Help please with understanding a borrowed neutral

[baza]

Hello everyone
Could someone explain why the following happens with a borrowed neutral?
Did a cu change today and installed a dual rcd board, Previously the up and downstairs lights were on the same mcb. I was half expecting the rcd/s to trip when I separated them, which they did. I dont need help in how to remedy this, but my question is when disconnecting the link at the back of the switch feeding the strappers linked to the other switch, I would have thought that I had removed the inbalance and the rcd would remain on.
Thanks in advance

 

[Stroppy]

Commercial/ Industrial is way easier and more rewarding. WHY does every new member on here want to become a flippering Domestic Installer?

For some of us (ie: Me) we didn't really have a choice. My first job as an apprentice was with a domestic installer, so that is the route I have found myself stuck in. I say 'stuck' as I have tried to move to commercial/industrial, but having no experience with that side of things (3 phase, singles, cable tray, steel, etc) no-one wants to give me a chance as a 3rd and now 4th year, so I seem to be doomed to be a domestic, which to be frank is a pain in the ar$e, hacking holes in door frames, I want a socket in the middle of that wall but don't damage the wallpaper or axminster carpet, fix my light which I've smashed off and snipped all the cables off in the grade 1 listed ceiling blah blah...

 

[guest119]

Oops, Ive just landed on a Electrical Trainee thread :banghead:

Boydy

 

[electric will]

sorry bud. That does not help in the slightest! Hang on, i can hear a beer calling me from the fridge........

That should help,as that wiring diagram makes it clear, with the lighting circuit your working on isolated, if you disconnect a neural that another circuit is using (borrowing) that disconnected neutral will become live, through a filament in a lamp etc

 

[Knobhead]

For some of us (ie: Me) we didn't really have a choice. My first job as an apprentice was with a domestic installer, so that is the route I have found myself stuck in. I say 'stuck' as I have tried to move to commercial/industrial, but having no experience with that side of things (3 phase, singles, cable tray, steel, etc) no-one wants to give me a chance as a 3rd and now 4th year, so I seem to be doomed to be a domestic, which to be frank is a pain in the ar$e, hacking holes in door frames, I want a socket in the middle of that wall but don't damage the wallpaper or axminster carpet, fix my light which I've smashed off and snipped all the cables off in the grade 1 listed ceiling blah blah...

Now where did you want that socket?

 

[SirKit Breaker]

It was common practice many moons ago. The lighting circuits would be supplied with an individual live to upstairs and downstairs lights, then a neutral would be run around all of them and back to the board, so 2 lives and 1 neutral, each live was then terminated into separate fuses, hey presto 2 lighting circuits. Then someone invented the RCD and updated the regs, and now we have lots of borrowed neutral issues because of this. I once had a DIY charlie job to sort, he had put an outside light up, dug through the outside wall into the kitchen light switch, tapped into the live, and put in his switched live, but the light had no neutral, so he moved the light up the outside wall till it was in line with the socket in the bedroom, dug through the wall and took the neutral from the socket, turned his light on and lost power to half his house, then he rang me.

Cheers...........Howard