Going by that picture, it'll be the big red switch at the left hand end.
Fuck knows why that double width black switch with what appears to be an
earth leakage reset button has been marked as "MAIN SW" in black marker
pen is supposed to do. Feed another consumer unit perhaps?
Each consumer unit only has a single double pole isolating switch to
shut off the feed coming from the meter. Once that's shut off, everything
is then isolated and safe to work on.
You can normally assume that it's safe to work on if pulling the big
switch kills the supply to every electrical circuit in the house.
Obviously, if you have *any* lingering doubts as to how the unit has
been wired up, you need to test with a meter or mains voltage probe to
verify that the "BIG SWITCH"(tm) has indeed isolated everything from the
mains supply and hasn't been bypassed in some 'cunning' way from an
alternative link to mains voltage via some other route. Normally, the
meter tails going into the terminals of the "BIG SWITCH"(tm) are the only
available access to the incoming fused supply cable in a domestic
 Whatever measuring device you're using to test for mains voltage,
it's important to verify that it will give an indication of mains voltage
before you start testing. You can use a multimeter set on a suitable AC
volts range, (you have to set it on a range that exceeds the mains supply
voltage, typically the 300vac or next higher ac voltage range on the
meter) to probe for mains voltage on the PC end of its mains lead with
You can usually push the meter probe tips into the IEC Live and Neutral
socket holes where they'll normally stay wedged in without risk of
shorting out before plugging the 13A plugtop end into a switched 13A
socket so that you can switch on without actually having to handle the
test leads. Once you've established that your measuring device is working
as advertised, you can then use it to verify that the CU is indeed free
of any nasty surprises before physically handling its wiring.
Incidentally, the best way to probe for (in this case unwanted) mains
voltages is to clip the common test lead onto an earth terminal so you
only have to hold onto the other test probe's insulated handle whilst
carefully poking around inside the CU making sure to keep the rest of
your body isolated from earthing (or even other possible sources of live
mains) contact. It's best to let the meter do the work rather use
yourself as a test probe. :-)
Unless a crazed lunatic installed the CU (or reconfigured it into some
unsafe state), pulling the "BIG SWITCH"(tm) (in this case, rather
conveniently coloured red), is more than sufficient to render the typical
domestic CU and all the fixed wiring it serves (ring main(s), lighting
circuit(s), Electric shower, cooker point and immersion circuits, perhaps
even another CU) entirely safe to work upon.
If this all seems too complicated and you have any doubts, there's no
shame in letting discretion be the better part of valour and employ a
professional electrician to do the work for you.
That is NOT always true.
2 counter examples: Economy 7 type split tariff with dual meter feeds;
CU with UPS integration feeding a separate busbar.
Everyone's correct in that it is *usual* for the big red switch to cut
off the whole box, but it is impossible to be sure that is the case here
as someone could have done something exceptional.
On Wed, 18 Nov 2015 07:49:24 +0000, Tim Watts wrote:
Thanks for the 'Heads Up' but I think I managed to cover my arse ("Ass"
for the benefit of our American cousins) on this one quite nicely by
using the weasel phrase, "Unless a crazed lunatic installed the CU (or
reconfigured it into some unsafe state)". :-)
I recall one new CU I bought came complete with "inspected" and
"passed" stickers all over it, but was wired such that turning on the
RCD would have shorted the incoming supply! So a sanity check of the
interconnects (not to mention tightening the screws) is always a good
investment of 5 mins.
 Can't remember if it was a Contactum or a Hager now....
If I dont understand the wiring when I open it I wont touch it, already
done that with trying to change a thermostat/timer for the gas heating,
the wiring was altered and didnt make sense so got an electrician in.
Even when I change a plug socket or light switch I use insulated
screwdriver and thin nose plyers to handle the cable even though the CU
is turned off, I wont touch the metal.
I learned my lesson as a 7 year old when I took a bulb out a table lamp
and stuck my fingers in. Dont ask cause I dont know why :-)
As I said before it should be easy to establish what isolates what simply b
y switching each switch and seeing which circuits are dead. The scenario th
at is the main concern as implied by the sticker on the top is that the RCD
has been incorrectly wired as a main switch in other words to the input si
de of the main switch. If that is the case then switching off the main swit
ch will not isolate all the circuits. A main switch should just be that and
all circuits should be isolated using it, I think the CU needs looking at
to check that the main switch does isolate all circuits and at the same tim
e to place the shower MCBs so that they are protected by the RCD leaving ju
st the lighting circuits unprotected by the RCD. If the main switch does is
olate all circuits than the only dangerous area not to poke around in is wh
ere the meter tails connect to the main switch, although depending on the C
U those terminals may have an additional cover as protection.
Quite understandable, the phrase, "Once bitten, twice shy." applies
here. At that tender young age, I could imagine it must have been quite a
As for not asking why you removed the lamp and poked your fingers into
the empty socket in the first place, I've no need to. It's just the sort
of thing any child with an enquiring mind would do. It's the way we gain
experience as juveniles to learn how to survive in the wider world around
I didn't suffer my own "Light Socket Moment" until I had developed a
much greater resilience to such electrical 'insults' as a teenager. Mind
you, I did a similar thing with my homebrewed 2 valve Top Band 10 watt
transmitter's power supply when I bridged the HT smoothing cap terminals
with my thumb when examining it whilst it was unplugged from the mains
supply, thus learning the value of including HT smoothing capacitor bleed
resistors in the circuit and the hazards of such stored potentials in the
HT modules of SMPSUs when they don't (or the resistor has blown open
In my particular case, the residual voltage was the no load 500vDC peak
from the fullwave rectified 350vAC secondary windings which had blown two
rather neat looking pin head sized holes in the pad of my thumb. And, it
still had some 300 odd volts left over which I thought, considering the
capacitor was only 16 microfarads, was quite an impressive result! All
that pain and I *still* hadn't properly discharged the cap!).
Agreed. FWIW *I* would pull the fuse. The other gadget which I am very
fond of is the "volt stick" which is like a neon screwdriver that works
through insulation. Same rule applies as with a neon or any other
indicator, be *really* sure it is working before you rely on it, but
very handy when poking around in rats nests of wiring where it might be
possible that you have only isolated some of it.
They are also very handy for a quick check on whether a plug fuse has
failed, or the wire, or the switch in the appliance, etc. They let you
do a lot of diagnosis without opening anything up.
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