Septic tank circuit

[Ianm]

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?

 

[Murdochcat]

Photos ?

 

[Ianm]

Photos ?

I can take some if that would help

 

[JohnW2]

... 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?

On the basis of what you've told us, it doesn't (need two MCBs).

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?

Again on the basis of what you've told us, that would seem to be the obvious way to do it. In fact, you don't necessarily need an outside switch at all (unless you want one close to the septic tank - that could be 'inside', perhaps close to the CU.