Exporting PME to Socket in high street.

It does feel like TT'ing the sockets etc. in this situation is likely to be somewhere between difficult, impractical and impossible. Just too much PME influence around (both below ground and within reach above ground). To cover most bases you'd probably have to look at something like a buried grid extending under the whole area that could potentially be served by the sockets (and even that approach tends to raise issues at the extremities unless you have more than the usual amount of space to play with).

I can't help thinking that the least worst solution in this kind of case would be to use the PME earth but with the addition of an open-PEN device - if you have 3-phase available the 3-phase versions (that reference against an artificial N point) don't have the blind spots that the single phase types are notorious for. Not a directly documented BS 7671 arrangement admittedly, but BS 7671 does allow for novel approaches provided they provide a solution that no less safe than direct BS 7671 compliance.

   - Andy.

BS 7671 does not recognise O-PEN devices for anything other than EV charging equipment installations at the moment.

There is much debate on whether they would be the right thing for other circumstances, for a number of reasons.

Section 717 (for mobile and transportable units) and 711 (exhibitions, shows, stands) are probably the closest in Part 7 of BS 7671 to your application, and both do not, in general, permit TN-C-S supplies outdoors.

Thank your for reply,

I have made the cert unsatifactory in both these areas across the diffrent sites.It will require a bit of planning to sort this out. TT it the way forward but as implied from another member the PME influence may be a issue.This is going to be left to a better person than me to sort out. Again thanks for all the responses.You have to love TNC-s..

M.L.R said:

TT it the way forward

BS 7671 requires that simultaneously accessible exposed-conductive-parts are connected to the same earthing system (Regulation 411.3.1.1).

TT is not always a conformant solution, in a 'sea of PME'.

Separate TT earth electrodes, not bonded, is not the "same earthing system".

Gkenyon, the mud just got thicker. i will report it back and someone with more knowlage than me can take a look and propose a course of action. There will be be a work round some how. 

It may be that there is no conformant solution.

It will be very difficult, if not impossible to accurately measure (or measure in compliance with BS7430) the TT electrode resistance if there are many other metallic services etc. in the nearby vicinity.   In these situations, I do wonder if it's better to keep the PME and install a local earth rod that will help to tie-down the local touch voltage in case of any issue upstream.

In reality  you don't really need to know the electrode resistance to the plate at the end of the universe in any case. 
It makes sense for verifying TT on a remote farm building, but as you observe in the cluttered case what you want is something that gets closer to an equipotential zone. After all what we want to minimise is the shock hazard in the event of faults in either the installation or the kit that is plugged in to it. We would not need earthing at all if we can eliminate (reduce to acceptable) the chance of a fault by another method. There is a reason for double insulated power tools and garden mowers etc... it neatly sidesteps any question of the integrity of the supply earth by not using it at all.
I'd be tempted if I had to to verify the electrode by Zs style reading of current from phase to electrode. Shades of the old 100W lamp test of the old water pipe earth - if the lamp between L and E lights at full brightness, there is an earth connection.

All this is old fashioned and dangerous thinking of course but it is sometimes useful to remember where we came from, and what preceded the current PME situation when it was not TN-S.
Mike.

mapj1 said:

In reality  you don't really need to know the electrode resistance to the plate at the end of the universe in any case. 
It makes sense for verifying TT on a remote farm building, but as you observe in the cluttered case what you want is something that gets closer to an equipotential zone.

Could it not be argued that where TT earthing is used to reduce the risk of open PEN, it has to be verified to be a genuinely independent zone of earthing?

TT does not exactly reduce the risk (probability) of an open PEN, it might reduce the danger, as the metalwork connected to the TT electrode now tracks the voltage of the ground the victim is most likely to be standing on. Unless the victim is handling cables the bring in a earth potential from a long way off, the most helpful thing is usually for feet and hands to be at similar voltage.
If by independent zone, you mean the floor area the victim is standing on, yes I agree, but if that floor is in effect a voltage at or near the local PEN, then you really want your earthed appliance to be connected to it.
M.

mapj1 said:

TT does not exactly reduce the risk (probability) of an open PEN, it might reduce the danger, as the metalwork connected to the TT electrode now tracks the voltage of the ground the victim is most likely to be standing on.

Yes, I think we're saying the same thing. In my org. Risk = Probability x Severity. 

By "independent zone", I mean the zone of earthing that the TT electrode is associated with does not interfere with the zone around the PME earthing electrode that is carrying neutral load current from the circuit with an open PEN.

mapj1 said:

TT does not exactly reduce the risk (probability) of an open PEN, it might reduce the danger, as the metalwork connected to the TT electrode now tracks the voltage of the ground the victim is most likely to be standing on.

Yes, I think we're saying the same thing. In my org. Risk = Probability x Severity. 

By "independent zone", I mean the zone of earthing that the TT electrode is associated with does not interfere with the zone around the PME earthing electrode that is carrying neutral load current from the circuit with an open PEN.