Why does RCD trip when fused spur switched on

[Petej999]

So you think replacing the FCU will fix the problem? By "wiring within the FCU" I take you to mean the fuse and internal metalwork that makes the connections. The wiring as in the two legs of the ring, remains the same whether the FCU is off or on, doesn't it?

A fault with whatever is connected to the LOAD SIDE of the FCU

 

[johntheo]

Yes, it was consistent for a long period, but once it went it was gone.
I was also suspecting water ingress as showers had been in use earlier & there was a previous occurrence some months ago where I believe the same circuit tripped as a result of a leak from the shower waste - on that occasion there was water through the ceiling & I'm told it was repaired.

Just very confusing in that scenario as to why the fault would go away with the FCU switched off (no neon BTW). I appreciate RCD faults can be a bit weird, but I always like to understand a fault rather than just fix it.
It's a split load consumer unit. No RCBOs. One of those MK ones, that has a mid position on the RCDs

Yes, "RCD faults can certainly be a bit weird" like mine, maybe some of the very very experienced people on here might like to (further) comment or whatever on/in the thread, my post,14,
Earth to neutral fault???? - https://www.electriciansforums.net/threads/earth-to-neutral-fault.195016/ with concentration on any conceivable outside cause.