"DrJones" wrote in message
oups.com...
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?

Not enough information to offer any opinion at all, but probably damaged
insulation or a connection error. Ideally a megger should be used to find
the problem/ check the insulation at 500v but otherwise get a £4.99 test
meter from Maplin and isolate the fault by opening the ring circuit at the
farthest point and check backwards for shorts on each half of the ring.

john