I have a 9.8KW shower on a Niglon combined 63A MCB/RCD sub main attached via Henley blocks. I test the RCD about once a month and it trips fine. Recently it's tripped the MCB too when I test it and I'm sure I've read somewhere that it shouldn't. This doesn't happen every time though, which is even more confusing. Everything re-sets and works normally afterwards, so need I do anything? The MCB is rated at 50A and the whole thing is wired in 10mm cable.