Strange RCD tripping scenario

[Rkidcd]

Any suggestions as to possible causes of RCD tripping in the below domestic scenario would be helpful:

Origin:
single phase TT supply into an external cabinet housing cutout and our installed 100A 100mA Type S 2Pole RCD. Earth Rod has also been sunk here.

House:
3c SWA supply to house from above to a TP&N board with single phasing kit. All circuits from this board are on 30mA RCBO’s.

We have been turning circuits on once tested over the course of last 2 weeks and the board has sat with all but 2 circuits on for nearly a week without issue but all of a sudden today we have had nuisance tripping at the RCD in the cabinet.

We have insulation tested the sub-mains again as a result of this and everything seems fine. So at the minute I’m unable to find the cause, none of the RCBO’s have tripped at any point.

Any suggestions?

 

[TJ Anderson]

Yes you need to gain access to the elements terminals and isolate them first.

Or.....stop wasting time and suss it out quick.

Put your earth leakage clamp around L&N at stare at screen whilst someone else is running around turning stuff on. You will pin point it very soon, If you don't have one buy one. They are cheap.

 

[weevilward]

I agree with the earth leakage clamp approach to see where the leakage is coming from. Could be cumulative across that many final circuits, without tripping individual RCBOs. I've seen 'normal' leakage on a 7-way board cause a 30mA incomer to nuisance trip, so it's feasible that all your final circuits could do the same to a 100mA. Unlikely, but feasible.

One last thought - I fitted a Lewden (aka Control Gear) 100A 100mA type S a few years ago and kept getting called back for nuisance tripping, when the 30mA downstream never batted an eyelid. All IR tests >99M. I replaced it with an MK type S and never been a problem since. Funnily enough I don't use Lewden RCDs anymore!