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?

 

[Octopus]

I’ve thought about trying a 300mA - there’s over 20 Rcbo’s

Wrong approach imho

Changing parts instead of investigating and testing is rarely a successful approach...

 

[Strima]

Any test results?

I’ve thought about trying a 300mA - there’s over 20 Rcbo’s

Until you confirm the cause of the tripping there's no point swapping parts out in a vain hope it might work.

 

[Dave OCD]

You'd have to see if the disconnection times could be met based on Ze, have you I.R tested every circuit L-E and N-E ?

 

[Deleted member 105166]

You say it's been ok for 2 weeks, then the nuisance tripping has started. You also mention the cabinet with the 100mA RCD is external. Is this tripping happening at the same time of day/night? Have the atmospheric conditions changed? I'm wondering if the cause may be condensation related.

 

[Rkidcd]

You say it's been ok for 2 weeks, then the nuisance tripping has started. You also mention the cabinet with the 100mA RCD is external. Is this tripping happening at the same time of day/night? Have the atmospheric conditions changed? I'm wondering if the cause may be condensation related.

It could be something as simple as this Tony

 

[Rkidcd]

You'd have to see if the disconnection times could be met based on Ze, have you I.R tested every circuit L-E and N-E ?

Initial verified them all yes - not since tripping though as only happened this afternoon - gonna have to delve into it tomorrow

 

[marconi]

Re OP's #1: Which two final circuits?

You mention a TN&N board converted to SP. How many final circuits are there?

Suggestions:

Test the IR of electric ovens, hot wire element electric hobs, washing machines and tumble dryers and dishwashers, immersion heater.

Take a close look inside any outside light fittings and switches.

Pour 2-3 gallons of water around the earth electrode.

Check tightness and solidity of all connections.

Carefully remove sockets and look behind them for securing screws close to live conductors.

 

[Octopus]

^^ AND ramp test the RCD.

 

[Wilko]

Hi - Agree Murdoch and Strima I would not change out 100mA S type without a test result showing it's at fault. As others have said, it's quite likely a N-E fault exists. If it's in a final circuit unfortunately the single pole rcbo won't remove it from play.
Some questions and thoughts -
What parts of this installation are new?
Any patterns to the tripping?
What is the site Ze? Locate the rod and confirm the impedance of connection from rod to MET and CU earth bar with wander lead. Repair if required.
How long is the underground SWA linking cabinet to CU? What protects this cable? What were the cable IR results please? Carefully check all connections from cutout to CU and the polarity.
Happy Hunting

 

[Octopus]

^^ and don't forget to check the IR between L - SWA and N - SWA

 

[shaun1]

You could ramp test the RCD first with everything off and then with everything on to get an idea of leakage current on the installation.

 

[Deleted account]

I vote it could be accumulation of I.R. Do a bulk I.R test across all circuits.

 

[sam400]

Suggestions:

Test the IR of electric ovens, hot wire element electric hobs, washing machines and tumble dryers and dishwashers, immersion heater.

Out of interest how do you test the element in the appliance? Do you have to dis-assemble?

Cheers

 

[marconi]

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

 

[marconi]

Beko Tumble Dryer Heater Element | eSpares - https://www.espares.co.uk/product/es1435433/tumble-dryer-heater-element?pageNumber=1&PartTypeId=1584&ManufacturerId=927

espares and similar companies do short video clips showing how to gain access to heater elements and swap them. The link above is for a BOSCH tumble dryer for example. ( I know the link says BEKO but scroll down to see its a BOSCH machine).