ive wired my cu like the picture below, but the rcd trips out every time, ive checked all connections & they seem ok, but ill check again, does everything look okay or am i doing something wrong, thanks. ps please excuse the drawing
No image not allowed on this. Normally RCD trips because:-
You have something faulty
You have selected wrong neutral
Someone else has selected wrong neutral
So lets look at basics, If power out through RCD = Power in through RCD it will not trip. So if everything turned off and it still trips then you have leakage through wiring and you need to get the mega out and test all cables. If on other hand only trips when you turn something on then most likely neutral fault.
This could be a CU but also could be anywhere else in house. One should not borrow neutrals but it is done.
Before fitting an earth leakage trip most electricians do a mini inspection and test to see if anything is faulty before fitting as easier to tell customer before starting they have wiring problem than after. Even so sometime we get caught out and have to move things back off the RCD protection.
This is why the new 17th is going to be a problem. Using RCBO's does help as it is only the fault circuit that trips but in old houses you can have assorts of faults including where someone has drilled through wires and repaired with tape under the damp plaster. Multi-meters will not find these type of faults needs a 500 volt insulation tester. It is because of the expensive meters required that normally these type of jobs are not done as DIY and building control have to be notified and all the paper work submitted to them under Part P.