unusual lighting circuit arrangement

[Electric Magic]

Went to do some pre-work inspecting and testing for a small job on a lighting circuit I am undertaking next week. Relatively new dual split load consumer unit, 1 lighting circuit for upstairs and downstairs, 1 line wire, and seemingly 2 neutrals (one of which is a single). I initially thought perhaps the single might belong to another circuit and had been put in the wrong place on the bar but thought I should test to make sure. So I tested continuity between N&E at the upstairs fitting I am going to work on - there was continuity with one neutral but not with the single. Then I insulation resistance tested the circuit LN-E but with the single still attached to the neutral bar and got around 1.5Mohms. Then I removed the single neutral from the neutral bar and tested again and I got 109Mohms. My tentative conclusion is that the single neutral is most likely part of the same circuit and that a single line wire is connecting two of the lights in the circuit. Also, the landing light is fed from the hall switch (2 way switch via 2 strappers) but the line and neutral for all the other upstairs lights (and the neutral for that light) are fed via a T&E to the landing rose, which I am guessing comes straight from the CU (ran out of time to test, will test when I go back).

A feed from the hall switch to the landing light and neutral taken from upstairs is fairly common (a lot of installs like this in the 60s and 70s so I hear as are posts on here about the related RCD issues when a CU change is done), but out of curiosity, if it was done to save cable why did they not take the T&E to a downstairs light first (its on the way from the CU) and if they were using sheathed singles to save money why did they not take the T&E to a different rose upstairs and run a single+cpc back to the landing light?

 

[davesparks]

Is there an external light by the front door? Ive seen a few where an outside light has a single neutral from either a rose or the CU and a single s/l from the switch to avoid taking a feed outside or a neutral to the switch

 

[Electric Magic]

I don't think so but I'll double check when I go back

 

[hasel5]

And ? whats

 

[Electric Magic]

And ? whats

Did you hit post before you finished that one?

 

[Richard Burns]

Initially test for continuity between the two disconnected neutrals; unless the actual light is on with an incandescent lamp then there is likely no continuity (but a high reading, hundreds of ohms if the light is on).
If there is continuity (tenths of ohms) then the single is redundant and there is no problem.
The likely thought is that this is a neutral for a remote light and (if there is no continuity between the neutrals) the light is the end of a run, so check the light wiring on each light looking for a single neutral and a single line incoming.
Alternatively switch off all lights, disconnect and then join the line and single neutral together and test continuity, switching on one light at a time until you get continuity through the lamp. Then check the wiring for that light.
But always start with the remote lights, outside, in under stairs cupboards, in new extensions.

In your case as there is one circuit for upstairs and downstairs lighting then crossovers between upstairs and downstairs are immaterial, just a nuisance if you were to try and separate upstairs and downstairs.

Normally the switching approach on landing lights would be to use two wire two way switching and use the supply line from downstairs and this then takes the switch line up to the upstairs switch so a short run to the landing light and use the upstairs supply cable for neutral and earth, any other method would require another core to be run from downstairs to upstairs and since crossing neutrals has no apparent electrical effect (without RCDs) why not; not allowed, not easy for the future, but simple at the time.