To export an earth or to TT from an RCBO?

[nothanks]

I have a couple of external sockets that are fed from an RCBO which also provides power (via about 30m of buried 3-core SWA cable) to a class II Well pump and controller. The SWA armour and earth conductor are connected to the house earth solely by the CPC in the T&E cable. I'm wondering about putting a small weather-proof CU at the Well so I can use its supply for a remote socket, garden lights, and maybe a gate controller. The question is whether to TT the remote CU (but the SWA will still have to be fed from the RCBO) or whether to add a 10mm earth cable between the MET and the SWA armour so I can use the exported earth. The house has a dedicated TN-C-S supply from a 2-wire overhead feed, but I have fitted an additional ground rod so (hopefully) the loss of the supply neutral should not cause a safety issue.

What would the experts do?

[ARW]

You cannot make a TT supply from a 30mA RCBO.

[nothanks]

I can see a possible issue with discrimination, but if the RCBO (or RCD) is just measuring L-N current imbalance then why would there be a functional problem?

[ARW]

Other than discrimination it would work.

[nothanks]

So, the question becomes: which is the lesser of the two evils, to export an earth or to have an RCBO and RCD in series? I'm leaning towards the latter because it's in the nuisance category, whereas exporting the earth leads to a possible (but unlikely in this case) safety issue. Hmmm, further thought ... if I were to export the earth I would need to lift a lot of floorboards to run an earth wire back to the MET from the SWA termination, but these are the same boards I would have to lift to run a dedicated T&E cable back to a spare MCB in the CU. My conscience is now telling me that I should run the T&E and do a "proper" TT job from a dedicated MCB.

This DIY group has been useful yet again. In asking and discussing a question it forced me to think about the problem in a different way.

[ARW]

Why is the SWA RCD protected?

And you do not have to export the earth as a main equipotential bond unless there are extraneous conducive parts.

[nothanks]

In "plan A" it was because there's an external socket (fed from an RCBO) on the end of the house nearest to the Well - a convenient point to take the SWA from. However, I've decided to "do it right" and lift the floorboards to put in a dedicated T&E feed to the SWA from an MCB.

Currently the pump and controller are class II, but if I have a small CU at the tail of the SWA then I can't predict what might be connected in the future. But that all goes away if it's TT'd.>

[ARW]

I don't think I have seen a class II pump, but there is no reason why they should not exist - the number of pumps fitted in my lifetime (sump and pond only) is about 10.

I agree about the TT then. You can then pretty much fit whatever you want.