I am running a new light from a ring main socket. I have run 2.5 to a FCU with a 5amp fuse then 1.5 to a switch and from the switch to the light. When I put the fuse in at the consumer unit the light comes on but as soon as I throw the light switch the fuse blows in the FCU. I have double checked the wiring and everything is fine. Brown from ring to live in on FCU same with blue, earths connected flying earth to metal box. From FCU to L1 in switch and blue to com then on to light. Can anyone help, it would be much appreciated.