Help required with a Domestic Ring Final fault

[claret73]

Evening Chaps...
Had a 2nd call out today to a relative (let's say they're a client) for a fault on the downstairs Sockets.

In a nutshell the downstairs sockets are tripping the RCD. Been there nearly all day & cannot fathom out what might be the cause?

So, the board is a 16th MCB Only. There is a 63A 30mA RCD between the meter & the Main Switch which protects all circuits. It is only the downstairs sockets tripping the RCD. The circuit concerned is 2x Rings into the MCB. 1 does 3x sockets in the front room, the 2nd does the dining room, kitchen & outside socket which runs out an armoured to a 2nd weatherproof box.

I disconnected the outside socket from the spurred cable in the dining room. Switched back on, still trips. Split out the circuits having done a continuity to find the 2x legs. Found out which was the tripping circuit & ignored the other. All Appliances were disconnected.

Tests I performed:
End to end to check continuity on Line & CPC
Line to CPC Insulation resistance
Ramp Test on the RCD

The board is boxed into a corner & the Neutral bar is very difficult to get access to, so was not too happy with taking out the Neutral connections.
However I reconnected the circuit & did L-CPC, N-CPC & L-N Insulation resistance.

The meter read >99.9 (Megger MFT) & so all Cables were deemed OK?
Took all sockets concerend off & checked for nicks/damage & tightness of the terminals. All OK...

So I don't know what the problem is? It's still tripping. I put the other Ring back in to give them some power.

Any ideas what I may have glaringly missed??? Any help greatly appreciated...
Thanks
Andy

 

[topquark]

Do you have a clamp meter at your disposal? If so I'd be tempted to try and confirm your suspicion about the RFC using that (and eliminate the other circuits as well if possible). As the RCD is before all of the circuits you are probably dealing with a combination of leakage faults. You have a ramp test from the RCD so you'll know what it's tripping threashold is.

 

[truckster]

Gotta get them neutrals out.

 

[claret73]

Thanks again Lads! Going back on Friday...Not got a clamp meter to hand...(Ahem!)
Got a call today to go look at another fault job, tripping MCB on a lighting circuit!! Ohhhh Nooooo! It's really not my forté...

 

[topquark]

Faults like these will become more and more prevalent, anyone local to you that could lend you one?

 

[Big n Daft]

It sounds to me like a nuetral. I'd IR test the whole shooting match at the DB then disconnect that outside pir/light thingy. If that doesnt sort it then its testing all the circuits in a methodical manner at the DB.
Best of luck.

 

[i=p/u]

a clamp meter for 99 squid comes in handy, but hey all circuits off and turn on 1 by 1,then test each circuit n-e 1 by 1 , and try new rcd after that bill them say they need new house

 

[coomers]

I was chasing my tail a few years ago everything un plugged except a brand new 1 mtr 3 way extension turned out to be a surge protector in lead realised after taking last socket off!

 

[claret73]

Well Guys, went back to this yesterday. Decided to dead test at the board again in case I'd missed something. With it all connected & dead proceeded with IR Tests again on each circuit. All Perfect...So, bit the bullet & 1 by 1 started disconnecting the sockets. Narrowed it down to a spur from a JB which was feeding the dishwasher. This cable was inaccessible as it was jointed above the ceiling. Tracing it back I could just see it clipped behind the sink unit level with the floor on the outside wall. Either it's a fault at the JB which I can't get to or being on the outside wall is susceptible to the damp conditions??? Anyhow, with that out of circuit I managed to power it back up...

Just really glad it was a non costly fault for them in the end...Pain in the A**E! Though...
So Thanks for all the help here for everybody who replied...