rcbo's tripping randomly

[fylthespider]

Hi all, new to the forum and got a cracker for you.
story so far...
100 year old mid terrace house. was asked to inspect/test 30year old wiring. turns out ok so recommend new cu, all rcbo's and DNO to install PME.
shower, kitchen rm, house rm, lights up and down.
all goes well, everybodys happy.

about a month later i get a call.... the dreaded nuisance tripping ARRGH!!!!

so, perform another full test and pat test all appliances and again, all good. insulation test good, no shared/crossed neutrals/earths etc.
so leave with instructions to keep a log of tripping.

after about a month i went back to find completely random tripping, any and all rcbo's in any combination at random times etc.

tested again and still no joy.

tried substituting mcb's in various combinations, isolated each circuit one at a time, all the usual tricks.
even had shower rcbo trip when outgoing cable isolated ?????.

starting to loose the will on this one.

any ideas ?
only thing i can think of is transients on the supply as its the only thing common to all rcbo's, but of coarse the DNO dont want to know.

I rate myself as a good sparky, been part p since it started, 2391 since 2001, Ive never been stumped before but this got me tearing my hair out (and i havnt got that much left)
only good thing is the customer is a friend so she's not dancing on my neck, but i got to figure this out cos its been going on for months.

any thoughts appreciated cos i'm ready to give up and get a job at tesco's.

cheers phil.

 

[fylthespider]

not done a ramp test but they do hold at 15 and 20 ma so dont think its the rcbos. had new incoming cable installed and all connections look ok and are tight.
been back to basics as yes, its often the simple stuff that gets you.

thing thats got me foxed is how can an rcbo trip when theres nothing connected to it???? ( the shower circuit)
Only thing its connected to is the incoming line and neutral. so, no current able to flow, yet still it trips.
which makes me think maybe some strange transient on the supply. dont know and think its time for a pint.

Thanks for the ideas tho.

cheers phil

 

[Richard Burns]

I do not know the answer here so do not take this as necessarily correct, but from reading other threads on here there is a suggestion that some RCBOs can trip because of a large current flowing in an adjacent MCB/RCBO from induced current in the coil.
I would have thought that this would be addressed by manufacturers but you never know.
If possible it may be worth removing every other RCBO across the board and try the remaining circuits and see if they trip.
This would at least address the disconnected RCBOs tripping.
Might be worth a try.
Tell me to shut up if I am talking rubbish!

 

[topquark]

not done a ramp test but they do hold at 15 and 20 ma so dont think its the rcbos. had new incoming cable installed and all connections look ok and are tight.
been back to basics as yes, its often the simple stuff that gets you.

thing thats got me foxed is how can an rcbo trip when theres nothing connected to it???? ( the shower circuit)
Only thing its connected to is the incoming line and neutral. so, no current able to flow, yet still it trips.
which makes me think maybe some strange transient on the supply. dont know and think its time for a pint.

Thanks for the ideas tho.

cheers phil

Any chance of proving the RCBO at home (in a temp supply to a temporary board)?

 

[fylthespider]

already swapped the rcbo's from another job where they were fine, so im 99.9% sure its not the breakers. nice idea tho.

hmmm, magnetic cross talk. now that sounds possible, worth a try. might even try steel screen between the rcbos.
its another straw to clutch at lol.

cheers. will let you know.

 

[topquark]

already swapped the rcbo's from another job where they were fine, so im 99.9% sure its not the breakers. nice idea tho.

hmmm, magnetic cross talk. now that sounds possible, worth a try. might even try steel screen between the rcbos.
its another straw to clutch at lol.

cheers. will let you know.

What I was thinking is, if you can prove it in a temporary supply and it's fine, then you could take the temp supply to the premises and connect to the meter and use it to prove whether it's the DNO side or the house holders side (rather than having to take the sherlock approach and prove it's not the house side).

 

[Richard Burns]

Make sure you do not drop the screen onto the busbar!

 

[Knobhead]

I would be very reluctant to put shields in. Anything goes wrong and your necks on the line for deviation of the manufacturers design.
Did you try recording the N-E voltage?