Testing for a borrowed neutral (I know I've already searched)

Good morning all. I am new to this and I am changing my db for my nic eval. 

The house has a single rcd protecting all but the lighting circuits. 

The lighting is wired in singles (live and earth together and separate neutrals). There is 3 core used to connect the intermediate switch with the other two way switches on the landing circuit. Power is fed from upstairs to th ha switch and the cable (common and switched line) from the landing switch runs into the loft. 

After I had examined this I thought I was ok and closing all lighting circuits on both floors ( so lights were all on) and testing at the db indicated no link between the circuits. 

I am replacing with an 18th edition board with RCBO's shortly. Is there any other way yo check for the borrowed neutral issue or am I now being paranoid?

Thanks for your help....

In Training said:

Good morning all. I am new to this and I am changing my db for my nic eval. 

The house has a single rcd protecting all but the lighting circuits. 

The lighting is wired in singles (live and earth together and separate neutrals). There is 3 core used to connect the intermediate switch with the other two way switches on the landing circuit. Power is fed from upstairs to th ha switch and the cable (common and switched line) from the landing switch runs into the loft. 

After I had examined this I thought I was ok and closing all lighting circuits on both floors ( so lights were all on) and testing at the db indicated no link between the circuits. 

I am replacing with an 18th edition board with RCBO's shortly. Is there any other way yo check for the borrowed neutral issue or am I now being paranoid?

Thanks for your help....

Click to expand...

Meant to say the hall switch not th ha switch! Big fingers little phone!

link L&N at the suspect light. continuity between the live for  circuit and neutral of another. should be open circuit

I did continuity between L of downstairs and N of upstairs and between L of upstairs and neutral of downstairs and it was off the scale. All lights were on before I isolated. 

Sometimes it can be a bit of a sod getting readings through some type of these modern energy saving lamps..

Also some decorative fittings are more hassle than its worth to open up and access the terminations..

The old days of reading 50 or 60ohms through filament lamps dangling from pendant and roses are dwindling fast..

So assuming you have Global IR'd everything on all light circuits to prove no nasties to earth either L or N..

Then I tend to do some live tests.... 

Before dismantling anything I generally make sure every single light in the property is switched on....

Keep all light switches in the ON position for the duration of the checking..

Turn each circuit off one at a time and write a note of exactly which lights have now gone off..

(so you know where each lamp gets its live from)

Then once you know the probable circuit layout...

Leave one light circuit connected at a time and disconnect the lives & neutrals for all other light circuits..

Check that all lights on the one still energized are still working..

AND with a voltage tester verify none of the other disconnected wires are now live.. 

If you find no disconnected conductors becoming live when they shouldn't then they should all be independent of each other..

Repeat the process for the other circuits..

Also when actually putting them back onto the new RCD...

As a belt and braces check..... 

Connect one circuit at a time with all other lives & neutrals strapped down to earth..

Any cross connections should trip the RCD if other conductors are at earth potential.

Once all the circuits are back onto their new RCD/RCBOs if all lights are still switched on and nothing trips then jobs a goodun!

BUT make sure you have identified and accounted for every light..

Nothing worse than finding a light later that you had forgotten and it trips out!!

Also..

I know you said this is your house...

but for future info be aware of builders / conservatory fitters doing a light bridging between a light circuit and a socket circuit!!

Not all borrowed neutrals are Light to Light circuits.  

Guinness   

You can always remove the  lighting neutrals from the bar  and test between them , / power off  ,  /   landing switches  on .  Should be open c/t . 

Or just connect them ...if the RCBOs trip put  you'll have to separate  the neuts or put them all on the same circuit . 

Or if its easier ...rewire the hall  or ha  light switch to the upstairs circuit .  

Be warned ... a lot of domestic wiring has at least one fault on it ,  either original or from a rewire .

The most common is the "Borrowed Neutral "  which began to disappear in the 1980's .

The other most common is the "open " ring main   I'd say .

Then we get the DIY "experts"        We won't go into those .   

Thank for the comprehensive information. I will definitely do some more Dead testing before removing the board. Have zs tested the circuits to ensure compliance.