MCB -> RCBO I'm missing something silly

[timhoward]

Simple job, swap an MCB to an RCBO on a lighting circuit. It's got me totally stumped!

IR test L+N -> E is fine on the circuit. So is Zs. All 2-way switches operated.
Instant trip when changed to RCBO though, and tried two RCBO's
Ramp tested them both, all good. All tests pass on them with no load.

Also (in desperation, shhh!) tried connecting lighting circuit via plugtop / flex / wago's into a socket from the RCD side of the board. It didn't trip (and works). Then tested that RCD - all good.

My brain gave up at that point. I initially wanted to say there's a shared neutral with something on the RCD side of the board.
But if there was that would trip the RCD side when the circuit was on an MCB the unprotected side...

All I could think of was incredible coincidence of two strangely duff RCBO's that pass all tests until there's a load on, or a strange transient fault?
Now had to put back MCB and carry on with day.
Going back tomorrow, any thoughts of other helpful tests or recollections of more unusual faults welcomed!
(it's always things you plan to be quick that go pear shaped.....!)

 

[snowhead]

I'll risk 50pence on it NOT being faulty RCBO's.

Have you tried the RCBO in the RCD side of the board, the same side you have the plug / flex setup?

 

[telectrix]

shared L from downstairs lights to landing light. will trip RCD when a 2 way switch is operated to turn light on.

 

[littlespark]

Not trying to teach you the basics or anything, but did you take the circuit neutral out the bar and put it in the rcbo, neutral fly lead into non rcd neutral bar?
and have you got the right neutral for the circuit?
Sometimes there’s a mix up and say, circuit 3 breaker is paired with circuit 4 neutral?

 

[timhoward]

shared L from downstairs lights to landing light. will trip RCD when a 2 way switch is operated to turn light on.

Thanks.

 

[timhoward]

I'll risk 50pence on it NOT being faulty RCBO's.

Have you tried the RCBO in the RCD side of the board, the same side you have the plug / flex setup?

Didn’t try that. Thanks

 

[timhoward]

Not trying to teach you the basics or anything, but did you take the circuit neutral out the bar and put it in the rcbo, neutral fly lead into non rcd neutral bar?
and have you got the right neutral for the circuit?
Sometimes there’s a mix up and say, circuit 3 breaker is paired with circuit 4 neutral?

No worries, yes I did all that.
I verified the N with R1+R2 and it would have tripped when I moved to the RCD side if I’d got the wrong N.
It’s one where you can’t see the cable sheaths so a very good point to double check.
I’ve already decided when I go back I’ll label all the Neutrals on the unprotected side.

 

[timhoward]

Ah I think one has explanation has occurred, hours later...
Sequence is key.
1 - Rcbo issues
2 - R1+R2 done at convenient double pole bathroom fan isolator to confirm correct N. I also turned it off out of habit....
3 - tried cct on RCD side, fan isolator still turned off I think.
4 - by then I was running late and annoyed and I didn’t retry it on the other side.
So Earth leakage issue at that fan is a possibility. I’ll feel rather silly if it was! Find out tomorrow.
(I’ve also just tried the rcbos on another job and they are fine, as suggested.)

 

[pc1966]

With the MCB in and powered you could put a clamp meter around the L&N for the RCBO use and see what sort on imbalance is present. Might point to a specific cause (e.g. if light-bulb magnitude, etc).

If you do L+N to E IR testing with the board off then any linked L or N that is good won't show up as an IR fault. So you might want to do the L+N IR at 250V to the N terminal strip and to the L busbar (power off, obviously!) so see if there is a clear link.

 

[i=p/u]

A live from circuit and neutral from other .

 

[timhoward]

Thank you all for the suggestions.
I'm 100% I had the right live and neutral.
Now I've remember that mid-point I turned a fan isolator off and never turned it back on I'm hoping that will turn out to be the easy answer.
The shared Live is a great shout, and it's conceivable that the switch positions led to the differing results. I'll hold that thought in reserve.
(Btw when I said R1+R2 above I meant continuity tests involving the Neutral.... )

 

[timhoward]

Update - It was the fan.... DOH! It was soaking wet inside due to poor installation (pointing uphill!) and condensation from it not working.

 

[NateCreekside]

At least it wasn't an issue you caused!

 

[Pete999]

Simple job, swap an MCB to an RCBO on a lighting circuit. It's got me totally stumped!

IR test L+N -> E is fine on the circuit. So is Zs. All 2-way switches operated.
Instant trip when changed to RCBO though, and tried two RCBO's
Ramp tested them both, all good. All tests pass on them with no load.

Also (in desperation, shhh!) tried connecting lighting circuit via plugtop / flex / wago's into a socket from the RCD side of the board. It didn't trip (and works). Then tested that RCD - all good.

My brain gave up at that point. I initially wanted to say there's a shared neutral with something on the RCD side of the board.
But if there was that would trip the RCD side when the circuit was on an MCB the unprotected side...

All I could think of was incredible coincidence of two strangely duff RCBO's that pass all tests until there's a load on, or a strange transient fault?
Now had to put back MCB and carry on with day.
Going back tomorrow, any thoughts of other helpful tests or recollections of more unusual faults welcomed!
(it's always things you plan to be quick that go pear shaped.....!)

Borrowed N