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Deleted member 9648
Ramp test RCD's and IR test the whole installation. It'll either be a faulty RCD or a N-E fault anywhere on the installation, assuming neutrals are correctly connected in the DBYeh I understand this however with the switch disconnected and a choc block with just a flex with light fitting there should be no extra load on circuit ie should work fine on the 16amp mcb and not trip.
I would understand if the shower was on and tripped but with a light bulb on didn't as would prove that there was too much drain on circuit to trip rcd of current load was overloading circuit.
However if rcd was faulty then it would surely trip with load of house appliances and rooms etc this is reason was asking here of anything else that could be worth testing before paying for it to be investigated to be told well yeh its something we have over looked .