help with RCD

[me29]

Evening, need a bit of help / suggestions on this fault.
Got called to a house which keeps having issues with the main switch MK 63A RCD tripping.

There is a kitchen circuit with only 4 sockets on it. One has freezer and cooker plugged into it, another a fridge and 2 above counter. The cooker works fine 9/10 but occassionally trips when switched on by the cooker controls. The issue is that only the RCD trips (not the breaker) and not all the time.
I had the kettle, toaster, microwave all on and it was fine, then turned them all off and just turned on the kettle and the RCD tripped, then was fine and tripped when toaster was turned on. There seems to be no issue with the appliances as all work fine on other circuits.
I gave the RCD a auto test and ramp test and the RCD would no trip at all and not even at 5x. So i changed the main RCD, but when i powered it back up the main RCD would not reset at all. I turned off all the breakers and it reset and stayed on when i turned on all the breakers back on.
I tried other appliances and the issue was no different, (it would just trip randomly but only when appliances were plugged in), only this time every time the RCD tripped it would only reset if i turned off all the breakers and then reset and turned them back on. The old RCD would just reset after each trip.

The setup is main fuse into meter into stand alone RCD into consumer unit.

Any suugestions would be greatly appreciated.
Sorry if the explanation is a bit confusing.

 

[RF Lighting]

What were the insulation resistance readings like for the circuit?

Did you test the RCD in isolation?

 

[Lectrician]

My money, as always, is on a N - E fault.

Why do people start with a ramp test to the RCD's when trying to find nuisance tripping? Get out yer megger! It is more than likely going to be an insulation issue!

 

[RF Lighting]

I'll take that bet and say appliance fault.

Your megger will tell you within moments whether it is a fixed wiring or an appliance fault you are trying to trace.

 

[hobbsie]

have you tried dropping sockets off to see of any sign's of damage, but then again u meeger it you know if theres a fualt there, remember to disconnect rcd if you megger from socket side not fuseboard side.

 

[GaryMo]

Why do people start with a ramp test to the RCD's when trying to find nuisance tripping? Get out yer megger!

He might have got his Megger out to do an RCD ramp test!

 

[me29]

Thanks for your replies, i am pretty sure it is not an appliance fault as it does not seem to matter which appliance is switched on, it has been the toaster, the kettle and the microwave which have all tripped the RCD. And all work fine on other circuits.
I have not tested the insulation as the circuit is fairly new, although i will test this to see the results and let you know.
Is the general consensus that there is a wiring fault?
Would a screw / nail be the cause which is my next search?

Thanks

 

[Spark123]

It could be

 

[hobbsie]

could be screw depends on your readings when you megger if you get a dodgy reading i would first drop the faceplates off to ensure no cables have been caught or ask client what works they have had dne have they put ang pictures up etc etc.

 

[Lectrician]

could be screw depends on your readings when you megger if you get a dodgy reading i would first drop the faceplates off to ensure no cables have been caught or ask client what works they have had dne have they put ang pictures up etc etc.

Why do you keep going on about dropping fronts off? That is time consuming and only looks at a tiny portion of the circuit. Get your megger out, test for insulation, interpret the results. Once you know what you are looking for THEN you start to remove fronts and split the circuit down to locate the cause.

Don't forget to isolate everything on the circuit before testing - Combi boilers, timeclocks, tv boosters etc. Unplug everything. Don't just turn off.

 

[ban-all-sheds]

have you tried dropping sockets off to see of any sign's of damage, but then again u meeger it you know if theres a fualt there, remember to disconnect rcd if you megger from socket side not fuseboard side.

http://www.google.com/support/toolbar/bin/answer.py?hl=en&answer=32703

 

[hobbsie]

i never said dropping fronts off 1st if u read i said after u have meggerd the circuit then try dropping them off, dunno y people get so upset when other people try helping.

 

[ban-all-sheds]

i never said dropping fronts off 1st if u read i said after u have meggerd the circuit then try dropping them off, dunno y people get so upset when other people try helping.

http://www.google.com/support/toolbar/bin/answer.py?hl=en&answer=32703

 

[hobbsie]