MCB -> RCBO I'm missing something silly

[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