Borrowed neutral ?

[duncanhead]

This is a strange one, but may have a simple answer..
upstairs light radial is from a RCD, 18ms at I
downstairs light radial is via a rcbo, 28ms at I
upstairs hall landing is switched by two switches, (live) connected to down radial, neutral from ?
if a load (bulb) is turned on (or a megger connected on volts range) the RCD trips... (up stairs), but not the circuit it's on.
wiring at the switch upstairs is 2 wires from down, one going up (to loft) and thus to fitting
Is it using the neutral from the upstairs light radial?
A test soon will reveal...
comments welcome..
If it is a borrowed neutral then what size of wire do I need? ie is it 4mm2 ie to the same specs as a cpc (PVC insulated) as a "single" ?
I'd need less than 10m then and would take only an hour...
PS- the semi next door had their downstairs hall light taken out of service, probably due to the same issue on a new CU install.

 

[sparkyspike]

You say that the RCD trips when a load is on the upstairs light. But then you say the RCD doesn't trip on the circuit it's on. But as the upstairs light is on the RCD, then that doesn't make sense. Unless it's tripping the RCBO?

Why do you think you have a borrowed neutral?

How long has this been a problem?

If you do need to run a new neutral, you can't use 4mm singles. You can use 1mm or 1.5mm singles in conduit or use insulated cable.

 

[17thman]

It may be an arrangement like this that somebody recently uploaded.

I have seen that a lot of people suggest that you need to run a neutral from the up mid rose (from the lamp return neutral) to a convienient point on the downstairs lighting circuit and then have the upstairs rose effectively by passed by the upstairs lighting circuit. I have done this.

 

[SparkyFromTheValley]

so at the CU is there not a neutral wire for the upstairs lighting?

 

[ericmark]

Years ago upstairs and downstairs lights were supplied from the same MCB/Fuse. However today when we wire they are often kept to separate circuits. I will guess someone at some time has changed the consumer unit and separated the upstairs and downstairs feeds.

However years ago it was also common to connect up two way lights with twin and earth. In order to save running triple and earth the line supply for second switch would be taken from another switch in the same plate.

This has resulted in the supply changing according to position of the two way switches but the neutral will not. Really borrowed line but called borrowed neutral.

I tried to explain it here

but I see some errors I will try to correct when I get time.

 

[ColJack]

your drawing is inaccurate..

it shows L1 and Com linked on the switc for 2 way..
it's L1 and L2, with the supply to one Com and the switched live from the second Com...

 

[ericmark]

your drawing is inaccurate..

it shows L1 and Com linked on the switc for 2 way..
it's L1 and L2, with the supply to one Com and the switched live from the second Com...

I did say some errors I had noticed them and have corrected if you refresh you will see now correct.

 

[ColJack]

no, at time of posting, is still the original drawing, even after refreshing..