Rcd tripping problem

[Braz1845]

Hi guys
this is my first post,so please go gentle.
i have recently finished a large domestic installation,which was put into service, after initial verification and all subsequent tests carried out with satisfactory results.
Now unfortunately I wasn't the last trade on site and now I have a problem with Rcd tripping once or twice a day( no set time of day)
I carried out Tests on RCDs results fine.
I tested A ring main circuit IR between L-N,L-E,N-E all clear but when I tested the disconnected Cpcs to neutral and got a reading of 4meg,also a voltage of 49V between the same Cpcs and neutral.

I left the circuit disconnected for 3 weeks (until able to return) and the RCDs have been fine( no tripping)
the installation is new and is TNC-S.

Ive spit the circuit into 2 radials,and get this same IR reading on each leg ( with the ring split at mid point)
Starting to do my head in now and wondered if anyone has any pointers for me before I get totally confused

thanks

 

[MDJ]

What Circuit? Socket outlets by any chance? collective earth fault from plugged in appliances more than likely if the RCD is not faulty and the circuit checks out okay.

 

[Marvo]

A few things; firstly if the circuit was disconnected then ignore the 49v you got, if yu see a weird voltage between wires that makes no sense then confirm it with an analogue voltmeter before you go chasing the problem. Secondly 4 megs N-E wires isn't anywhere near low enough to allow 30mA leakage and cause the RCD to trip, rather connect everything back up, double check you don't have any crossed neutrals in the CU and use an earth leakage clamp meter around the Land N of each circuit to establish exactly what leakage they have.

 

[showerman]

Hi , interesting when you said that you were not the last trade on site .
I have had, on several occasions, a problem caused by the tiler removing
sockets ( fixing screws not connections!) doing his bit then refitting unit
BUT just catching the neutral with the screw and it seems to be ok until
something is switched on and the neutral then comes into play , hope this helps.

 

[SPARTYKUS]

Hi , interesting when you said that you were not the last trade on site .
I have had, on several occasions, a problem caused by the tiler removing
sockets ( fixing screws not connections!) doing his bit then refitting unit
BUT just catching the neutral with the screw and it seems to be ok until
something is switched on and the neutral then comes into play , hope this helps.

yes! Worst faults in the world, little nasty intermittent critters where somethings just quite not touching / touching.

 

[Braz1845]

What Circuit? Socket outlets by any chance? collective earth fault from plugged in appliances more than likely if the RCD is not faulty and the circuit checks out okay.

No appliances connected to circuit,(yes it's a ring main final circuit)
The consumer unit is made up with 2 banks of mcbs ,each bank protected by a Rcd
ive switched the RCDs and found that the same Rcd tripped even though it is now not protecting the suspect circuit,I presume it's either a faulty Rcd(which actually tests out fine,)or I have a neutral fault between my Cpcs of the said circuit and a neutral of another circuit,if you understand me!!

 

[Braz1845]

I know that it shouldn't trip the RCD but on a new installation I would presume IR of infinity,not 4meg

everything seems to point to a neutral fault but can't get my head round the fact than when circuit is disconnected Rcd hods ok
I'm probably guilty of over thinking this one,but I can't help but think why do I have a reading between a disconnected Cpcs and neutral bar at CU (when L,N,E are all disconnected)
thanks for response

 

[Braz1845]

Thanks shower man yes I've had that in the past with painters,but alas it's not that this time.

 

[Braz1845]

Sorry for my ignorance but by land neutral do u mean earth?

 

[overskilled]

The neutral of this circuit is deinfetely connected to the correct neutral bar?
Continuity check out ok?

 

[Braz1845]

The neutral of this circuit is deinfetely connected to the correct neutral bar?
Continuity check out ok?

Yes it is pal,I've got 2 CUs stacked and yes continuity ok on all 3 cables L,N,E

 

[Octopus]

Well..... if there is nothing connected and no appliances in the house then the next step is to remove all the sockets 1 by 1..... however could you be barking up the wrong tree, you have a dual board so what are all the other circuits doing?

 

[Braz1845]

Good point and it's something I've thought about,but the fact is that with this ring circuit disconnected the problem disappears,but like I said earlier it could be a fault between either the cpc of the ring circuit and a neutral of another circuit or vice versa.
how that could happen beggers belief,
im due to return to site in a couple of days,so I think I'm going do IR test between cpcs of ring circuit and neutrals of all other circuits.

 

[moospark]

Sorry for my ignorance but by land neutral do u mean earth?

Mate, I believe he means 'live and neutral'. Space bar trouble.

 

[overskilled]

Other option..."someone" has had sockets off entirely and messed up the terminations?
Line/Neutral wire come loose and touching an earthed back box?
Bad socket outlet?