RCD Tripping

[willy.d]

Help needed please!

RCD supplying three ring final circuits. Everything unpluged, turned off etc. Reset all MCB's and RCD. Then, on any of the circuits apply a load (different loads, kettle, tv etc) and the RCD trips?

This is an existing install working fine untill now. I'm thinking the RCD is defective?. Only had an hour there for a quick poke around and back tomorrow with my megger for a closer look but some pointers would be very welcome!

Thank you

 

[ebow72]

Have you just installed a 17th Edition CCU?

 

[High Tower]

Check no interconnections, crossed neutrals, then ramp the RCD

 

[willy.d]

Thank you.

No, nothing installed, called by the client because of the fault. There have been no problems with the existing installation up to now at all (so they tell me anyway)

 

[FatBob]

Swap the RCD to eliminate

Measure earth leakage current and see where it is coming from.

 

[High Tower]

I think you will probably find that the RCD is faulty, when you ramp it it will probably top out at less than 15mA, i doubt that its a particular item of leakage as shutting the circuits down would have shown a difference

 

[Octopus]

If the RCD tests look OK maybe you should swap each of the circuits, in turn to the other RCD to see if any one is at fault.

 

[ebow72]

This is why I value coming to this site. Everyone goes out of their
Way to help. I look forward to the day I can offer advice as detailed
as the ones on this site. Such a source of information I can refer too,when I need too. Thanks!

 

[voltage]

This is why I value coming to this site. Everyone goes out of their
Way to help. I look forward to the day I can offer advice as detailed
as the ones on this site. Such a source of information I can refer too,when I need too. Thanks!

+ 1 Totally agree some great people on this site with a wealth of knowledge.

 

[BlueToBits]

Willy.
I would suspect a low IR between neutral and earth. Plugging in an appliance load would then cause leakage down the earth. No load =no leakage and that's probably why the RCD does not trip off-load.
Typical common causes are compression of cables in accessory back boxes, Cables trapped behind accessory fixing screws, Waterlogged jointbox under sub-floors, Fixings for pictures, shelves etc in walls above sockets, cables crushed in joist notches under floor boards, cables melted on heating pipes, Waterlogged outside lights fed from single pole switched spur connection units, Damp garage or outside wiring accessories.
One of my most disgusting causes was a semi-decomposed, half cooked mouse stuck in a joist hole after electrocuting itself knawing through PVC insulation.

 

[telectrix]

N/E probable cause. only find out with testing circuits.

 

[frustrated]

Don't forget to check the insulation of the sockets themselves, old ones develop a low insulation resistance over time especially if the walls have suffered from damp.
Home this helps.

 

[andekoch]

Changed a light fitting the other day. Mob off and tested no power. Next thing rcd trips and takes out computers.
Problem was I touched neutral and earth at same time so rcd saw a leakage. Pain in the but and embarrassing when told the customer he could carry on working, fortunately I told him to save his work just in case.

 

[Gazzdid]

Willy I was called to a very similar fault a few weeks ago and after asking all the usual questions I.e have you hung any pictures, had any work done, drilled any Walls I was told no, I explained that RCD's don't tend to suddenly go faulty in my experience and it's possibly something like an old appliance gone faulty I.e cooker. I got there, investigated and ate my words the rcd had indeed gone faulty! I was able to easily verify this before buying a new one as it was a dual RCD split load board so I swapped the RCD's round.
Anyway if your sure everything is unplugged (don't forget hidden treasures and spurs off the ring like boiler spur, extractor hood etc) then prob best to try replacing the RCD. if the new one trips then it's almost definatly a neutral-earth fault. Then
It's a case if finding out which circuit the fault is on, then when you've narrowed it down to a circuit you can investigate that circuit to find what's causing it. Switch/socket being screwed back and one of the pins damaging a neutral cable is a common one. This is more common in single boxes as obviously there's less room in there = more chance of a cable sitting behind a pin. Hope this helps don't forget to let us know how you get on!

 

[FatBob]

Remembering how the thread started
RCD supplying three ring final circuits. Everything unpluged, turned off etc. Reset all MCB's and RCD. Then, on any of the circuits apply a load (different loads, kettle, tv etc) and the RCD trips?

Because the fault manifests its self on any of the 3 circuits, it most likely will be the RCD. If not, I cannot imagine the scenario which would involve those three ring circuits.

 

[nickblake]

It could if a neutral to earth fault has developed , think about it you have 3 ring circuits everything un plugged , plug in an appliance on any of the 3 circuits and the RCD trips , so look at the common factor here neutral and earth , all the neutrals and earths are linked neutrals on their bar earth on their bar if you have a fault on any of those three circuits N to E then when the load is applied the fault is picked up via the neutral s and earths of all circuits ,
What is needed to be done is to disconnect each circuit from the board neutrals and lives then reset the RCD and try again till you eliminate the circuit that causing the problem or simply test the IR on each of the circuits L E NE LN , even doing a continuity test on these may pic up a fault , and do as the other guys have said if all is well check the RCD tripping time , ramp test , etc but i would place my hard earned cash on a NE fault on one of those circuits , as i have said test each circuit individually even if its removeing the Neutrals from each circuit till the RCD stops tripping then test the circuit thats causing the fault

 

[FatBob]

Thanks Nick, you are of course correct

 

[willy.d]

##Update##
Ok, so went back yesterday. First thing, I ramp tested the RCD, which tested fine (so no quick fix unfortunately). I then indeed found low IR between Neutral and earth on one of the three circuits.
Next, I dropped all the socket plates off the wall (client had recently replaced many with stainless flat plate sockets, and some were very 'busy' behind) though nothing obvious in terms of pinched cables etc found. I then re-tested and the fault had disappeared, sockets back on the wall one by one, MCB's & RCD reset, power back on, happy client. I would have been happier if I knew which one caused the fault though.....

Many thanks again for all your contributions, very much appreciated.

 

[nickblake]

Willy i bet you moved the fault you said you had a socket that was busy , what could have happend is the faceplate screw caught the neutral causing a short , simple fault ,and what would have happend is when you removed the faceplate it removed the fault when you put the face plate back it missed all the cables hence no more fault

 

[RogertheBodger]

Isn't Electric wonderful?

Good thread, good help, good result.