The EV code of practice basically says that we have to assume that even TNS supplies will be PME somewhere or may be converted (or partially converted) to PME at some point in the future
Part of the problem is that we define things slightly differently from the DNO, we all know that TNS is a separate path to the star point of the transformer and TNCS uses the neutral. The DNO don't tend to talk about TNS and TNC-S, they talk about CNE and SNE (Combined and Separate Neutral/Earth) which at first glance you might think, well CNE is TNCS and SNE is TNS, however not all SNE is TNS in truth. The DNO will assign the SNE deisgnation to a supply where the cable off the joint in the street into the building has a separte earth and neutral conductor, even if they are joined to the same conductor in the cable under the road, it does have some advantages, as long as the joints are made to more or less the same location on the street cable (and of course the same one) you can have multiple services in building that shares common steelwork with a much reduced chance of currents in the steel than if you brought them in as CNE (which they often won't do for this reason), but its not true TNS, they will however put a label on the cutout (on new supplies) saying its separate neutral earth. Do you tick TNS on the cert?, you don't know whats going on under the road, even if its an old PILC, if they have to do a joint, they'll put neutral to earth.
Then you get the whole PNB debate where the place *is* on its own transformer, had a discussion with on site maintance on an industrial site about this, he was worried about car chargers on some of their buildings because its PNB on some of their transformers, my view was that its still TNS, its separate from where you first tie down the stair point to physical earth and certainly doesn't come with the risks, that PME throught a housing estate does....
The only other place I've encounted the needs to be guarenteed TNS requirement is for petrol filling stations.
Few things that I've never yet worked out:
1) Why does the design of electric vehiles have to be such that the CPC of the supply point is connected to the vehile bodywork, could we not just have the wiring in the car effectivly class 2, and the output of the charging circuit in the car electrically separate?
2) Why do we worry about EVs or portable office cabins on TNCS, yet we are perectly happy to connect lighting columns to it (providing we meet some minimum sizing on the cpc)?
3) Why is TT so quickly pushed as the solution, I've seen more cases of dangerous conditions cause by a fault to earth on TT where for various reasons an RCD did not operature than I have of a dangerous voltage on metalwork off a TNCS system, in fact about the only time i've seen the latter (sufficient enough to give someone a shock) it was caused by the former in an adjacent property on a rural distribution network*
The fault path back to the neutral side of the transformer was driving a current through the DNO earth electrodes (which was only 2 seemingly poor rods, on at tx, one at end of run - the consumer side was a better electrode, being a metallic framed barn). This created a voltage drop across the DNO electrodes, meaning there was 190v between the neutral at the transformer and the physical earth, the same neutral which then provided a PME earth* in an another property which pulled alll the radiators upto 190v above the concreate floor below
*It actually wouldn't have mattered in this instance if it had been true TNS over PME, the ground potential had been displaced from that of the stair point of the transformer
