Tracing a RCBO switchoff problem

[Rogeranerley]

In trying to meet current regs in installing lighting in a new shower room, I've fitted a 6a RCBO to the non-RCD side of my cu linking the upstairs lighting circuit. All seemed well until I switched on a 2 way landing light that's on the same non-RCD buzz bar: at which point the RCBO clicked off - shutting down all of the upstairs lights.

However, once the RCB0 is switched off, there are no problems with the landing light wired to the 6a MCB on the non-RCD side.

In order to try and confine the areas to visit in correcting this problem, am I right in thinking that a fault in the 2 way wiring of the landing light could be causing the RCBO (connected through the same buzz bar) to click off.....or is there a more obvious answer?

Thanks

 

[Adam_151]

You'll find that the landing light is taking its neutral from the upstairs lights and live from the downstairs lights. The RCBO for the upstairs lights is tripping from this inbalence when the landing light is switched on. But because single middle wide RCBOs generally do not break the neutral, the light is not stopped from working.

Be careful when working on the circuit, you could potentially get a shock from the neutral of the upstairs lights with the circuit switched off due to the feed from the downstairs lights.

This arrangement is known as a borrowed neutral, and while against BS7671, there are many instances of it out there, a lot of which are on landing lights (the whole street is probably the same!). The cure is to re-feed the neutral from the landing light from the same circuit it obtains its live from

 

[Luckyphil]

More than likely you have a "borrowed Neutral, In older wiring on a 2 way Circuit , Landing / Hall you`ll find the Landing Light may be fed off the Downstairs Light , via a Link across the 2 way Switch in the Hall , So I effect you have the Feed from the Downstairs Lighting Circuit , and the Neutral is on the Upstairs Lighting Circuit , So you need to either run in a separate feed or , ideally have the Landing and hall Light on the same Circuit , which is better , If the Power is off to a Circuit both sides of the Switch will be dead

Lucky

 

[Luckyphil]

Just a tad quicker than me Adam

Lucky

 

[Rogeranerley]

Thanks for your quick answers guys.....and the caution on a live neutral!

The house is 100 years old and, from the lack of any earths on the light circuit cables (since added) looks have first been rewired in the early 60's.

I did wonder whether there was some cross feed, and have now found a 'live' feed running between the 2 x 2 way switch in the hallway.

I think I'll aim for bringing the whole hall/landing rig onto the downstairs circuit . With luck, I might be able to use the existing cables to avoid cutting new channels in the wall.

Thanks again