RCD neutral fault.

[La Poste]

Greetings.
I am an electrician wannabee and am doing a spot of labouring for an electrician. Maybe not a wise career move at this time but it's too late to change now.
My electrician boss and I were having a discussion about why an RCD would trip even if all the MCBS connected to it were switched off and he said it was caused by an earth neutral fault.
I asked how can there be an earth neutral fault when all the circuits supplied by that RCD were switched off and hence no power was going through the RCD. He didn't know the answer and neither do I.
I was wondering if anyone could shed some light on this.
I thought RCD's measured the balance between phase and neutral and tripped if their was an imbalance IE a phase or neutral earth fault only if the circuits are on, so how can an RCD trip if all the circuit MCB's it supplies are in the off position?
The electrician removed all the neutral wires from each circuit one by one until he found the circuit that was causing the RCD to trip, all this was done with the MCB's in the off position.
The main switch to the fuse board was in the on position.
Thanks.

 

[newfutile]

neutral and earth are already connected upsteam (at least in england anyway and i imagine else where as well).i cant think of any upstream fault that would affect an (RCD) Residual Current Device .
a downstream neutral to cpc fault will only trip an (RCD) Residual Current Device if a circuit with at least double the actual tripping current (maybe more due to unequal cable sizes) is powered on.this could be as little as 40mA (assuming a 20mA actual trip current)

 

[littlespark]

Um…. Original thread is from 2010.
Might be a bit late to add anything.