Bathroom fan blows RCD switch

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 same...
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).
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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.
Matt
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paulfoel wrote:

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[1], 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.
[1] Two way light switches have three contacts, but can be used in ordinary one way circuits:
http://wiki.diyfaq.org.uk/index.php?title=2_Way_Switching
--
Cheers,

John.

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John
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.
Rob
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robgraham wrote:

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.
--
Cheers,

John.

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The really clever way is to replace the fuse wire with a length of paper clip. Don't laugh, I know a case where that was done.
--
Keith Willcocks
(If you can't laugh at life, it ain't worth living!)
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[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....
george
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snipped-for-privacy@hotmail.com wrote:

Ideally with a sensitive resistance meter (or a type all pro sparks would have as standard kit), but a reasonable digital multimeter will also give you a good indication.

In best blue peter tradition, here is one we did earlier:
http://wiki.diyfaq.org.uk/index.php?title=Electrical_Circuit_faults
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Cheers,

John.

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wrote:

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 work?
--
Keith Willcocks
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Keith Willcocks wrote:

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.
e.g.:
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 would open)
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Cheers,

John.

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John Rumm wrote:

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 connection.
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.
--
Sue










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Palindrome wrote:

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 system.

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.

Indeed!
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John.

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I had to ask ;o)
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Keith Willcocks
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Keith Willcocks wrote:

It was a good question really. As a quick confidence check, the test you described is not bad, you just need to be aware of the limitations.
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Cheers,

John.

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Yes. Sorry you're right.

Its a pull switch and has got an L1, L2 and one other terminal. The fan is a manual type.
I've been connecting one red wire to L1, one to L2, and black the other terminal?
Wrong?
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paulfoel wrote:

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 common terminal.
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....
--
Sue


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Sue,
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 COM?
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 ???
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paulfoel wrote:

Nope, it matters not.

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 to COM.
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 shorted out.
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.

House *Sockets* are on a ring main.
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paulfoel wrote:

Sounds like it...

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.
--
Cheers,

John.

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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.
Adam
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