - usually meter tails are too large to get two sets in one pair of terminals on a main switch and a henley/service connector block is required instead).

Not an acceptable solution on a number of counts...

Not only that, but MCBs are single pole devices, so you have not achieved isolation by turning it off.

Well it depends on what problems you are trying to solve. For connection of the outbuilding you could simply ignore the existing CU and add a new switchfuse at the head end as described above.
Do you get nuisance trip problems with the main RCD at the moment? If not, then the main potential problem is loss of lighting in the event of a trip.

While on that subject you might also want to look at the over current protection on your lighting circuits. This is currently implemented with two 16A type B circuit breakers. This would require that all lighting wiring is done in 1.5mm^2 cable or bigger and not 1.0mm^2, and it would preclude use of any light fittings that have mini BC or small ES fittings since they would not be adequately protected by this MCB (where 6A would be required). A pair of 6A type C MCBs would be more appropriate unless they are very large and heavily loaded circuits[1]

Yup, if the RCBO fits. You would also need to ensure it is not powered from your current CU since you would have no RCD discrimination between that and the new one.

You probably answered the question yourself in the first post - how much do you fancy the prospect of navigating your way about a shed in the dark with a spinning power tool in your hand?
It is a case of designing a system appropriate for the circumstances, and trading off cost and inconvenience as you go. For example, I have a feed to my outbuildings - one of which is used as a workshop. So similar circumstances. My house is a TT install, so I also need a RCD at the head end to give adequate earth fault protection for the submain. Hence I have small CU with a time delayed RCD as its incomer, and a 30A HRC fuse to protect the submain. In each building there is a non split load CU, with 30mA RCD on the incomer, and ordinary MCBs for lighting and power. There is a non maintained emergency light, powered from the lighting circuit. So in the event of a fault there is discrimination with the house RCD, and we eliminate any possibility of the outbuilding contributing to the likelihood of a nuisance trip in the house. However I chose to use emergency lighting rather have the cost (15 Vs 100) of a pair of RCBOs for the workshop (since the workshop is also TT it would need RCD protection on its lighting circuit).
A neighbour had a much simpler requirement - the need to run a propagator heater in a greenhouse. There it was easier to have a dedicated 30mA RCD protected switch fuse at the head end, and the submain feeding directly into waterproof double socket enclosure at the greenhouse. The only issue to address there was removing the possibility of the outbuilding circuit causing problems back at the house.
[1] To estimate the total load, total the power of each light fitting - count each light as 100W unless the actual load is greater - in which case count that.
--
Cheers,

John.

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John Rumm wrote:

As a tangential point of information the 17th edition regs will at last permit the use of SBC (B15) and SES (E14) lampholders on 16 A circuits [reg. 559.6.1.6 in the DPC]. This is for lampholders that meet the current standards, viz. BS EN 61184 (T2 temperature rating) for B15 & B22, and BS EN 60238 for the ES types.
Also the present polarity requirement (L to the end contact, N to the screw thread) will be removed for for compliant E14 and E27 lampholders.
--
Andy

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John Rumm wrote:

Yes. I'm comfortable with the power requirements for cable area etc, but bonding seems to be a bit more mysterious. From what I read on the Web I'm not the only one that's unsure. I want to avoid having an earth rod if at all possible, partly for reasons of cost but also simplicity and maintenance. However, being a metal shed placed straight on the ground (well, slabs in concrete), I imagine bonding is going to be very important here. I went and looked at the meter box today (as I said, it's separate from the CU) and it appears that I have a TN-C-S system as the house earth goes into the fuse block on my side and only a live and neutral go in from the supplier.

Yup. As Andy suggested, I think this is the way to go. I'd originally steered away from it because it looked like it was placed as the "rolls royce" option on the wiki page, and I don't consider mine a "rolls royce" installation. But in fact, in my case it's the simpler option after all.

Well, I don't know what's wrong with it then :-) . It's done it for years; he's just got a sign over it saying not to press the button.

Fortunately not. It did go when a bulb blew a few weeks ago, but that's it. Mind you, I only moved in in September.

Hmm, ok. Obviously I don't know what cable the lighting circuit is done with; I suppose I could open up a fitting and have a look as a sample. Upstairs at least I think has been re-done, presumably when the extension was added (which now contains the CU). I'm pretty sure I don't have any mini-BC lamps (didn't even know that format existed); SES I know I have had to buy but if I remember rightly that was only for a plug-in table lamp.
That said, I don't think my circuits count as "large and heavily loaded". It's a modern end-of-terrace, originally with two bedrooms since extended to three. I've had a quick whiz round and I think I have just under 1000W of (potential) lighting downstairs and 900W upstairs.

