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.

Been stuck with this issue going round my head for a good few weeks now with little advicee on how to tackle it or make it safer. Did think about a O-Pen device, but got hung up on the 50v touch voltage for some reason.The Guide to highway Electrical street furniture section 4.2.1 (mentions about a earth rod at max 20 ohms, to limit the touch voltage to 100v on the lighing coloumbs . The O-open device will operate at 70V. 

So my thought is now,Still using the single phase TNC-s system. (As the design planning of the electrical system will be immense, impractical and the cost avoidance required)

Fit a earth electrode at the pillar to 20ohms (follow the lighing section, this 20ohm electrode is also used for other situations within guidance note 8) 

Fit a O-pen on the mains.( which will pose it own problems retro fitting into a existing system)

Or let the highways and council sort it out with a electrical designer, which seams the most logical to me.

Well in that sense it sounds you could just report back that it fails to meet the  BS7671 derived good practice advice for such situations, and you do not want to recommend a course of action. This is because there is no straightforward and cost-effective  way to improve matters that meets that guidance- which is probably how it ended up in the state  it is now !!

They either have to re-engineer things significantly, probably at an unjustifiable cost  or accept the small risk of an imperfect installation.

Mike.

Cheers all for the feed back, there are above 15 installations like this. We will get there some how.

(mentions about a earth rod at max 20 ohms, to limit the touch voltage to 100v on the lighing coloumbs .

The touch voltage and the electrode resistance are directly related via the current that flows to Earth via the Electrode. For simple single phase setups, that's only really limited by the load (plus the resistance of the electrode itself of course). For a typical lighting column with a a load <1A,  20Ω would give a touch voltage below 20V which is usually fine. If your sockets are potentially feeding loads of a few tens of amps though, it gets a lot more challenging. A 20Ω electrode only needs 2.5A to flow through it to reach 50V and 5A to hit 100V - and these voltages tend to persist indefinitely as neither overcurrent nor residual devices see anything unusual.

   - 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. 

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.