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. 

  •   ... just to be clear, not picking on you. I'm seeing the term used quite a lot, but it does have a specific meaning, which has nothing to do with earthing.

  • I mean if I TT, it’s generally done at a DB, not at the electrical socket/accessory itself. I prefer PME any day over TT. 

  • Fully understand ;-) it wasn’t taken that way. 

    Thanks. 

  • with this socket then connected to a local earth rod.

    With any TT arrangement it is essential that there can't be a fault from a pre-RCD line conductor to the TT earthing system - usually that's achieved by making all the pre-RCD wiring double/reinforced insulated between live and earthed parts or just keeping the TT earth well away. Conventional wiring in the back box behind a socket typically  doesn't achieve that - even if the c.p.c/Earthing conductor is insulated, there's typically only single insulation between the line conductor and the earth straps etc. on the back of the socket.

    Having an additional RCD upstream certainly helps to mitigate that problem (as long as the TN side RCD is co-ordinated with Zs of the TT system - which is likely not an issue with 30mA devices) - although it's not clear that relying on such a setup complies with the letter of BS 7671 (esp.'Every exposed-conductive-part which is to be protected by a single protective device shall be connected ... to a common earth electrode')

       - Andy.

  • 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.

  • The other way is to bring the an armoured cable - with the PME earth on the armour, in via a suitably large plastic stuffing gland, so the armour at the load end is not connected to the box, or accessible to casual touch. Once in the box, the phase and neutral cores, and perhaps a bit of the the bedding jacket as well, depending on how things are best arranged for strain relief and IP ratings,  then carry on to an RCD of some sort, ideally all in one non-conductive enclosure. I'd imagine a plastic outdoor RCD socket box could also be wired this way.
     The armour can then be reached by meter probe for testing but is cut back or sleeved /taped so that  much like the unused PME terminal at the farmhouse, goes no further.

    How the two RCDs know that the one at the socket should trip for faults on things plugged in, but the one at the house should not trip for that but only trip if the armoured cable is damaged is not clear. BS7671 may think that happens but I suspect both trips operate  at once ;-)

    Mike.

  • The rcd at the furthest, hot tub, socket would only trip on a fault from that appliance or its lead. The house one would probably also trip. But the socket rcd won't trip if there is a fault upstream in the armoured cable or anything plugged into the other intermediate sockets. So there is some form of discrimination.

  • 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".

  • "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.