To be honest, at the moment I'm not too fussed. The only power tools I currently have (apart from a couple of sanders which can't really do any harm) are a handheld drill and circular saw. Both of them I'm happy enough to just hold in place while they spin down. But. I'm 25, and have always been keen on making and repairing things with my hands. I haven't really had the facilities before, but have just bought a house and am putting up a shed. Somehow I suspect it won't be long before more serious tools make an appearance :-)
This is why I'm putting in more of a supply than I currently need (an extension lead plugged into the living room sockets is probably all I need right now!) and also why I'm considering things like integrity of lighting.
Pete
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Pete Verdon wrote:

Metal shed! In that case you really have no option - it must be TT. Similar arguments apply as are applied to caravans, where it is illegal to use a TN-C-S/PME supply. Inside the metal building forms a nice equipotential zone but outside there could be a dangerous touch voltage between the metalwork and the local ground.
To be clear, the armour of the SWA submain cable is earthed at the supply end only, and the cable is sized to ensure that the supply protection will operate (fuse will blow) in the event of an earth fault on the cable, e.g. if someone digs through it, at its far end.
In the shed the armour is left unconnected and a separate earthing conductor (16 mm^2 unless protected in conduit) is run to a local earth electrode, which could be a 4 ft. rod, as a minimum, or 8 ft. or more if the soil is poor. The connection to the electrode must be accessible for maintenance, so use an 'earth pit', available from any electrical wholesaler.
At the shed end preferably use a plastic CU housing, taking the SWA into it via suitable sized plastic cable gland (stuffing gland) not a normal brass SWA gland. Strip the cable so that no armour is exposed, and to be safe, apply a short length of heat shrink sleeving over the end of the armouring, overlapping onto the sheath and bedding layers. Then connect the line and neutral wires to the incomer device on your CU.
If a metal CU housing is used you have to meet Class 2 insulation requirements between the incoming conductors and the metalwork for the wiring ahead of the first RCD (or all RCDs if they are parallel fed). This is because a s/c between incoming L and the housing would not blow the supply fuse, leaving the whole shed at or near mains voltage. MK do an 'RCD insulation kit' to achieve this with their CUs, alternatively you could use site applied secondary insulation.
As well as connecting the the earthing conductor from the electrode to the main earth terminal in the CU you'll need main bonding (min. 6 mm^2) to the building structure. This could be connected using a crimp ring terminal bolted to the metalwork, and labelled "Safety electrical connection - do not disconnect". If you have any service pipes entering the shed (e.g. water) these should be plastic, to avoid inadvertent connection to the supplier's earth.
As to arrangement of the CU, the options are:
(i) a single 30 mA RCD as main switch and one MCB for each final cct. (accept lights going out if RCD trips);
(ii) a 100 mA S-type (delayed) RCD as main switch, feeding lights via an MCB and power via either a 30 mA RCD in turn feeding one or more MCBs, or via individual circuit RCBOs;
(iii) main switch incomer and one RCBO for each final circuit.
[Dad's installation]

I'm wondering whether the device in question (the downstream one) is an old earth-leakage circuit breaker (ELCB) and not an RCD at all? That would account for the symptoms observed.
--
Andy

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Andy Wade wrote:

Depends how old is old. Last time I was there I had a look at the cable to see what size conductors he's using. As well as the size (2.5mm) the cable also had the year 1997 stamped on it, so it can't be any earlier than that. The "RCD" is built into a normal-looking metal 2-gang socket.
Pete
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Pete Verdon wrote:

That rules out the ELCB hypothesis then. While the test button on a normal consumer unit type RCD doesn't introduce leakage to earth, it's always possible that the one on an RCD-socket could do so, as a way of verifying that the earth is connected.
--
Andy

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Andy Wade wrote:

Or it could just be that the whole thing is knackered :-)
Pete
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Pete Verdon wrote:

That is probably the most plausible reason...
--
Cheers,

John.

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On Dec 13, 12:01 pm, Pete Verdon

Hi,
If you're running mostly portable power tools, they're often double insulated these days and so don't have an earth connection, and so are unlikely to nuisance trip.
cheers, Pete.
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