Two RCBOs tripping together, no cross-connection. Need help…

[weevilward]

Are you sure it's not a ring on two RCBO s.

100% sure. No continuity between the separate circuits. Also different cable sizes (6mm, 2.5mm. 1.5mm)

 

[weevilward]

If it worked before, then it’s a new fault developed. Wrongly connected would show up straight away.

anything new been added to any of the circuits?

Smart meter? Long shot, but it’s happened before.

Individually, we might think an outside light or socket taking in water, or rodent damage… but for all 3 at same time?
What is the likelihood of rodents? Near fields?

Rodents - Very unlikely. Urban property. Kept immaculate. Recently gutted and refurbed (2 hrs ago ish)

 

[weevilward]

Have you checked for crossed over neutrals?
Or one I had some years ago where there was an unterminated cable waaaay up high for an outside light that caused intermittent rcd issues, only tripped when it reined from a certain direction caused the end of the cable to get damp. took ages to trace

Checked for crossed neutrals, it’s not that. Can’t rule out an unterminated cable somewhere but the tripping doesn’t link with rain. And that would only take out the one RCBO.

 

[weevilward]

do a 500 volt insulation test make sure nothing is plugged in or switched to on like the boiler control

Ha, this house is full of tech! I wanted 500v test but there are smart switches, smart appliances, wall Mount TVs… I bottled it.
Surely the 250v test would reveal a problem ? All readings were >99M

 

[littlespark]

Rodents - Very unlikely. Urban property. Kept immaculate. Recently gutted and refurbed (2 hrs ago ish)

2 hours! That’s really recent!

I know, you meant 2 yrs

 

[Intoelectrics]

If the circuits are completely separated as you claim, yet both trip simultaneously then there has to be a common factor that you have not inspected. If the lights do indeed dim just before the RCBO's trip then this may point to an intermittent imbalance mains issue. Have you checked the incomer and associated connections?

 

[yorkieone]

If it worked before, then it’s a new fault developed. Wrongly connected would show up straight away.

anything new been added to any of the circuits?

Smart meter? Long shot, but it’s happened before.

Individually, we might think an outside light or socket taking in water, or rodent damage… but for all 3 at same time?
What is the likelihood of rodents? Near fields?

house mice don't need fields lol

 

[weevilward]

If the circuits are completely separated as you claim, yet both trip simultaneously then there has to be a common factor that you have not inspected. If the lights do indeed dim just before the RCBO's trip then this may point to an intermittent imbalance mains issue. Have you checked the incomer and associated connections?

I think you’re onto something there – I got the customer to ask around in the street last night, and one of his neighbours two doors down had exactly the same issue at exactly the same time, so I am thinking there is a problem with the supply in The Street. I’ve got him to raise the issue with the DNO. The only bit I have not checked is the switch fuse and sub main from my customers meter to his CU.

 

[timhoward]

All fine for a week or so.

This is possibly the biggest clue.
As well as pursuing investigating a supply issue....

I'd be thinking whether anything might be out of circuit when testing that is in circuit during the fault.
Do they have any outside lights or anything time clock controlled?
e.g. an outside light switched by a double pole switch on the ring that is also linked to a PIR fed by the lighting circuit with all neutrals joined up, or something similarly special.

I'd also take a look at the boiler wiring if it's on the downstairs sockets circuit as when combi's are fitted all sorts of delightful things are done by our friends who fit them.

Have you tested the cooker itself? A cooker with a slowly developing N-E fault touching a bonded gas pipe behind can be the source of interesting fault chasing as the thing triggering it can be anything that is earthed.

I'd also try to be thinking about whether there is anywhere that the circuits run together. I remember one very obscure fault where cables had been pulled hard and they'd cut into some metal trunking, and vibrations changed the IR from absolutely fine to practically zero.

Best of luck and do report back!

 

[littlespark]

house mice don't need fields lol

Of course they don’t. But there is a higher chance of rodents in an old cottage, on a farm, full of holes than in a new inner town development.

Looks like there’s a common problem for the whole street, so no point guessing now. Lol