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Replacing old light swtich

Postby jondyke » Fri Jul 18, 2008 10:46 am

I have taken an old light switch off to replace it with a new one and the wiring is a bit confused to say the least. It is a 3 gang switch, 1 controlling the outside light, one the hall light and one the upstairs light (there is also a single switch up stairs). Current configurations is:-

Vertical switch with 1 on top and 2 on bottom.

L1 - Red L2 - Black
Common - brown (linked though from BOTTOM with brown wire)

This top switch appears to be connected to the upstairs light.

L1- Blue L2 - Yellow
Common - red (linked though all 3 commons with brown wire)

The bottom of the switch allows the downstairs and outside lights to be switched on and off independently.

How would I wire a new 3 gang switch with 3 lots of l1/l2/common (3 for each switch)?
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Simply Build It

Postby ericmark » Sat Jul 19, 2008 8:16 am

Two way switching. (note space did not work with drawing so using full stops to space out so ignore full stops please)
..............\__________________/ centre cable either connected to upper
Lamp___/__________________\ or lower at each switch.
Lower connects to lamp and the operation is when both switches are up or down the light will be off when one is up and other down light is on.
In most cases a triple core cable will be feed between switches but where multi-gang switches are used then a twin core cable can be used and the “Line” is feed independently to each switch. This practice is now causing problems since with stairs lighting it is sometimes found the two supplies come from different overload devices and where earth leakage protection is added latter this will no longer work.
Where switches are marked L1, L2, Com the centre wire is com. Swapping L1 and L2 would not stop it from working as long as the Com is connected the Com.
It will be noted this is not how we are shown to connect two way switching at school with the school boy method a connection block would be required.
All cores should be red if old colours or brown if new colours but it would be harder to identify each wire so red – black or red – yellow – blue in old colours ore brown – blue or brown – black – grey in new colours are used with red or brown sleeves on each cable. Since the cable colours for each core should be the same there is no convention for what colours do what under the red or brown sleeves if the electrician ever fitted the sleeves in first place.
To fit two switches close together in a ganged switch often the switches are reversed as to position of com and L1, L2. but not always. Because of this electricians often test the switch to ensure they have correct pair to each com. The USA use colours blue and yellow to mark the terminals which have nothing to do with our colour codes. Also some switches call it L1, L2 and L3 and don’t use “Com” as label.
Reading your post you say how commons are linked. With non two way switching this may be OK but with two way switching it would be L1 or L2 linked not “Com” as a result I would wonder if the electrician had used some non standard way to wire up the switches. And I would be making very careful notes and ringing every switch. If you use school boy two way switching then “com” would have a “Line” feed on one switch and “com” to feed light at other switch.
Line ___________
___/....................\_______ Lamp
.....\___________/...............You will note Line at one switch and Lamp at other which makes it necessary to run a wire back to first switch and fit connector blocks so is not normally used by electricians also if the Lamp is not returned as when using it for stairs lighting the fact that the feed and return cables are not in same sheave can cause interference on radios etc. But it still will work.
Under normal circumstances you do not find neutrals in the switch box so wrong wiring will not normally blow anything but in resent years 12 volt lighting has become popular which results in no ceiling rose to act a junction box so we are now getting neutrals feed into the switch box so there is no guarantee that neutrals will not be found.
Hope that helps Eric

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