In our 70s vintage house there is a light switch on the wall (not near
the door) with a piece of red tape on it, obviously saying "Do not
switch me". I decided to see why. It turns out to be one of two
switches (the other at the door) controlling the main room light. I
tried switching it when the door switch was off. The light came on
briefly as the switch passed the midway position, then went off again
when the switch was fully switched. In other words, both positions of
the switch left the light off. Hmm. I removed the screws holding the
switch plate, and replaced the switch with an identical one. Same
behaviour. Hmm. I then discovered that I could make the light go on
and off by moving the switch plate slightly (when off the wall). Sounds
like a wiring fault, but unfortunately the wires between the end of the
sheathing and the switch are clearly intact. Hmm. Out with the
voltmeter, to test the voltage between phase and earth and see if it is
affected by movement of the cable. This is where it gets really odd.
The voltage between the hot wire and earth cycles between 230v and 190v
with a period of about 7 seconds. WTF??
Correction: looking at the circuit for a Traveler or Common system
(which I presume this is)
I see that of course there is no earth wire. I don't know what the wire
colours signify, but since there are only three conductors at least two
of them have the potential to be hot.
I need to think about this :-]
The lamp (fluorescent) is in the circuit - I'm not sure why that causes
a fluctuating voltage, but I'll believe it.
Anyway I now understand the odd switch behaviour. It's just the crappy
70s switch design. The replacement one I tested with is the same
vintage, no doubt installed somewhere in the house at the same time.
The contact made by the spring action was fine, but the contact made by
the rocker switch depressing the contact arm was not good. I have
cleaned the contacts and bent the arm slightly - now it works as it should.
Another correction. I now have the switch working OK, but the circuit
wiring is odd. One of the two incoming conductors is always live,
unaffected by the door switch position. Now I have taken the door
switch plate off, and it has far more conductors wired into it than can
be explained by the two room lights. To my great surprise I have
discovered that the circuits for the front door light (outside) and the
hall lights are all powered from connections made at this switch (of
course they have their own switches). The way the thing is wired up it
is not functioning correctly in multiway mode for the bedroom light.
Very odd. I will try to sort it out.
It was definitely mis-wired, so the multiway system didn't work even
when the switch was fixed. I finally figured out how it all should go
together (not at all complicated, in reality, but sort of new to me),
and now it all works.
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