wrote:

Rob, I don't quite understand your point about upgrading the earth
wire. I had thought that a larger earth wire and possibly an extra
earth rod would always be a good thing, are you saying that a larger
wire with (presumably) lower resistance would increase tripping of the
RCD or is there a technical problem that I don't understand ?

I think he is saying that the size of wire to the main earth electrode
is not actually that important since it only has a small part to play in
the grand scheme of things.

Say your earth electrode (rod) has a fault loop impedance of 10 ohms
(which is not bad as these things go). A 6mm^2 wire (the size specified
in the in-site guide for TT systems), would introduce a further 6 milli
Ohms/meter extra. So to all intents and purposes the size of the
earthing conductor makes no significant difference to the total earth
fault loop impedance.

Adding additional earth rods *may* help reduce the total earth loop
impedance (if far enough apart), but again that may not be much use
unless you have a very high (or seasonally variable) earth loop
impedance to start with. In the case of an outbuilding it can be
worthwhile making the outbuilding a different TT system from the house -
i.e. its own earth rod etc. You would then need to ensure the CPC of the
submain to the outbuilding is not connected at the destination end.

--
Cheers,

John.

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