2way to 3way switching in extension

[iwire4]

I have recently changed the consumer unit on a clients house. I even carried out insulation resistance testing on the existing circuit prior to installation to prevent any problems later. All was well until the light in the stairwell was switched on and it tripped both RCD's! two circuits (upstairs and downstairs lighting on separate RCD's) I looked at the wiring for the light and could see it had been modified through a junction box. It had the live feed from the downstairs lighting circuit and the neutral was from the upstairs circuit. It would appear this was done when the new extension was built to enable the lights to be switched on and off at three locations. I scratched my head and tried to work out a 3way switching solution with the existing wires. I was stumped. I have returned it to the existing two way switching. This means that there is no tripping but it has left my customer unable to switch the hall light off from the new extension. I have tried to work it out in the meantime to no avail, hence this post.
I'm sure this is regular occurrence when a house is extended?
The downstairs switch has 3 core and earth to the switch in the upstairs hallway. Twin and earth runs from this switch to the light rose (conventional 2way wiring). The additional switch in the new extension has 3 core and earth to the attic (not connected at present). I have to use what is existing as I can't replace the wires without major re-decorating required.
Has anyone come across this on their travels? Can it be done with what is in place?
RCD's are great but bring with them their own little problems.
Ian

 

[telectrix]

the dreaded borrowed neutral. easiest way to fix is to put both lighting circuits on the same RCD if a proper fix entails major redecorating.

 

[R G]

Agreed with above, but also you would have to put all lighting on same MCB , as you cannot share neutrals on seperate MCBS either , poss upgrade to 10A MCB

 

[itselectric]

You will need an intermediate switch. also you will need to be able to access the exiting 3 core with the view to connecting the new three core to it, or the new three core will have to go to one of the switches. If you can fulfill these requirements then it's just a matter of juggling the wires at the switch

 

[iwire4]

Thanks everyone. No magical cure out there then. I don't feel so bad now. Will take on board your comments and give the good news to my customer. Cheers.

 

[MDJ]

Tels right, end of, put both feeds on the same rcd, infact you could break regs by putting them on the same mcb also haha, but don't tell anyone I mentioned it.

 

[markythesparky]

There's a guy on here who could help with this 2 way switching lark.

PM lofty84!

 

[R G]

There is another guy who would do it all live, ..........Martom ..................
not sure if he would be alive after though

 

[markythesparky]

I wonder if Martom is Loftys boss?