John Rumm wrote:

Well it only needs to muster a feeble 8K ohms to be able to trip a 30mA
RCD.

Although the limit of 1667 ohms to comply with BS 7671 [413-02-20] is a
bit more relevant and ensures that the touch voltage won't exceed 50 V.
If the supply feeds a greenhouse then section 605 applies and the
limits become 25 V and 833 ohms [605-06-01].

If you achieve the OSG recommendation of 200 ohms in normal conditions
then I really don't think you'll need to worry too much about the ground
drying out or freezing in extreme weather, unless you're on very rocky
ground (literally).

If a 100 mA is involved the figures become 500 and 250 ohms, so the
margin is reduced somewhat. IME (which is limited to chalky and clayey
soils) you're unlikely to have any worries if an 8 ft earth rod is used.

The thing to bear in mind is that with a PME system and an exported
earth, there is the possibility that you will pass no leakage current in
the case of a disconnected CNE conductor to the property. That can leave
phase voltage on your exposed metalwork indefinately. Hence why
maintaining the equipotential zone in the outbuilding is important if
you are exporting the earth.

Absolutely. And where that's just not practical - the aluminium
greenhouse being the archetypal example - go for TT.

--
Andy