When is a PEI not a PEI?

I see what you're getting at now ... so there's potentially a debate around whether 'not installed' equates to 'not required' (it could be argued, converse to your assertion, that if I don't need protective bonding in a building, because there are no extraneous-conductive-parts then I have indeed installed protective earthing and bonding to meet the requirements of Regulation Group 411.3.1).

Arguably, if none of the accessible metal parts are extraneous-conductive-parts, and all others that might be affected by potential are exposed-conductive-parts (or connected to exposed-conductive-parts) then it doesn't really matter.

So I guess it just depends which perspective you take (or the guidance takes ... GN2 appears to err on the permissive side, for example ... para 2 page 47 of the 9th Ed 2022).

AJJewsbury said:

way.

(quite what "connected to Earth by a low resistance to meet the disconnection times..." is supposed to mean is a quite different point of contention)

Your not the only one, the NICEIC Technical Helpline admitted that they had no idea!

Do we not think it is just a typo and missing the word 'enough' ?,

As in "a low enough impedance to ..."

Or in other words a decent earth is needed as ADS must work promptly with all possible combinations of internal and external supply and all the possible variations in Zs and pssc that implies.

It could have been clearer, and certainly if that is not what was meant then it really does need to be clarified.

M

Or in other words a decent earth is needed as ADS must work promptly with all possible combinations of internal and external supply and all the possible variations in Zs and pssc that implies.

Part 2 defines "Earth" as the conductive mass of the Earth - but the earth loop on a TN system doesn't include The Earth - it's metallic all the way back to the star point. The actual resistance of the system to Earth on a TN system can be reasonably high (anything up to 20 Ohms, often in the region of 1 Ohm) so not really comparable with Zs when you're using overcurrent protective devices, not that the N conductor need any have particular resistance to enable ADS as it's entirely uninvolved.

   - Andy.

well yes, we have noted before that the standards are not good at separating earthed objects that are also metallically connected and those that are earthed only by being stuck into terra-firma. (The obvious example is a  failure to distinguish between bonding to a water pipe that may carry significant diverted current voia a sub-ohm impedance to the neighbours , and to something like a bus shelter that will be  its own electrode of some tens of ohms at least. Both are identically classed as "extraneous conductors" variation in prospective current, and sensible cable size may be significant ..)

However, if in this case BS7671 actually mean provide an electrode or equivalent buried metal suitable to operate the RCD or what have you, then that needs to be made explicit, and depending on the ADS arrangement, perhaps quite substantial undertaking. I would not have read it that way.

M.

It also begs the question for a TN system why isolate N and leave the c.p.c. connected if they're connected together anyway (typically at the origin) and so likely both be subject to a very similar undesirable voltage compared with terra firma.

   - Andy.