Got a bathroom fan thats been there since the house was built (only 7
years ago). Its got a pull string switch as you go into the bathroom
and the fan is on the opposite outside wall.
Been working fine until one day when I turned it on it tripped the
circuit breaker. My circuit breaker is in the garage and has got a
main switch as well as switches for upstairs/downstairs lights,
upstairs/downstairs sockets etc - it tripped the upstairs lights one.
Tried again - tripped again.
So far, I've :-
1. Replaced the pull switch - still the same.
2. Replaced the entire extractor fan unit with a new one - still the
I'm afraid thats the limit of my electrical knowledge now. Anyone got
any ideas ?
Ah, the guy who used to do my MOT uses that method of fault finding. Very
OK, looks like the MCB is tripping rather than the RCD (the "main switch" in
consumer units), so it looks like you are dealling with a short between live
Does the MCB trip when you pull the pull-switch with the fan disconnected?
Yep. Its not the main big switch just the one for upstairs lights...
Not tried it with the fan disconnected...
One thing - there are 3 fans fitted in three of the bedrooms (running
on this upstairs light circuits) fitted by me. They're all working
fine, but could there be a possible wiring problem on one of them that
is causing this???
If so, how do I know which one? (I guess I could start at the bedroom
next to the bathroom!)
Many bathroom fans were supplied with an external timer unit, so that
they would remain running for a few minutes after the light was turned
off. Newer ones tend to have the timer unit built-in.
If your existing fan unit didn't have two reds (one switched, one
unswitched) and a black (plus a green/yellow) and it stayed on after the
light went off, then you have an external timer unit.
The timer unit typically uses a capacitor/resistor series circuit to
lower the input voltages to that used by the electronics. The resistor
can overheat and damage the board with time, which can allow tracking,
leakage current and circuit breaker trips.
It would be easiest to simply replace the timer unit, if that has
failed. However, the wires going to the timer unit can be taken to a fan
unit with built-in timer, instead.
You said that you had replaced the switch and replaced the fan. But does
your system have a seperate timer unit and have you replaced that?
Modern bathroom fans tend to have an integral timer. Older ones often
had a seperate timer unit - somewhere between switch and fan. Just a
small plastic box, often with a clear cover and a screwdriver slot for
setting the on time. With wires from it going to the switch and other
wires going to the fan.
Just a thought but you don't say if the light is on the same pull switch as
the fan (most are). If so check the light bulb sometimes the cheeper ones go
short when they blow.
Or you may have a problem with the light fitting not the fan.
OK. So the upstairs light circuit has got on it:-
- 3 combo fan/lights (3 bedrooms)
- 2 normal lights (1 bedroom, 2 bathrooms)
- 2 fans (2 bathrooms)
The one switch for one of the fans is causing the trip when its turned
However, is it possible that the fault could like elsewhere in the
There are many more explanations that are possible, but unlikely:
1) The total load is enough to trip the mcb. So try the switch with
everything else on the circuit switched off.
2) The mcb is faulty and tripping on the fan motor start up - even with
everything else off. So try swapping the mcb with a different one.
3) The fan switch is a two pole one and wired incorrectly. So it shorts
the supply when pulled. Check the wiring.
4) The replacement fan is also faulty or wired incorrectly. So try it on
a 3A fused mains lead.
5) You have been doing DIY and put a nail through the fixed wiring from
switch to fan. So replace the cable.
6) Rats have eaten through the cable in the loft/ behind the wall.
Replace the cable.
So, try the following in sequence:
1) Disconnect the output cable from the switch and try the switch. If
the mcb trips, the switch is faulty or wired incorrectly.
2) Reconnect the output cable to the switch and disconnect the wires
from the fan, suitably insulating them in a choc box multi way
connector. Operate the switch. If it trips, you have a cable fault - eg
rats have bitten it.
3) Reconnect the fan and operate the switch. If it trips, swap the mcb.
Sue, I must ask you this.
You have started to do that cleaver =?UTF-8?B?UGFsaW5kcuKYu21l?= smiley
thing again with your "from" header.It certainly makes your postings
but it was only when you stopped that I realised you were called Palindrome,
and not Plindr me which sounds more like a request than a name.
Quite an enigma, are you going to leave it as it is?
The scenario I'm thinking of is that the original fan went faulty -
hence tripping the mcb. However, in putting in the new switch and new
fan, you have introduced a new fault. Say by mis-wiring the double pole
switch. It can be easy to do - the terminal layout on the new switch may
not be identical to the terminal layout on the old one..
It is potentially lethal - as has been the things that you have already
done, such as changing switches and mains fans. If you are at all unsure
about what you are doing with any household electrics and feeling
nervous - your instincts for self-preservation are kicking in with good
reason. Listen to them. Do an evening course at your local college..
then you will be happy taking on anything.
Then you changed it. Are you absolutely positive that you wired the new
All of these possibilities are remote. But there has to be something
Shame! That was one of the more realistic possibilities..
It has to be something. If the mcb is ok, the switch is ok, the fan is
ok, there is nothing else switched on - then it has to be the wiring.
So, have you tried it?
They are normally used to join wires /cables together *inside* equipment
- where there *can't* be any pull on the wires and *can't* be any
possibility of touching them. But they can also be used on the end of
unused wires inside boxes, to insulate the wire end - or used to
temporarily insulate the ends of free wires which aren't connected but
may be going to be made live.
It isn't difficult but it has to be done with absolutely no mistakes,
first time. The inside of the box will be live in places at all times.
Doing an evening course in domestic electrics makes sense. House
electrics really isn't the thing to learn by trial and error. It only
needs one teeny mistake to end up with someone dead or with the house on
It sounds like you ought to get an electrician in, this time..
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