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
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
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
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
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
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
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.
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
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
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.
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.
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
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.
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
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.
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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