TT’ing external socket - series RCDs


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. 


  • There is certainly no prohibition on RCDs with identical trip thresholds feeding one another - indeed if you are supplying a caravan or a canal boat it is de-rigeur. In your shoes I'd only do it if there is someone there to reset it without too much aggro - if the first RCD is 100m away in a locked cupboard I may be a bit concerned about the ease of resettting versus the rising chances of a trip at an awkward moment. An all RCBO board solves this but is over the top for adding the odd  garden skt.

    ~The exploding hot tub sounds fun though and perhaps far more dangerous than the electrics.

  • I had to re-read the final part to understand the last part of your sentence ;-) 

    So TT’ing a single socket is fine (whilst of course ensuring all other PME’d stuff is >2m away).

    RCDs of the same spec in series is always talked about as a big no no, but I see no issue with it. 


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