I have run a new 3 phase supply from a distribution board on a tt system to a new sub board from there I have wired a lighting sub circuit but when I switch on the lights it knocks of the rcd on the main board , why is this .
Many thanks , john

sounds like you have a ground fault...
check for burn marks at every connection.

I guess I'm dumber than a bag of rocks this morning but I sure could use a tad more information on your problem.

For example, when you flip the switch, does the downstream panel trip a breaker or is it only the main panel that trips?