Fault finding problem

[Gavthesparky]

I am having some problems at discovering a wiring fault between neutral and cpc on a ring circuit. The RCD dedicated to the various socket circuits at this house trips as expected because of the fault but also trips when the downstairs lighting circuit which is in the same board but not protected by the rcd is energinised. I have disconnected line, neutral and earth of the offending socket circuit from the board and now the lights are not affecting the RCD but I am still getting a reading of 1V in the dead sockets. Any ideas to what might have caused the problem, and also ideas on how to find the fault without pulling the place apart.

 

[Sparky Ninja]

Are you sure the light circuits neutral is connected correctly at the CU?

 

[telectrix]

sounds like a borrowed neutral. i.e a neutral taken from a circuit on the protected side to a circuit on the non protected side. or vice versa. landing lights are the most common cause. neutral from upstairs cct. and live from downstairs cct.

 

[Gavthesparky]

I am happy that the neutral is wired correctly at the board. I think it is being shared elsewhere along the circuit. Odly this installation has been working fine for the last 4 years from commision then pop the washing mackine trips the RCD and suddenly a whole load of problems happens.

 

[pushrod]

Is this happening after a board change or just something you've been called out to. The one volt is probs just an induced voltage from cable running near each other.

 

[Sparky Ninja]

If more than one circuit starts to get affected, it may be worthwhile carrying out a PIR on the installation.

I had a situation like this once where the shower install was at 136V to earth, it turned out to be damaged insulation on some switchdrops on another circuit. Individual testing of the circuits is the best way to find these problems.

 

[pushrod]

are both lighting circuits on the same rcd? if you put them on the same side and the tripping stops that is a sign that it is a borrowed neutral- it is not however a long term solution.

Have you tested the rcd?

 

[Gavthesparky]

This is something that I have been called out to. I have managed to get the lady up and running with everything except the offending circuit which for you interest is wired in a 16th edition style with only the socket circuits wired through an RCD. I am returning on Wednesday to do a PIR.

 

[pushrod]

I am having some problems at discovering a wiring fault between neutral and cpc on a ring circuit. The RCD dedicated to the various socket circuits at this house trips as expected because of the fault but also trips when the downstairs lighting circuit which is in the same board but not protected by the rcd is energinised.

What have you done to confirm this fault - insulation resistance or continuity? If you are getting a low continuity reading you can check from either side of the ring and you can then use the resistance value to calculate the approx distance to the fault.

Are you certain it is not a dodgy appliance still on the ring?

 

[Gavthesparky]

So far all I have done is an insulation test which came back as 0 ohms between neutral and cpc. I feel sure that I have now isolated all appliances and the next plan is to do as you suggested and do a continuity test between the two offending cables to try and locate the fault.

Thanks for the feedback

 

[nickblake]

The chances are its a trapped neutral in the socket check on continuity and see if you get a reading if you do , and you have a plug in socket for you meter plug the neutral into live and earth into earth as you would normally then go round each socket and test at the socket the lowest reading at a socket could be where your fault is , could also be in a jb under the floors , unless its concrete that is, once you find the fault re do your IR using the normal methods

 

[telectrix]

So far all I have done is an insulation test which came back as 0 ohms between neutral and cpc. I feel sure that I have now isolated all appliances and the next plan is to do as you suggested and do a continuity test between the two offending cables to try and locate the fault.

Thanks for the feedback

might be a silly question, but did you dis. the neutrals from the CU terminal block when IR testing?