RCCB tripping - help needed..

[mattydubster]

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

 

[Evans Electric]

Where is the Neutral for the AC  unit connected ?

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

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 ?  

 

[mattydubster]

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...

 

[Murdoch]

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 ...

 

[mattydubster]

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.

 

[Sidewinder]

TT or TN system?

 

[mattydubster]

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

 

[Murdoch]

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

 

[Evans Electric]

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? 

 

[mattydubster]

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. 

 

[Murdoch]

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

 

[mattydubster]

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.

 

[Murdoch]

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.

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

 

[Evans Electric]

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. 

Ah  thats an original Starbreaker  MK 1  then . 

 

[Doc Hudson]

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.

 

[NozSpark]

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

 

[mattydubster]

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...

 

[mattydubster]

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

 

[Murdoch]

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

 

[kerching]

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

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!)