When you turn it on using the pull switch the RCD switch (for upstairs
lights) trips on the consumer unit.
I've changed both the fan and the actual pull switch but still the
I've even disconnected the fan and the switch itself sets it off....
Any ideas ???
Basically, the switch has got a terminal for N (so I attach both black
wires to this), and a live 1 and live 2 (to which I attach one each of
the red wires).
I'm looking forward to the responses to this one. I did something
similar once upon a time; I replaced a wall switch and forgot to look
at how the wiring was set up. So when I fitted the new one I assumed
all the blacks to neutral, all the reds to live. The switch didn't
last more than half a milli-second.
Do you by any chance mean MCB (i.e. miniature circuit breaker) rather
than RCD? It would be relatively unusual to have individual RCD
protection on a lighting circuit. (although that will change shortly!)
Sounds like you are using the switch to create a short circuit.
Depends on how it is wired and the type of switch. How many terminals
does the switch have?
From your description it sounds like three or four terminal switch.
This would suggest either it is a two way light switch, with a COM,
L1, and L2 terminal, or a pull 2 pole isolator switch with 4 terminals a
L1, N1, and L2, N2 (or might be marked L & N In, and L & N out, or
Supply and Load).
The fact that you have two cables entering the switch would suggest one
is a permanent feed, and the other needs to be a switched output to the
fan. (is this a fan that just runs when turned on, and then stops when
manually turned off? (i.e. no overrun timer etc)). If this is the case,
to work with a conventional light switch you take a red wire to COM, and
the other to L1 or L2 (does not matter which). The blacks you join
together but *DO NOT* connect to the switch. If it is a 2 pole switch
you can take the pair of wires from one cable into L & N terminals on
the supply side, and the other to the load side.
 Two way light switches have three contacts, but can be used in
ordinary one way circuits:
I do admire your patience.
Having had a moan in these pages about electricians, it does worry me
hugely that there are people working with electric wiring who just
have no idea what they are about. It is just as well there are MCB's
etc to protect them from themselves. On the otherhand if they had to
go and spend the time rewiring a fuse each time they were stupid, they
might stop and think a little bit first.
They might stop and think they need thicker fuse wire! ;-)
(I stood in a clients new office as the sparks was attempting to trace
the source of a short in a lighting circuit once.... what do you think,
careful low ohms measurement to calculate a distance to the short? No
chance, just keep disconnecting lights on the circuit, turning on the
MCB, and listen to it going "pop" after each fiddle). Needless to say
the short was in the first leg of the circuit before it reached any
light - caused by a builder screwing coving into place.
[careful low ohms measurement to calculate a distance to the short? ]
aha- how do i do a careful low ohms measurement?
ive just found this group,
got a new old house last month,
trying to decipher the wiring,
how to do a low ohms measurement,
where ti find clues please....
I have just had an interesting read through the above page. It reminded me
that, 40 odd years ago, in an office building I worked at, the electrician
would come round regularly with a 13 amp plug with a short wire and
lampholder attached. The two wires from the lampholder were attached to
the Line and Earth terminals of the plug and if the bulb lit when it was
plugged in he was satisfied that the Earth circuit was intact. Obviously
with a ring circuit it only indicated that at least one leg was working but
with radial it was a good indicator that the Earth was OK. Is this still
an acceptable practise and indeed, with modern consumer units, will it still
(If you can't laugh at life, it ain't worth living!)
There are a couple of problems with that approach. The first is that it
would immediately trip a RCD should one be protecting the circuit, and
since there is a move toward all general purpose circuits being
protected in this way, it will become ever less useful.
The second more subtle problem, is that even where there is no RCD, the
lamp only gives you a go/nogo indication (mostly). If the earth
connection was very poor, then the reduced brightness would probably
alert you, but if it was not so severe, you could end up "passing" an
earth that was present, but with sufficiently high resistance to prevent
the operation of a circuit breaker.
A 100W bulb will draw say 400mA in operation, which gives it an
effective resistance of ~ 600 ohms. Say you have a earth connection with
10 ohms of resistance, your bulb would still light up like a 94W bulb
(i.e. in isolation it may look fully lit)
A Live to earth fault in an appliance connected to the socket would
however see a 10 ohm resistance. This would only draw about 24A, not
enough to either open the circuit MCB, or keep the touch voltage on the
case of the faulty appliance to a safe value. (obviously a RCD if fitted
You could have added that this will dump going on for 6kW into the
area of high earth resistance - likely to be a loose or corroded
Which will probably get very hot, very quickly.
And either cause a fire or go totally open circuit - or both. In the
latter situation, the touch voltage on the case of the faulty appliance
will rise to full mains value..
RCDs are such a good idea.
Yup, quite possibly. Depends a bit on if the 10 ohms comes from a single
point connection, or a whole chain of bad ones, or indeed a poor earth
as supplied by either the electricity supplier or as a result of a TT
It would probably be pretty close to it anyway - a 24A load will not
pull the supply voltage down much unless it is a very long circuit.
Yep. As described by other posters, the two black wires should be joined
together using a "chocolate block" connector. One red wire should go to
L1 or L2, it matters not which. The other red wire should go to the
How it may have been wired in the first place is that the black wires
may have been in the L2, the red wire, FROM THE SUPPLY, to L1 and the
red wire, going to the fan, to the common terminal. However, this does
require identifying which red wire goes to the supply and which goes to
the fan. Get it wrong and the supply is shorted out. This isn't
something that I would do as it could confuse someone in the future....
Thanks. I think thats why its not working then !!!!
So if I dont connect the black to anything (apart from each other),
does it matter which red (i.e. supply or fan) goes to L1/L2 and or
Why does it make a difference if the black wires are connected in?
Also, I thought this was a ring main? Wheres the two supply wires ???
Because of the way that the switch works.
Initially the switch joins COM to L1.
Pull the cord and the switch disconnects COM from L1 and connects it to L2.
The switch never joins L1 to L2, but only connects one of them at a time
If you have a supply wire connected to COM and black wires connected to
either L1 or L2 - then, in one position or the other, the supply will be
If you have a supply wire connected to L1 and the black wires connected
to L2, then the switch can never short out the supply.
No. As long as one goes to COM and the other goes to one of the other
two, you will be fine.
If you have mains live connected to COM, and fan switched live connected
to L1 and Neutral connected to L2 then in one position your switch will
join live to switched live and the fan will run, in the other it will
join live to neutral and your MCB will get the hump!
What you are aiming for is:
o L2 No Connection
Live in -------o COM
o L1 ---- To Fan
Neutral ---------X--------- To Fan (Where X is a join)
As has been pointed out, you could twiddle the connections round to make
use of the other terminal on the switch for joining the neutral wires,
but this becomes a non standard way of doing things that could (and
apparently has!) lead to confusion
Neutral ----------o L2 ------------------ To Fan
o COM Live out ----------- To Fan
Live In ----------o
But this relies on getting the switch connections the right way round,
so that the "shorted" position connects the neutral to the currently
disconnected live on the fan.
"Ring mains" for a domestic property is a bit of a misnaming anyway -
ring mains are used in power distribution. Ring final circuits are used
in properties for provision of general purpose power circuits (i.e.
sockets). Lighting circuits are wired as radials - i.e. a linear or ad
hock branch topology.
A couple of things to add to John and Sues diagnosis is that a switch wired
with the neutrals conected to L2 is that when a switch gets older it may not
always break before make and just the act of turing the switch on or off may
cause a quick short circuit.
Secondly, if the fan is not a timer over run model and there is no fan
isolator then the switch should be a double pole switch.
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