If all your testing reveals no apparent fault, then the tripping could be caused by the double pole switch in the CCU opening the neutral pole before the live pole ...
[FONT=&]In the real world, the situation is complicated by the residual capacitances and leakage paths.
N-E residual capacitance exists in all installations, but the leakage through this capacitance is negligible because of the low potential (almost zero) between N-E. When the N pole of a double pole switch opens, the voltage across this capacitance will rise suddenly with a subsequent increase in N-E leakage current through this capacitance. This increase will be at a maximum if the switch N pole opens before the L pole. As the switch contacts open they will arc, causing HF voltage spikes, raising the N-E leakage even higher. If the switch has a slow break characteristic, often found in domestic switch gear, the effect will be even greater.
This phenomenon is most commonly witnessed in distribution boards with a separate front end conventional RCD. Where opening the DB Main Switch causes a high voltage (near full mains potential) to charge the N-E capacitance of the whole installation, easily causing enough leakage current to flow to trip the RCD. [/FONT]
