I have had a problem for a while with the the RCD tripping on the consumer unit. I think I’ve finally isolated the problem, it seems to be the outside, single mcb consumer unit that is the isolator for the septic tank. It’s quite old now and I think it’s letting in water, I’ve put a bucket over it temporarily and the RCD has stopped tripping. The motor is rated at 75 watts and it’s on a dedicated circuit from the main, indoor consumer unit with a 6 amp mcb. This was installed by a qualified electrician, so I have to assume it’s right way to do it. My question, though, is why it effectively needs two mcb’s, one outside and one inside when it’s the only draw on the circuit? I suppose the real question is whether the external isolator could just be a weatherproof double pole switch, if there was a problem wouldn’t the indoor mcb just trip anyway?