Neutral or live fault ?

Hi all , I am new to this site and just wanted to see if anyone has ever come across this fault before. 

So I have just done a full inspection on a property and have a fault on 2 lighting circuits , when I turn one of the circuits off and disconnect the neutral the live becomes live until you turn the other circuit next to it off. 

I have tried this the other way round by doing the same with the other circuit when the first circuit is built back up but the circuit is ok and doesn’t come live. 

Also on the first circuit when it becomes live , both neutral and live both become live to earth but not when testing between the live and neutral. 

I know this may sound confusing the way it’s worded but I tried to word it as simple as possible .

almost certainly a borrowed neutral, usually in a hallway light.

:Welcome:  to the forum. What's your background - it will help us help you answer the question.

Well this is what we thought in the first place , although it’s not the typical hallway light as these 2 circuits are on the same floor of a big house which have a lot of 2 way and intermediate lights. 

Luke p said:

Well this is what we thought in the first place , although it’s not the typical hallway light as these 2 circuits are on the same floor of a big house which have a lot of 2 way and intermediate lights. 

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Either you are matching the wrong neutrals to the wrong lives or there is one (or more) lamp(s) getting a live from one circuit breaker and the neutral from the other circuit. Are they twin & earth or singles? Just as when you remove the neutral from the CU on a circuit still live with lamps switched on the neutral appears live with respect to earth or suppliers neutral. If this neutral is also joined to another circuit, whose live & neutral are disconnected then both L & N on the other circuit could appear live with respect to E & suppliers N via reverse paths through lamps that are still switched on. But the voltage between L & N would appear as 0v if they are both at 230v respect to E.  Draw it out and I think you will see how your symptoms could occur.

Doc H. 

Thanks for the reply , they are all twin and earth and no singles but there are a lot of 2 ways / intermediates so we was also wondering if maybe one of the lives of one circuit was mixed up with one of the strappers of another circuit in the same switch although they are all working fine. 

I am back there tomorrow so will do some more investigation.

First of all get a note pad and pen to write down exactly what results you get from the following tests:  Then I would start with everything connected at the fuse box go round and switch on every light on all light circuits. Then go back to the fuse box turn all lighting circuits off, disconnect the L & N for every light circuit, then do a continuity test between each combination of L & N for all circuits. eg if you have 3 lighting circuits called 'a' 'b' & 'c' test:

La -> Lb

La -> Lc

Lb -> Lc

Na -> Nb

Na -> Nc

Nb -> Nc

Test on continuity and IR test @ 230v. Obviously there should be no continuity and all IR's should be out of range. But if you have any continuity paths that will indicate circuits that are crossed/joined somewhere.

Assuming you do see some incorrect continuity between circuits, reconnect all the cables, still keep all lights switched on. Turn each fuse Off one at a time and note exactly which rooms / lights are powered from each fuse. Then with one circuit at a time energised, removed the neutrals from all other light circuits. Then check if any lamps from that circuit have now stopped working. This will indicate a lamp that is powered from one circuit but returns via the neutral of another. Repeat that process for each circuit. Write down ALL results as you go along.

At the end of that process you should know what, (if any), circuits are crossed and which light positions are involved. the next option will very much depend on what results you have now found, (or not)?

Doc H.       

It can get really complicated , as The Doc has said ,     I'm also thinking  that  when  the floor was wired  they forgot to take neutral through one part so " borrowed " one from the adjacent circuit .  

     No one would ever know until  (a)  You came along to do an inspection  or (b)    Someone isolated one circuit  and  started disconnecting  and received a shock from  the other circuit  returning down  that neutral .     

Never assume  stuff  has been done properly  by the way .    

With housing borrowed neutrals , if the customer doesn't want to pay to have it corrected , we tend to put them on the same circuit , if  feasible,   usually  is with all the LED lamps these days. 

I was always taught when testing for "Dead"    to check the neutral  too for this very reason .

Oh and welcome to the Forum ......don't be afraid to ask if in doubt . 

This can  also be encountered where you turn off one lighting circuit to change a fitting (loop at light)  Remove the neutrals and one then appears to be live.  Be very careful.

Some will tell me off for this, but a neon screwdriver is a bloody good tool for this job, it will show instantly if one of the neutrals appears to become live.

Evans Electric said:

Never assume  stuff  has been done properly  by the way .    

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The golden rule in my book is "ALWAYS assume circuits are wired incorrectly, until you have personally proved otherwise". And if taking advice from anyone, listen to their advice, then go and prove what you have been told before accepting is as fact."

Doc H.

Doc Hudson said:

The golden rule in my book is "ALWAYS assume circuits are wired incorrectly, until you have personally proved otherwise". And if taking advice from anyone, listen to their advice, then go and prove what you have been told before accepting is as fact."

Doc H.

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Now that is bang on.

Thanks for all the posts guys

Be sure to update us as you go

Doc Hudson said:

The golden rule in my book is "ALWAYS assume circuits are wired incorrectly, until you have personally proved otherwise". And if taking advice from anyone, listen to their advice, then go and prove what you have been told before accepting is as fact."

Doc H.

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A brilliant philosophy, if I may say so.

ProDave said:

Some will tell me off for this, but a neon screwdriver is a bloody good tool for this job, it will show instantly if one of the neutrals appears to become live.

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Lets face it  Dave , if you use your test lamps ...to earth ..it'll just trip out  on modern boards .    A neon CAN be useful. 

don't have that problem with my volt probes. I use a fluke voltage indicator a lot as well - it has torch built in, so ruddy useful....

@Megger Jase

@megger mike

if Carlsberg made a voltage tester........https://isswww.co.uk/fluke-t150

some people may prefer a different tester and that is there opinion, admittedly it is wrong, but it is their opinion  :innocent

I have the cheaper version of those, but never had a problem with any voltage probes.

That carlsberg does narf look familiar, probably seen it before. 

Tony S said:

The golden rule in my book is "ALWAYS assume circuits are wired incorrectly, until you have personally proved otherwise". And if taking advice from anyone, listen to their advice, then go and prove what you have been told before accepting is as fact."

Doc H.

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The Doc is right there .     I should have  recalled it earlier today  TBH.

Did various small jobs for customer  , mostly fitting new light fittings...then a 2 gang dimmer switch to change , not working .

A bare earth wire had blown it to bits when someone  pushed a bit hard on the knob .

I was puzzled by the wiring in the switch ....a pair of red/E  for the centre light  ...OK .then a pair of 2.5mm  reds  for the wall light  .  Wall light  turned out to be wired directly from  the ring  main  , in 2.5 mm  back to the light switch .  

Forgiven Deke, everyone has an apprentice day! Just done make it a habit