I'm pleased to see in this one thread several hobby-horses of mine that have grown in the last year or two for various reasons.  I'm intending to write them up in detail, and will pass on the links here if that happens (by my getting some time during this period of less work activity).   But here are some notes, welcoming comments.


The "PME thing":  yes, I have long taken an international interest in this, and it seems mainly the UK has this strong and legislated worry about particular applications of TNCS systems. The risks are of course considered elsewhere, but I'm not aware of such a number of prohibited applications (e.g. caravans, petrol stations) or special regulations elsewhere.  Some countries don't use TN* or TNCS much (FR, IT, etc).  Some force the use of TN(usually CS) such as the US and Sweden. Those ones admittedly tend to have 'could be balanced' supplies i.e. split phase or 3-phase. That's far from a cure in all conditions, but it does reduce to some extent the probability of harm. It also avoids the chance of reversed LN polarity going undetected for long! In Sweden many cars have engine heaters that connect the chassis to PE.  They're used with no concern about whether it's TNCS - which it almost always is. In older customer-installations the 'C' part (PEN) may be all the way up to a shared N and PE bar in the fusebox for outgoing final-circuits.  I've seen installations even from the 1990s where a cooker is fed from a TNCS supply coming into an individual flat, and stands next to a sink with no bonding of the water to the PE (there was continuity from sink to cooker, but clearly running through the PEN back to something like a building bond or a pump). But the electric shock deaths per capita are lower than in the UK, and I don't find vehicle-related ones for years back.  That doesn't mean I would touch that cooker and sink together without my gloves on, but I realise I'm probably being oversensitive compared to many other risks with roads, food, etc that I don't know so directly about.  EVs could plausibly be more dangerous than just engine-heaters, being in use during all of the year, including times of likely bare-foot walking (which anyway would raise a question about the safety of 70 V ac).  In countries that 'like' TNCS there aren't exclusions for camping sites, docks, etc etc. In the US I know of a few people now campaigning to get home docks permitted to be, basically, TT: there's very strong resistance from the establishment. They were surprised when I pointed out that their proposal is a method from an IEC standard, used exclusively in some countries, and that the method they're forced to use would be forbidden in the UK. (If you don't know of US systems, consider that in most states there is a  MV [medium-voltage e.g. 13.8 kV] multiply-earthed neutral distributed, to which transformer primaries are connected LN and LV neutrals are required to be bonded, except special exceptions for some farms. Imagine the potentials that can arise on PE during MV faults, or from MV load 3rd harmonics, etc., besides from the LV system. It doesn't take many volts to make a part-submersed person holding a ladder or boat-lift very uncomfortable or uncontrolled.)  Funny world.  Sometimes there are technical reasons for regional differences, which here could include that US RCDs [GFCIs] are electronic and voltage-dependent.  Often, I suspect, it's as much or more a historic matter of what fear has come out on top in the traditional compromise of the pro and con of different earthing systems. 


The attention to TN-C-S is interesting too because it ignores dangers in alternative systems: broken flaky old connections to lead cables in TNS systems together with an earth fault (or leakage), or TT-system RCDs that fail or that can't respond to the current from a particular type of fault. Although load current alone won't cause danger in these systems, the first fault of the connection or RCD could go unnoticed for a long time, until an insulation fault or high leakage does happen. Are the details well enough known to justify special measures on TNCS instead of other systems? (Or special measures at all?)  An old paper (Gosland 1950) does a courageous job of analysing risks (for overhead supply) with what data the author could find. It's probably not very useful now, except as fun, but I really liked the author's response to a comment about the uncertainty of input data: "...refer to the fact that these are based on scanty and divergent data, if not assumed. Nevertheless, any engineer attempting evaluation of the relative merits of methods of earthing must have quantities of this kind at the back of his mind, and there seems to be advantage in stating explicit figures, so that the foundations of opinion may be exhibited and discussed."  I share the UK sentiment of fear of TNCS, and implement this fear in my greenhouse (TT).  But when trying to rationalize the fear, it's not clear that it's justified.


