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.

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.

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.

Indeed. This is rather like the EV earthing discussions from a while back. My initial thought for EV charge points was to TT and as long as you kept the TT electrode pretty close to the area the vehicle would be standing on all should be well - as the electrode would follow any local ground potential changes (such as from nearby PME metalwork) - in effect exchanging 'true Earth' for equipotentiality. It was pointed out however that there can be very significant voltage variations even within quite small distances - there was a diagram somewhere, but the general gist was something like if a buried pipe or cable was at a hazardous voltage (230V say, due to a broken PEN) there might be half that voltage on the surface directly above it, and half again a couple of metres further away. A TT system that in effect takes the potential from some point(s) underground and transfers it to the surface a several metres away doesn't really ensure sufficient equipotentiality.

The open-PEN device approach on the other hand - when it detects the problem - disconnects the c.p.c. to above surface exposed-conductive-parts (as well as all the live conductors of course), leaving them floating (or at worst influenced by the ground they're directly standing on). So in a situation where you can't be sure that the voltage on the protective conductors is similar to that of the ground next to it, letting it float is probably the least worst option.

  - Andy.

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.

Indeed. This is rather like the EV earthing discussions from a while back. My initial thought for EV charge points was to TT and as long as you kept the TT electrode pretty close to the area the vehicle would be standing on all should be well - as the electrode would follow any local ground potential changes (such as from nearby PME metalwork) - in effect exchanging 'true Earth' for equipotentiality. It was pointed out however that there can be very significant voltage variations even within quite small distances - there was a diagram somewhere, but the general gist was something like if a buried pipe or cable was at a hazardous voltage (230V say, due to a broken PEN) there might be half that voltage on the surface directly above it, and half again a couple of metres further away. A TT system that in effect takes the potential from some point(s) underground and transfers it to the surface a several metres away doesn't really ensure sufficient equipotentiality.

The open-PEN device approach on the other hand - when it detects the problem - disconnects the c.p.c. to above surface exposed-conductive-parts (as well as all the live conductors of course), leaving them floating (or at worst influenced by the ground they're directly standing on). So in a situation where you can't be sure that the voltage on the protective conductors is similar to that of the ground next to it, letting it float is probably the least worst option.

  - Andy.

It is, metaphorically, a mine field (- as a professional aside a real mine field is not a place to be driving electrodes ! but you can sometimes detect where the ground has been disturbed to plant them from variations in ground conductivity. ). The problem is that while there are certainly are  rapidly decaying voltages in the area adjacent to any single live buried object,  once there are many such objects, the whole clump of soil 'enclosed' by them gets pulled up to the new potential - more like the field under around a grid electrode is fairly equipotential but at the edge of it there is a large step voltage. So for example it is very likely that the ground in the middle  of the road between houses on the same substation is indeed a large floating island at more or less constant voltage if the PEN breaks to the whole street supply, but in the same street if there  is a PEN fault in the supply to any one house but not the others it very much isn't.
The problem is that very often there is no record of what is below ground and even if there is, not how conductive it is, and of course the location and type of fault is also critical.

Mike