Your fault is on one of the following circuits
Sockets in Kitchen, Dining Room, Hallway, upstairs Landing and front bedroom.
you state that the mcb only protects those circuits , unplug everything from one room at a time and see if mcb trips , start in dining room .
Not very impressed with your electrician, what test has he/she carried out on your fixed wiring & rcd?
Whilst unplugging appliances or isolating them might establish the fault(s), but once that’s proved inconclusive, a more systematic approach is required. A back to basic testing should be started.
You might have an insulation fault, your RCD might be faulty, there might be too much cumulative earth leakage for your RCD. You might have a fault on the other circuits (supplied by the other RCD), causing the this RCD to trip. You‘ll be very lucky to locate this particular fault, without systematic approach.
Appliances etc, must be isolated with both live & neutral conductors, just turning off the mcb will not suffice.
I would suggest, that now your mcb has been operated several times, by this fault, it’s probably not functional and should be replaced.
If your electrician is not at their best, fault finding, you need to find one that is. It might take some time.
Yep I think you should pay for him to come and test. If the RCD reinstates as soon as it’s been tripped, with appliances totally disconnected, could point to a faulty appliance.
However it could be a neutral - earth fault on the fixed wiring, that only trips the RCD, once an appliance draws a load on that circuit.
An earth fault can cause a dead short; it could be the appliances you can’t access. Ive missed the bit, why you can’t isololate them and the uf.
They don’t have something like this FCU?I took a look behind one of the isolation switchs for the underfloor heating, and there were about 7 wires, 3 earths, 2 neutrals, 2 live i think - I just wasn't sure confident with what to do to remove the themostats out of the equation.
Switching them off, should isolate the ufh. Unless it’s been connected by someone not knowing what they are doing.
Hope your electrician locates your fault.
why dont you just remove the fuse from the fused spur before you try disconnecting the N and E and then monitor the behaviour of the mcb marked downstairs ring that keeps tripping........i would try this first.