You asked me what im thinking.What do you think?
Don't confuse yourself pal, you have two issues here -
1 - does the RCD work or not
2 - why does one circuit cause the RCD to trip
Sort out number 1 first and get the board happy with that circuit disconnected.
Then move onto sorting out number 2. IR tests are your friend, L+N to E.
The spark has extended the cpc using the wrong size cpc causing more fault current to go through the neutral. Causing the test button not to work.
If I change the cpc to the correct size the fault current will go to earth, the neutral will be what it should be, the test button will work.
Thats what I think.
Thats what ill be doing 1st thing Tuesday