I'm with Bernard that perhaps there is a N/E fault that is there all the time.But is occasionally causing trip when the circuit is in use.When the RCD is tested on that circuit the N/E current flow will balance/offset the test current flow in the RCD coil.So although the tester is allowing say the full 30mA to flow as a fault in the Phase conductor to earth there is also fault current of say 10 mA in the Neutral so a net of 20mA and not enough to activate the trip coil????
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