This back-to-BS842 (voltage-operated ELCB) method for EV chargers was fun to see when it arrived.  (Does any other country use this in EV charging?)   I prefer the simplicity of having separate detection of protective conductor current, and tripping all conductors (including PE) if that happens. That seems a good idea for any single socket that feeds outside-the-bonding equipment. I have my own implementation for an outside socket, as one of several "unconventional uses" of a cheap 4-pole type-A RCD.  Tripping is equivalent to unplugging. I later found that the principle is established as 'SPE' RCD (switched protective earth) but is only used in portable RCDs, e.g. the inline ones in an EV charger. Presumably, having this on a fixed socket or charger is disliked because "you mustn't switch the PE" is deeply ingrained.  But I think it would make sense more generally, for single items.  The impedance of vehicles to earth would not cause tripping in normal conditions in a properly operating system, from what I've measured (with my engine heater, even in salty snow).  The protection can operate through wet conditions if the chassis attains a dangerous voltage (I only tried 230V, not 50V), or might require current through a person in order to trip in drier conditions: but as it's only a protection for very unlikely situations it's not unreasonable to see it as supplementary protection that reacts to the shock.  I would use it on any earthing system, not just TNCS.


Yes, type AC RCDs do not seem defensible as a continued product. Last summer I reviewed prices in different countries to indicate that there's not a significant necessary extra cost for A compared to AC.  A better steel is needed, but the international prices make clear that this doesn't have much effect compared to the other factors. As long as most people use AC, then A is "special" and likely to cost a lot more. Having a smaller range of different devices is surely more efficient for production, stock and price. In those places where AC isn't available, A costs much the same as AC does in the UK.  There's practically nothing for which AC is justified, now that cookers and heaters and lights and almost everything else contain diodes as about the first thing at the input. It's obvious that type AC is not used properly in the UK, whatever regulations might wish, since almost every ready-made CU has AC only, and many shops have lots of AC and not a single A.  At least, that was what I found a year and a bit ago when I intended to replace someone's old RCD with two new ones on a visit to the UK.  I was amazed: after not finding suitable ones at several shops including screwfix, I got a fairly expensive quotation from CEF.  It's particularly silly when we have TT systems that depend on the RCD for more than 'just' supplementary protection. (I assume that type-B and AFDDs will also get a lot cheaper with more use, but these at least have some clear further circuitry and cleverness rather than the small construction difference of A vs. AC.)

I'm pleased to see in this one thread several hobby-horses of mine that have grown in the last year or two for various reasons.  I'm intending to write them up in detail, and will pass on the links here if that happens (by my getting some time during this period of less work activity).   But here are some notes, welcoming comments.


The "PME thing":  yes, I have long taken an international interest in this, and it seems mainly the UK has this strong and legislated worry about particular applications of TNCS systems. The risks are of course considered elsewhere, but I'm not aware of such a number of prohibited applications (e.g. caravans, petrol stations) or special regulations elsewhere.  Some countries don't use TN* or TNCS much (FR, IT, etc).  Some force the use of TN(usually CS) such as the US and Sweden. Those ones admittedly tend to have 'could be balanced' supplies i.e. split phase or 3-phase. That's far from a cure in all conditions, but it does reduce to some extent the probability of harm. It also avoids the chance of reversed LN polarity going undetected for long! In Sweden many cars have engine heaters that connect the chassis to PE.  They're used with no concern about whether it's TNCS - which it almost always is. In older customer-installations the 'C' part (PEN) may be all the way up to a shared N and PE bar in the fusebox for outgoing final-circuits.  I've seen installations even from the 1990s where a cooker is fed from a TNCS supply coming into an individual flat, and stands next to a sink with no bonding of the water to the PE (there was continuity from sink to cooker, but clearly running through the PEN back to something like a building bond or a pump). But the electric shock deaths per capita are lower than in the UK, and I don't find vehicle-related ones for years back.  That doesn't mean I would touch that cooker and sink together without my gloves on, but I realise I'm probably being oversensitive compared to many other risks with roads, food, etc that I don't know so directly about.  EVs could plausibly be more dangerous than just engine-heaters, being in use during all of the year, including times of likely bare-foot walking (which anyway would raise a question about the safety of 70 V ac).  In countries that 'like' TNCS there aren't exclusions for camping sites, docks, etc etc. In the US I know of a few people now campaigning to get home docks permitted to be, basically, TT: there's very strong resistance from the establishment. They were surprised when I pointed out that their proposal is a method from an IEC standard, used exclusively in some countries, and that the method they're forced to use would be forbidden in the UK. (If you don't know of US systems, consider that in most states there is a  MV [medium-voltage e.g. 13.8 kV] multiply-earthed neutral distributed, to which transformer primaries are connected LN and LV neutrals are required to be bonded, except special exceptions for some farms. Imagine the potentials that can arise on PE during MV faults, or from MV load 3rd harmonics, etc., besides from the LV system. It doesn't take many volts to make a part-submersed person holding a ladder or boat-lift very uncomfortable or uncontrolled.)  Funny world.  Sometimes there are technical reasons for regional differences, which here could include that US RCDs [GFCIs] are electronic and voltage-dependent.  Often, I suspect, it's as much or more a historic matter of what fear has come out on top in the traditional compromise of the pro and con of different earthing systems. 


