TT’ing external socket - series RCDs

Hi,

Following on from a previous post, I wanted to ask this separately for clarification as I usually TT distribution boards as a whole.

25A radial circuit leaves internal DB and loops between various sockets and FCUs (without integrated RCDs) within garden connected to the buildings PME supply. The “final” socket, away from any other class 1 appliances, has its CPC terminated in a plastic enclosure, with this socket then connected to a local earth rod. This socket also has a built in, 30mA RCD.

Does this comply with regulation - I think it does, but wanted to check. Generally I would TT a DB externally, however cost is an issue here. I don’t foresee this being any different than an EV charger with its own earth rod, and the PME disconnected. 

Does having 2 RCDs in series, cause any other issue than nuisance tripping? The house RCBO would likely trip at the same time as the RCD socket within the TT island. This is I think offering a belt and braces approach with redundancy. 

This far end socket will be used for a blow up hot tub, every now and again, and needs to be energised whist in use. It doesn’t make sense to install a dedicated supply for this, and it’ll be too costly. 

Thanks. 

Parents
  • I'm seeing the term used quite a lot, but it does have a specific meaning, which has nothing to do with earthing.

    Indeed. My seafaring relatives would insist that the proper definition of an island is all about water and has nothing to do with electricity whatsoever. Like so many words it's meaning depends very much on context, and just because it has one meaning in one context doesn't mean it can't have a somewhat different meaning in another.

       - Andy.

  • My seafaring relatives would insist that the proper definition of an island is all about water and has nothing to do with electricity whatsoever.

    Good job 'island mode' is defined in Part 2 of BS 7671:2018+A2:2022, so there is no confusion to those regaining their land-legs after some time at sea. Joy

  • Good job 'island mode' is defined in Part 2 of BS 7671:2018+A2:2022, so there is no confusion to those regaining their land-legs after some time at sea.

    "Island mode" ≠ "Island".

Reply Children
  • "Island mode" ≠ "Island".

    No, but IEV does define 'island' as a term for use in a power system: https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=603-04-46

    BS 7671 only defines 'island mode' because that's the only real reference to it in the present standard.

    An island in terms of the surrounding installations are supplied by the Grid, but in "island mode" the installation supplies itself, so metaphorically speaking, an "island" in a sea of connected-mode installations all around.

    Perhaps, not the best terminology, but it's there all the same.

    There are also definitions for 'intentional island' and 'unintentional island', neither of which relate to the usage in earthing systems ... which is really why I mentioned the terminology confusion when talking about splitting off to a TT earthing arrangement.