RCD tripping when test results are fine

[sparkiedave]

Any ideas on this one folks, ring circuit, couple of sockets on the top floor are spurred, (two individual spurs from from two seperate parts of the ring), insulation resistance of >500 between all conductors, RCD tests at 36ms@ x1, 13.6@x5, all normal so far, no tripping at half current, 27ma on ramp test, can plug stuff in downstairs with no problem, but on the top floor the RCD trips, I tried again with nothing else plugged in to avoid a build up of fault current through several appliances, no difference, disconnected all the other circuits, no difference, swapped circuit onto other RCD, (just through curiosity and just in case of a borrowed neutral somewhere), did the same,
am I being thick here?

Also trips when taking zs readings, but only on top floor!!
tried loosening socket fronts in case of trapped neutrals etc,

 

[RF Lighting]

Have you just fitted the cu by any chance?

Does the RCD hold when nothing at all is plugged in?

What brand is the cu?

Have you got a photo of the cu with all it's bits on show?

 

[Spark123]

What work has been carried out?

 

[sparkiedave]

Hager twin RCD board, (5+5), new CU, main bond gas & water, all tests fine, tried swapping to other RCD thinking that if it was a borrowed neutral it would show up by the neutral in the "borrowed" circuit being on the same RCD, gonna go back tommorow and strip the CU again, with a "fresh head" just can't get over the fact that it's only on the few top floor sockets, gonna take a wandering lead too, narrow things down, the test results showing up ok is causing a problem tho', cos the only thing that seems to show it is applying a load, going to try splitting it into two radials and testing them separately.
Yes, holds when nothing is plugged in, also when plugging in to the same circuit on the lower two floors

 

[RF Lighting]

Sounds like youve got a neutral and earth swapped somewhere. Can't be a short as you've checked that.

Are your neutrals in the rcd the right way round? MK used to be bad for that on the old single split load CUs

 

[ColJack]

stupid questions time... sorry but they have to be asked..

you HAVE checked and varified that these top floor sockets are actually spurs the socket ring and not off the lighting ( top floor sounds like loft conversion and if someone put a socket up there for a tv amp etc it may have been off the lights.. )..

you HAVE taken the fronts off the sockets in question and ALL the sockets in the rooms below to check the wiring is in the right holes? ( a N-E swapped at the end of a radial still reads as ok to earth when disconnected from the board.. if you have plastic / fast fix boxes then no path to earth is seen and the current returns via the earth not the neutral from that socket when something is plugged into it.. ).

 

[sparkiedave]

Yes to all the above, not on lighting circuit, one socket on top floor on ring, two more as spurs

 

[Spark123]

Have you got end to end resistance readings for the ring?
Having one socket on a ring is a bit pointless

 

[Taylortwocities]

It really does sound that you may have a neutral mixed up somewhere so one neutral is swopped with one from another circuit.
Could equally be a live swopped (line/phase for the purists).

I would take a careful look at the conductors and make sure they really go where you want them to.
Then check the continuity of each ring in turn. Best to check the obvious, simple stuff first before getting in too deep.

 

[ColJack]

unplug everything, switch JUST the socket circuit on and then run round EVERY socket in the house with a "socket and see" tester ( or similar ) and I bet you find one or more with the N-E reversed..

 

[Spark123]

I didn't think they could detect N-E reversal?

 

[ColJack]

don't they? i thought mine did.. hmm must look at it monday...

you can still use them though..

disconnect the earth from the board and make sure that the "earth light" on the socket and see is not lit.. if it's the "neutral light" then you know it's swapped.. .

remember of course to reconnect the earth when you're done..

 

[sparkiedave]

Thanks for all the suggestions, went at it with a wandering lead and found a N/E swap on a JB under a floorboard, originally didn't suspect any JB's might be there, just goes to show, can't take owt for granted!

 

[Taylortwocities]

Well done on the Sherlock stuff, Dave!

 

[maltaron]

Spark123, just for info RAPITEST socket tester does: Correct;No Earth;L-N reverse;N fault;L-E reverse;L/Fuse Fault.