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TT’ing external socket - series RCDs

vantech

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. 

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  • 0

    assuming the cable to this final socket is armoured, the gland and the armour would presumably be connected to the PME earth. I read somewhere that the shroud which usually covers it might not be suitable for ensuring it is not an "exposed" conductive part, so might need further thought

  • 0 in reply to OlympusMons

    Thanks, I would likely land the armoured cable into a separate enclosure and then take a 2 core from this enclosure into the socket using plastic conduit or a 2 core outdoor cable of some description. 

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  • 0 in reply to vantech

    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.

  • 0 in reply to mapj1

    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.

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