2 Lighting circuits sharing same 6amp MCB

[olsonn]

Looked at a job today and noticed 2 red line cables in one 6amp MCB which was marked up as Lights. The place is a small purpose built flat with a couple of lights in the entrance hall with stairs going up to the living area and the rest of the lights. The two way switch in the hall operates the hall lights in isolation and also the upstairs landing light. The landing light can also be operated by its own switch on the landing. Although the two lines shared the same MCB the Neutrals and CPCs were in seperate slots on the bars in the CU. I thought it would be a simple job to simply disconnect one of the circuits and put it on to the other RCD. This seems sensible given that the flat owner is in her 80s and lives alone and has to come down the stairs to reset the MCB when it trips in darkness. Well I connected one of the circuits to the other side of the board thinking this would solve the issue. I turned on the hall lights and they came on no problem. Ithen turned on the landing light from the downstairs switch and both RCDs tripped. I have put it back the way it was for now and it is working.

Does anybody know why it is doing this and how can I sort it out

 

[alanl]

Shared/borrowed neutral. You'll have to find where the neutral is connected, usually at the pendant at the top of the stairs, disconnect and make it safe, then run a neutral from the downstairs circuit to the pendant.

 

[olsonn]

Would this have been done by accident or was this once an acceptable way of doing this?

 

[needasparks]

Might have had a new circuit installed to make a spare way or someone else found out about borrowed neutral and doubled up when the board was fitted and did not want the bother. tends to happen when no test is done before a board change by a £150 out in an hour sparks.

 

[telectrix]

this was common practice in the 70's. the landing light has it's N from upstairs, but the L is fed from the hall switch. meant a 3 core between the 2 way switches wasn't needed. either keep the lights on 1 circuit, or run either a 3 core between switches or, if in tube, a single, then dis. the L feed from downstairs and feed as you would normally do from the landing fitting .

 

[ezzzekiel]

tends to happen when no test is done before a board change by a £150 out in an hour sparks.

Doubling circuits is a perfectly acceptable way of overcoming a borrowed neutral where tenant refuses further disruption or costs

 

[Knobhead]

Not necessarily a borrowed neutral, it was quite common practice to run a ring for lighting in 1mm T+E in the 60’s to early 90’s.

To my mind it’s still a bloody good way of doing things but I always used 1.5mm. There again I wasn’t tied by the constraints of finance. They were my own houses or friends.

 

[Knobhead]

Mods please delete, pushed the wrong button!