Throw all the small breakers to off and set the RCD to on - ideally it will not trip. One by one reset the MCBs to on until you find which circuit is causing the problem - might be power might be lighting.
When you know which circuit it is then unplug / switch off all the lights or appliances on that circuit - re-energise the MCB and turn lights / appliances on one at a time until the RCD trips.
Good luck...
Peccavi, March 2010