Re CU tripping on most circuits:

The other thing not yet mentioned is your house wiring may have
connected neutrals between different circuits, or have loads that run
from live on one circuit to N on another.

Or there may be silmilar N/E issues.

I would remove all problem circuits from the CU, disconnecting all 3
L,N & E, and test with a multimeter to see what connects to what. Each
circuit should be entirely unconnected to each other one, except for
earths which can connect to other earths no problem. For lighting
circuits, switching all the unpowered lights on would enable the meter
to spot lights connected from one circuit's L to another one's N.

You just need to work through the possibilities, confirming or
eliminating one by one. Thats if you want to run them on an RCD. I dont
know which circuits they are, but it may be fine to not RCD some or all
of them, depends.


NT