I expect the answer is somewhere in this forum but if someone can quote me a figure or point me in the right direction, I would be grateful. I am seeking to establish what sort of overcurrent would cause a 45 Amp Type B MCB to trip in approx 10 secs. The device is protecting a shower circuit wired in 10mm2 twin and earth. The circuit is further protected by a two pole RCD rated to trip @ 30mA. Presumably the MCB is tripping thermally rather than magnetically as it buzzes for a few seconds before dropping out. I believe this points to a fault in the shower unit itself (an Aqualisa rated at 10.5KW) rather than the associated circuit. The RCD has not tripped at all and I have already replaced the MCB and the double pole pull switch, to eliminate the possibility that either of those items was the cause of the problem.