N
Nigel
Let's just word this correctly, we are not using PME or suggesting it for the outbuilding which was the initial point in my post, we are extending the EBZ where the incoming supply is PME (noted you did say about EBZ earlier)
Also PEN conductors in a PME setup have multiple points of earthing along their length to protect against such network faults, it is TNCS that has an increased risk of ground faults but if bonded correctly then on these rare events the risks are minimised.
What can occur though is a PD between supplied earth and actually ground if you are in an isolated metal framed building where you could touch it and bridge between true ground and supplied earth, in these cases where the structural is exposed metal and it is raised up on something insulating where TT is not used then I have in the past implemented a dual arrangement of extending the BZ and adding Earthing rods. The strange thing is the BS7671 or guidance does not cover this latter scenario although it is a very real example and does pose a risk even from small network leakage through damaged cables of mainly the older kind where any steel ducting, armouring etc has corroded over time.
PS - if it is in guidance notes then this at most is just a simplified guide for most situations and often uses a hammer to crack a nut approach.
You keep saying about wording it correctly but throughout 7671 it is worded as using the PME earthing facility. I think it is best to utilise the terminology of the book that everyone should refer to to keep things uniform and clear.
As for where the PEN conductor is earthed, it is impossible to determine either if it is earthed, where it is earthed, how effective the earth is and also where the fault is on the PEN conductor. As such it is completely pointless bringing this up. It brings nothing to the design process.
I have utilised both before also. Although it is impossible to extend the EBZ in a situation where someone can touch a bonded metallic part and true earth. This is just not possible. So I utilised a perfectly good earth in the PME and installed everything to a TT so that in the event of a break in the PEN the installation would the TT.
Regulation 717.411.4 specifically states a 'PME earthing facility shall not be used as the means of earthing for an installation falling within the scope of this section'. There are some exceptions but certainly not to be used in a domestic setup.
Section 717 covers 'Mobile or Transportable Units' but also covers 'workshops' and 'offices' (detailed in 717.1).
I would be really interested in how you extended an EBZ but also determined that you needed earth electrodes??