TT’ing external socket - series RCDs - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

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. 


  • TT island

    If the installation is connected to the DNO supply, how is it an 'island' ?

    TT installations, with their own earthing arrangement, may operate in island mode, or connected mode, just as TN installations may operate in connected mode or island mode. Similarly, parts of installations that have their own TT earthing arrangement may operate in connected mode or island mode?

    Furthermore, anything connected to the general mass of Earth is connected to everything else connected to the general mass of Earth.

  • Sorry wrong use of the word “island”; so the “parts” of an installation in this instance would be the socket, connected to earth by means of a rod rather than the suppliers connection. 

  • A "tt island" within a PME supply is a vernacular term used to refer to somewhere you take the live and neutral (or even PEN) from a TNC-s supply, but do not use the neutral derived earth - rather using a local electrode. Such is common for EV chargers, and outbuildings on farms, where the DNO provided earth may be either used only for the domestic bits of the farmhouse, or more commonly round here, ignored altogether. The neighbours may use PME however, so it tends to join up a bit at the water mains etc.

    Furthermore, anything connected to the general mass of Earth is connected to everything else connected to the general mass of Earth.

    Indeed, but often only quite loosely, and where a voltage difference is impressed between two earthed objects from a low impedance supply, most of the voltage drop occurs within the near field around the point of contact, typically more than 60% within an excess radius  around the electrode (or item serving as an electrode) comparable to its principle defining dimension(s).
    Hence concerns about fences around the bases of poles to avoid surface step voltages affecting livestock, especially with longer body dimensions - cattle are easier to electrocute than sheep or dogs,  and in turn both of those are more vulnerable than chickens !!


  •   ... 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.

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


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