In my garage there is a light switch - it ought to be simple - the cable
from the power supply comes into the switch and then goes out to the light.
Most of the simple switches in the market seem to just switch the live and
you don't bother cutting the neutral.
However my switch has both wires cut and the switch has 3 terminals - one
called Com and the others called L1 and L2. Both neutral cables are
connected to L2 and the live supply is connected to L1 with the red light
cable connected to Com. When the switch is off it would appear that L2 is
connected to Com (so the light is off) and when the switch is on, L1 is
connected to Com so the light comes on. This seems to make sense and it
sometimes works ok.
However, for as long as I can remember, turning off the light switch
periodically breaks the circuit breaker - perhaps because of leakage of
current when the switch jumps across. To avoid this I have been leaving the
light on and just using the circuit breaker to turn the lights on and off
but I have now got around to trying to fix it. My first attempt was to
replace the switch with an identical one in case the switch was faulty but
that didn't work - indeed it just damaged the new switch and I have had to
revert back to the old one.
Please can you tell me is this a normal way to wire a light switch. If not,
is my guess above a reasonable explanation for the circuit breaker problem
(ie the switch should not be wired in this way)? In which case is there a
simple cure apart from starting with a fresh wire and buying a simple
switch - eg how could I join the neutral wires back together?
Alternatively if my guess is not right, how would I find out the cause of
the circuit breaker breaking.
What you have got is a 2 way switch, usually used for stairways etc.
where there is a switch at the top and bottom of the stairs. Generally
the black wires are not neutral in this case, they might be the two
parallel wires that go to the second 2 way switch, L1 to L1, L2 to L2,
and the second Com would then feed to the lamp, with the neutral being
connected to the other connection in the lampholder. I cannot help
further from how you have described it being wired, as the circuit would
need to be traced to see what else might be connected to the black
wires. A two way switch can be used as a one way switch by using Com and
either L1 or L2, but I cannot think why you have 2 wires in L2. If you
remove the 2 black wires from the switch and put them in a separate
connector, I don't think the light is going to work as then there will
be no feed to the lamp :-)
What I think he has is a mains cable (with L & N) coming in and a cable
going to the lamp going out. The black wire of the cable coming in needs
to be joined to the black wire going out. The red wire coiming in goes
to one side of the switch and the red wire going out goes to the other
side of the switch. Normally those black wires would be joined together
in a connector.
What it sounds like was done, is the the red and black wires coming in
were both taken to the L1 and L2 terminals of the switch and the switch
common taken to the lamp, along with a black wire commoned to the black
wire coming in. The switch is then alternatively connecting live or
neutral to the red lamp wire. This saved having to use a connector to
join the neutrals together.
That is correct. What I am wondering if this could be the cause of the
circuit breaker breaking. Assuming it is, I am trying to get hold of the
plastic joiners you suggested to see if that works. Thanks
Yes it could. The switch wasn't designed to have line and neutral
directly wired to it in this way.
What will happen is that, when the switch is wired normally, there is
always the resistance of the lamp between line and neutral to limit any
current. A tiny arc may form in the switch as it is operated, but the
current that can flow will be have been limited by the lamp and the arc
will soon disappear.
When the switch is wired as yours is, that tiny arc that forms when the
switch operates can create a conductive path direct between line and
neutral, within the switch. This can cause a very high current to flow,
but that current is almost immediately interrupted by the circuit
breaker tripping. With wear and age, the chance of this conductive path
being created grows. Hence the switch would probably have been fine,
when new, but will gradually cause the breaker to trip more and more
At least you have a breaker, not a fuse. With a fuse, a lot more time
and energy can pass under these conditions, before the fuse wire melts.
This energy release can do a lot more damage within the switch - it is
almost like having a tiny welding torch in there.. this can damage more
of the switch - removing bits of contact and burning bits of insulation.
The switch can start to be unreliable even when left switched on - or
This could very well be the cause of the MCB tripping. The switch may not be
"breaking before making" and would create a live neutral short that will
trip the MCB. Any DIY shop will 6 amp terminal block.
Sue's diagnosis sounds correct to me.
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