RCD tripping......yes, ANOTHER one!

[polo1]

Guys, what, if anything, have I missed?

Problem has apparently existed for over a year. Random tripping, no obvious pattern, but can be at 6am when only a fridge freezer will be operating (the f/f has been disconnected and still tripping occurs). It can go days/weeks and be fine with no tripping.

16th Edition board, RCD covering kitchen sockets, oven and shower. Ramp testing (25ma), tripping times(22.6 & 9.9) all good & individual circuit insulation res figures all good. All connections in the board, at the oven, shower and sockets re-made and checked. No sign of any water in the shower casing. Standing earth leakage with various appliances on, around 3-4ma.

Supply is TNS, Ze 0.19.

Not sure if this is a red herring, but the head is a very old cast iron three phase with ceramic fuses and solid neutral link - 1 phase serving the property I'm checking, second not used and third serving the property next door. The two properties are sharing the main earth (sweated lead). I'm thinking that an issue in the adjacent property may be a factor, but how/what do I check (family in this property are on hols so I've not been able to speak to them)?

Any thoughts/suggestions for further action and/or investigation gratefully received!

Regards.

 

[telectrix]

think you may be on the right track here, going by your test results. looks like a tricky one.

 

[Octopus]

Change the RCD?

 

[telectrix]

Change the RCD?

thought that after i posted. might be a good idea for the sake of £20.

 

[Octopus]

Had this a while back with a 100mA - tripping randomly over weeks - on one visit the RCD didn't want to trip on test - so customer agreed to replacing it and its been OK since.

 

[Guest111]

it's not a central heating pump is it?

 

[telectrix]

it's not a central heating pump is it?

nope, it's a RCD.

 

[captainkev]

you may have a faulty mcb, I have had this recently on a couple of static caravan pitch boxes containing a 25A RCD and 16A MCB, where there was random tripping, all tests like ops were ok, so changed RCD and was still doing it, changed mcb and it has resolved the issue.

 

[topquark]

I had this with a central heating three port valve. Very small leak in actuator head would take the trip out. By the time I cheched it, the leak had dried up!

 

[polo1]

Thanks for the suggestions guys. Change the RCD - yes will consider this, I just don't like swapping out things "just in case". Interesting point about a faulty mcb- didn't appreciate this could cause an RCD to trip.
The central heating & pump is fed from the non-RCD side.
Any thoughts/views about the shared main earth -can this cause a problem and if so, how?
Regards,
George

 

[Marvo]

I don't see how anything on the supply side of the RCD could cause it to trip apart from maybe a harmonic issue.....but saying that unless it was full of water I don't see how a faulty MCB could cause it to trip either so what the hell do I know?.

 

[davesparks]

One of the first jobs I ever did as a self employed spark was a new CU for my uncle.

All went beautifully, simple 6way board with 4 RCBO and 2 spares.

A couple of weeks later the downstairs lights tripped overnight. Strange as he is the kind of person who switches off everything when not in use (retired accountant)
I found nothing wrong and it reset ok.

This happened every other week or so for a while, so I dusted off my old mentor and dragged him round there to see what I'd missed. He could find nothing wrong.

So this silliness continued until a couple of months later when the DNO shut the road, dug it up, fettle something or other and put the Tarmac back. After that it never tripped again, I still haven't got an answer to what was happening/why it stopped happening. But I have since tested the RCBO again and it still trips in the exact same times as when I fitted it.

 

[Guest77]

Fault on a circuit perhaps that has been missed, maybe the oven, there could be a fault with the clock on the oven, even if the oven is not running the digi clock still runs, could be the neon in the shower pull cord switch flickering causing an inbalance, or just the RCD itself.

 

[Richard Burns]

Possibly a damaged cable under floorboards, if trapped it requires someone to step on that floorboard for the RCD to trip, if it is being eaten by mice it requires the mouse to come back (or if the current was too much, to decay a little more). If the outside light fills with water then it needs to have been raining. However the first one would be more likely if the IR results are all OK.
But this is only if you have tried everything posted before first.

 

[southsea1201]

Possibly a damaged cable under floorboards, if trapped it requires someone to step on that floorboard for the RCD to trip, if it is being eaten by mice it requires the mouse to come back (or if the current was too much, to decay a little more). If the outside light fills with water then it needs to have been raining. However the first one would be more likely if the IR results are all OK.
But this is only if you have tried everything posted before first.

I've had exactly this problem. RCD kept tripping despite excellent tests results. Was so embarrassed as I had put in 2 new boards for client in a large house and this saga went on for 6 months or so. I never found the solution but a young female guest did. She discovered that everytime she stood on a certain floorboard in the little used top floor bathroom the lights went out.
Took floor board up to find cable held by a clout in a notch. Over the years the sheath had opened and gently rubbed away some insulation from the neutral. Standing on the board caused contact between these conductors!!! Reset the RCD and all was well until the next time. Testing gave this circuit >200Mohms...so the chances of me finding it was remote. Pure luck that this young lady put 2 and 2 together!!!

 

[polo1]

Once again, thanks for all the feedback/suggestions. Good call on the oven clock - didn't consider that!.
The walking over floorboards I have considered, even tried a walk-around whilst I was there to no avail! Like others, I just hate not being able to get to the bottom of this type of problem.
I'll keep you posted if I manage to resolve it.
Regards