HELP! Voltage across neutral/earth on an isolated circuit

[tanglemist]

I am having terrible trouble trying to find the source of a problem which is
One of the ring mains in my house is reading an AC voltage of approx 8 Volts across Neutral to earth when the circuit is isolated at the CU. The supply is fine at 240VAC throughout but I have this reading on this circuit. I have checked other circuits in the house and they dont give any reading across neutral to earth when the circuit is isolated.

I am also suffering (link??) from an 'overactive' main rcd which has tripped on more than one circuit. P.S. This house was completely rewired 2 years ago and all CUs, sockets/fittings are new (electrician since 'disappeared').
Could we have a faulty RCD? Trips have occurred when using a number of different appliances in a number of locations in the house so no apparent link there.

 

[huwcecilmorgan]

Your problem could also be why my main RCD tripped when I was repairing a socket, which I thought was completely isolated. However, I believe the mcb controlling the ring main only isolates the live conductor, leaving the neutral connected to the neutral busbar. The main RCD tripped whenever I inadvertently touched the socket's earth conductor with the neutral, so I assume there must be a voltage (small, because I could not feel anything!) that upsets the RCD. Can an expert confirm this? And is it normal?

 

[tanglemist]

thanks for that. I knew the neutral and earth are both all connected at the consumer unit but I tried isolating other circuits and testing and got no reading at all. Ive read on here people mentioning 'inducted voltage'. Could it be that the reading is just picking up power from adjacent cables at a low voltage (and assumingly, very low current)? I tried a test further round the same circuit and where it had been reading 8VAV, it read 5VAC so that would hint that that was true. Am I right not to worry about this then, is it quite normal then?
If thats not a problem, whats the surest way of finding the nuisance tripping fault on the RCD? Its a split busbar CU with only the ring mains (about 6) fed from it. All outdoor stuff is fed from a separate CU fed from tails from the isolator and all lighting and alarms etc are on the unprotected busbar on the main CU. All sockets have been checked for damaged cabling / loose wires. Should I PAT test everything in the house as my next course of action? Is there a way of testing the RCD device itself without buying a new one? (apart from its own test button of course, which works - I just wondered if it could be oversensitive) Its the usual 30mA trip and 80A I think, a Crabtree. Theyre not known for such problems are they?
P.S. I have also checked sockets for damp / moisture as I know that can cause trips but theyre all bone dry and like I said, none of the protected supply goes outside at all

 

[huwcecilmorgan]

I extracted the following from a forum on the IEE site. It was in reply to a question such as you have posed, viz., how to test a suspect RCD. I Hope it helps!

"With circuit isolated, disconnect any current using equipment from circuit.
Disconnect outgoing cables from RCD and test insulation resistance between Phase/E and N/E to establish if it is the circuit that's causing the problem. If so, by splitting the circuit at a convenient midway point, testing again, and repeating the dividing of the circuit, you can narrow down the location of the fault and replace the cable.

If no problems are found on the circuit, then reconnect circuit, re-energise RCD and carry out RCD testing for proving.

The current using equipment would then need to be tested to establish if it is a fault on the equipment, assuming the circuit and RCD are fine. Note some electronic equipment produce high protective conductor currents and these can trip RCD's"