I am installing electric in my garage. I have used 4mm T&E (about 25m)
inside which is protected by an RCD on the main consumer unit. The RCD
also protects the socket circuit for the house. I am using a 16amp MCB
for the cable. As soon as I test the cable the RCD on the CU trips,
the MCB is unaffected. I then wired the T&E to the MCB used for the
house sockets, once again the RCD trips. I thought it could be the
cable so I replaced that but after testing it happened again. Any
ideas?

Regards

Dr Jones