Hi all, if you understand how an RCD works you might have better idea what is causing the trip. The principle is simple - the current entering the house and the current leaving the house should always be exactly the same unless there is a fault and some current is leaking to earth. the RCD compares the in and out currents and trips if it is not almost the same.
The leakage difference allowed for somewhere that a person might touch is 30mA which is not much, but this is the limit that you might have as an electric shock and not be dead !
So an appliance which has no contact to earth cannot trip an RCD, I would be very suprised if a ceiling lamp did it, PC's have filters inside which have constant leakage of maybe 1-2mA and other electronics devices might also have the same basic leakage ''built in'', motors (fridge/freezer) also have a leakage pulse at the instant the motor starts, you can see that your whole house might easily have a substantial leakage current without any real faults, It only needs something to add a small amount extra and the RCD will trip.