(Im)possible Three Way Lighting?

[Mikiec]

Hi, i'm a newbie on this forum but please don't let that put you off helping me!
I've wired up three way lighting circuits before, but I've come across a way of lighting I've not seen before in a very old house.
Basically I've got a two way circuit at the moment - two lots of two way switches controlling one light. I want to convert this into a three way circuit.
Normally I'd use an intermediate switch but the two ways are wired seemingly with the two commons linked together. It looks like one of the poles from each switch are connected together and to power, with the other pole from each switch again linked together and off to the switch.
I hope that makes sense.
How on earth do I add an extra switch without having to re-wire the whole thing?!
Any help greatly appreciated.

 

[fubar]

I think I know what you are describing, but could you do a diagram?

 

[MANDATE]

There are two methods of wiring a couple of 'two way' switches.
first method! switch cable to one switch only, red to L1 black to L2.
switches linked by strapper cable 3 core + earth. com to com (red). L1 to L1 (yellow) L2 to L2 (blue). intermediate switch uses yellow and blue wires only to form cross over
second method! No single switch cable as such, but the red wire goes to com on switch No 1. The black wire goes to com on switch No 2.
Now the strapper cable is 2 core + earth (red and black) these link L1 to L1 and L2 to L2 . intermediate uses these to form cross over.
I'm thinking you may not be familiar with both methods.
It becomes a lot clearer if you draw a diagram of each method.
hope it helps.

 

[Mikiec]

(I'm not entirely convinced that I'm right but it's the only explaination for the voltage readings at the terminals)

 

[Adam_151]

That is a standard 2way wiring arrangement with one difference, the "to light" cable is normaly on the other switch, on the terminal next to "V in"

you can cut into the two wires shown at the bottom of your diagram to install an intermediate switch

 

[plugwash]

yeah that is a workable though unusual method as it needs an extra core between the switches over the method of feeding one com and taking from the other

its basically the same as

but with the live and switched lives taken at opposite ends

note that to add a third switch you do not have to understand how the existing 2 way switching works you just have to remember that the combination of a 2 way switch and an intermediate switch is like a 2 way switch that can be switched from 2 locations

 

[Mikiec]

Thanks to everyone for all your help. I'll see if I can work out what I need to do.