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David Hansen David Hansen is offline
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Default wiring two gang two way light switch

On Fri, 19 Dec 2008 10:46:36 +0000 someone who may be alex6192
wrote this:-

The switches are in seperate locations. Both are two gang and both are
two-way. The switches control two banks of spot lights in the lougne (4
spot lights each). The box which has three wires (3 3 core wires) is in
one location and the box which has two wires (2 3 core wires) is in
another location.


Let me see if I have this right. All the switches are in the lounge.
In one box there are two switches (mounted in the one plate), one of
which controls half the spot lights and the other switch controls
the other half. In another part of the lounge there is another box,
which also contains two switches which also control the spotlights.
The spotlights are not controlled from another location as well. One
of the boxes has two three core + earth cables, the other has three
of these cables entering it.

If that is the case then this is what I think you have:

1) the box with three cables has one cable which contains the
permanent live and two switched lives to the groups of spotlights.

2) the same box has two three core cables which go to the other box,
each of which provides two way switching functions for one group of
spotlights.

3) the box with two cables coming in has the other end of these two
way switching cables.

To test this I would do the following, but only do this if you are
confident you can do it without too much danger:

1) separate out the ends of all the cables until all are easy to
test but not too close together.

2) turn on the electricity.

3) using a meter (or neon screwdiver if a meter not available and
desperate) identify which of the cores is the permanent live. Only
one core should be live, test them all to make sure.

4) turn off the electricity.

5) mark the permanent live, permanently.

6) using a terminal block connect the permanent live to one of the
other cores in the same cable (obviously not the protective
conductor).

7) turn on the electricity. One group of spotlights should light up.

8) turn off the electricity. Mark what that core feeds, permanently.

9) move the terminal block to connect the permanent live to the
other core in the cable instead.

10) turn on the electricity. The other group of spotlights should
light up. If it does you are almost there.

11) turn off the electricity. Mark what that core feeds,
permanently.

12) at this point if you have a meter make a flying lead and use
this to work out which ends of the other cables is which. To do this
connect the flying lead to say the brown of one cable, go to the
other box and test both browns. The one with zero resistance is the
other end. Mark both ends of both cables, A or B is preferable to 1
or 2.

13) you now have enough information to wire up the switches. If you
think you don't think about it yourself for a while and draw some
diagrams. http://www.ultimatehandyman.co.uk/two_way_lighting.htm
and the like explain how two way switching works, but remember that
in this case instead of a two core and earth cable coming in there
is a three core and earth cable because you have two groups of
spotlights. As you have marked the switched lives to the spotlights
you should find that the switches on the box with three cables
control the right set of spotlights.

14) if you didn't have a meter at 12) then connect the switches up.
If you are lucky then you won't have to do anything else. If you are
unlucky you will find that the switches on the box with two cables
control the wrong set of lights. If that is the case swap the cables
over.

Studily I didn't mark which wires went where when taking off the old
switches.


Most of us have regretted doing that.

btw the new switches are dimmers.


With "normal" dimmers only one dimmer is installed per group of
lamps. The switch with the dimmer turns the lights on and off and
controls the brightness of the lamps. Other switch positions simply
turn the lamps on and off. Even if the dimmer is turned right down
the lamps should glow enough when switched from another location to
be able to walk to the dimmer and adjust the brightness.

Dimmers take more space than a "normal" switch. Therefore I would
consider the box with two cables to be the favourite location for
the dimmers. I would place it there unless that did not find favour
with "the domestic authorities" in which case a deeper box may be
necessary at the location with three cables. The disruption this
will entail (assuming the walls are not plasterboard) may be enough
to convince "the domestic authorities" to change her mind.





--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54