The attention to TN-C-S is interesting too because it ignores dangers in alternative systems: broken flaky old connections to lead cables in TNS systems together with an earth fault (or leakage), or TT-system RCDs that fail or that can't respond to the current from a particular type of fault. Although load current alone won't cause danger in these systems, the first fault of the connection or RCD could go unnoticed for a long time, until an insulation fault or high leakage does happen. Are the details well enough known to justify special measures on TNCS instead of other systems? (Or special measures at all?)  An old paper (Gosland 1950) does a courageous job of analysing risks (for overhead supply) with what data the author could find. It's probably not very useful now, except as fun, but I really liked the author's response to a comment about the uncertainty of input data: "...refer to the fact that these are based on scanty and divergent data, if not assumed. Nevertheless, any engineer attempting evaluation of the relative merits of methods of earthing must have quantities of this kind at the back of his mind, and there seems to be advantage in stating explicit figures, so that the foundations of opinion may be exhibited and discussed."  I share the UK sentiment of fear of TNCS, and implement this fear in my greenhouse (TT).  But when trying to rationalize the fear, it's not clear that it's justified.


This back-to-BS842 (voltage-operated ELCB) method for EV chargers was fun to see when it arrived.  (Does any other country use this in EV charging?)   I prefer the simplicity of having separate detection of protective conductor current, and tripping all conductors (including PE) if that happens. That seems a good idea for any single socket that feeds outside-the-bonding equipment. I have my own implementation for an outside socket, as one of several "unconventional uses" of a cheap 4-pole type-A RCD.  Tripping is equivalent to unplugging. I later found that the principle is established as 'SPE' RCD (switched protective earth) but is only used in portable RCDs, e.g. the inline ones in an EV charger. Presumably, having this on a fixed socket or charger is disliked because "you mustn't switch the PE" is deeply ingrained.  But I think it would make sense more generally, for single items.  The impedance of vehicles to earth would not cause tripping in normal conditions in a properly operating system, from what I've measured (with my engine heater, even in salty snow).  The protection can operate through wet conditions if the chassis attains a dangerous voltage (I only tried 230V, not 50V), or might require current through a person in order to trip in drier conditions: but as it's only a protection for very unlikely situations it's not unreasonable to see it as supplementary protection that reacts to the shock.  I would use it on any earthing system, not just TNCS.


Yes, type AC RCDs do not seem defensible as a continued product. Last summer I reviewed prices in different countries to indicate that there's not a significant necessary extra cost for A compared to AC.  A better steel is needed, but the international prices make clear that this doesn't have much effect compared to the other factors. As long as most people use AC, then A is "special" and likely to cost a lot more. Having a smaller range of different devices is surely more efficient for production, stock and price. In those places where AC isn't available, A costs much the same as AC does in the UK.  There's practically nothing for which AC is justified, now that cookers and heaters and lights and almost everything else contain diodes as about the first thing at the input. It's obvious that type AC is not used properly in the UK, whatever regulations might wish, since almost every ready-made CU has AC only, and many shops have lots of AC and not a single A.  At least, that was what I found a year and a bit ago when I intended to replace someone's old RCD with two new ones on a visit to the UK.  I was amazed: after not finding suitable ones at several shops including screwfix, I got a fairly expensive quotation from CEF.  It's particularly silly when we have TT systems that depend on the RCD for more than 'just' supplementary protection. (I assume that type-B and AFDDs will also get a lot cheaper with more use, but these at least have some clear further circuitry and cleverness rather than the small construction difference of A vs. AC.)