RCCB tripping - help needed..

Hi everybody.  First time post so please be gentle...

I have just been to a house after a friend of mine has appeared to uncover a fault with a Crabtree 3 module RCD on a split load board that keeps tripping out when he plugs in a small air con unit that he has just installed. But with the air con unit unplugged it seems to have awoken an issue with the RCD

I have been and done an IR test (with air con still unplugged) on all circuits and the lowest reading between N & E is about 43MΩ so shouldn't give me a problem with nuisance tripping.  The RCD behaves quite erratically when all circuits are on, i.e. it sometimes works well on a ramp test and sometimes trips out on half rated.  So you'd think that I just change the RCD and everything would be ok.  I ordered one of Ebay and fitted it....and got the same problem.  Now I know that sometimes you can get dodgy stuff from Ebay, and you take your chances and all that, and I have now ordered a brand new one from my wholesalers but have 2 questions - 

1. The IR reading between the neutral bar of the non RCD side and the neutral bar of the RCD side is about 80MΩ.  Surely this wouldn't make a difference?

2.  Are these older Crabtree ones really that good?  Whenever I have any RCD issues it always seems to be flippin' Crabtree.

Also, I could get the RCD to trip out just by passing a  screwdriver tip by the front of the housing but without actually touching it. Obviously the coil wasn't liking metal close by.

Any help or opinions would be greatly appreciated.  Thanks

Where is the Neutral for the AC  unit connected ?

mattydubster said:

Are these older Crabtree ones really that good?  Whenever I have any RCD issues it always seems to be flippin' Crabtree.

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Hard to say , if its 1st generation Starbreaker then at least it was made in UK , I'd guess. 

Have you done any "Process of elimination "  with the circuits  in respect to the behaviour of the RCD ?  

Thanks for the reply.  

The AC unit was just plugged in, so was on a socket circuit supplied by the RCD, so all my tests have been done with the AC unplugged.

As for a process of circuit elimination, it has been a bit interesting - there are 8 circuits protected by the RCD, and when 7 of them are on, and it can be any 7, there is a good chance of the RCD passing a ramp test.  But when I put the 8th one on, which again could be any of them, it seems to play up a lot more. And it does also tend to trip out even when I do a Zs reading on the socket closest to the board.

I thought I was becoming a bit of a master when it comes to fault finding but when you have 2 rcd's in a row that won't work then you start to think it's something else...

Have you got an earth leakage clamp meter?

have you ramp tested the RCD?

if you have replaced the rcd, the the issue must be within the property ...

No unfortunately I don't have an earth leakage clamp meter.  and tbh thinking about it now I didn't actually do a ramp test, just a full RCD test - I will do a ramp test when I go back there after the new RCD arrives.  I think the biggest head scratching part is the fact that there's no fault on any particular circuit that's the culprit when I do my IR testing.  That's why I started to wonder whether the 80-90MΩ link between the protected & non protected neutral could be an issue.

TT or TN system?

TN-C-S.  The property was built in the late 80's so it's not exactly ancient and pretty much unmolested too.

Neutral fault on non protected side ................

So just to recap the history of this problem is :- 

1)   No problem for some years , RCDs not tripping .

2)   Your mate installs an AirCon unit and RCD  throws a wobbly . 

3)  Now even without the airCon  it still plays up . 

The board dates from the late 1980s  so it would be what I call  a MK 2  Crabtree  split load  (ie Modern) .

Did your mate mess with any of the neutrals  in the board? 

Yeah I did wonder about that but there are no real faults , I think 60 MΩ or so to earth.



The offending RCD above.

No he didn't mess with anything, he just plugged it in.

I think I can't do too much until I try the brand spanking new replacement one.  Along with a proper ramp test. 

welcome to the real world of sparking ........................ investing in an earth clamp meter is worth doing ................ then you can see whats going on.

Yeah I've been looking on t'interweb for one.  I've been a sparky for 28 years and seem to have managed without one, however I'm starting to see the benefits of having one even after all this time.  I will report back in a couple of days when I go back.

mattydubster said:

Yeah I've been looking on t'interweb for one.  I've been a sparky for 28 years and seem to have managed without one, however I'm starting to see the benefits of having one even after all this time.  I will report back in a couple of days when I go back.

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I would ramp test the RCD first ...........

mattydubster said:

Yeah I did wonder about that but there are no real faults , I think 60 MΩ or so to earth.

View attachment 10034

The offending RCD above.

No he didn't mess with anything, he just plugged it in.

I think I can't do too much until I try the brand spanking new replacement one.  Along with a proper ramp test. 

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Ah  thats an original Starbreaker  MK 1  then . 

You only mention IR tests N->E what about Live->Earth. Have you tested the RCD with no loads connected? What about cumulative natural leakage issues if multiple circuits are via same RCD? 

Doc H.

I think, as Doc says, that you have cumulative leakage......

I've had random RCD tripping before and changed the board for a RCBO one and it never tripped again - tested all circuits and none of them were faulty, some not ideal, but nowhere near low enough to be a fault

Yeah I thought there would be a good chance of a culmulative fault current.  When the new RCD arrives I will go back and start with a ramp test.  I have also located an earth leakage clamp meter I can borrow. I will ramp test with every thing plugged in and then with the circuits all unplugged, then also with all disconnected completely from the RCD.  Then we'll see...

Sorted!  

Ok, so I did a ramp test with everything disconnected and it popped out at 25mA then added the circuits one by one and got a leak of only about 3 or 4 mA, that's with all appliances etc connected.  Until.... I turned on the final circuit which is an old immersion heater circuit that supplies a lift -then I could barely get the trip to stay on and couldn't get a ramp test reading either.  Switched off the isolation switch at the lift end and everything was perfect again.  

So I switched everything off and did IR on the lift circuit with the lift isolator switched on.....L/E & N/E were perfect.  I then did a ramp test with just the lift on and it tripped at 19mA.

Did the only thing I could do - put the Lift on a separate RCBO  and then everything was completely fine!

Thanks for your help people

Out of interest, are you going to buy an earth leakage clamp meter now?

Murdoch said:

Out of interest, are you going to buy an earth leakage clamp meter now?

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I bought one years ago.  It paid for itself on its first outing, a faulty free standing oven. I even narrowed it down to the individual element.  It doesn't get used much BUT it will also function as a normal clamp meter , ohm meter and volt meter (well,I think it does as I have not used it in months!)