ProMbrooke:

I remember on another forum someone posting a TN-S overhead earthing supply from the 40s or somewhere about there. I feel like back then people understood electrical theory to a great depth.

Overhead TN-S:  I'm sure I remember references to this being used in the UK at some time. Just now, the only one I can find is in the work by Gosland, mentioned below: and there it's only an option, called "Direct Earthing to Separate Earth-Wire, Singly Earthed", without a claim that it was used in practice. I first thought it was mentioned in an old book, ~1929: Rural Electrification .. I don't see it there now, but the book should be fun for someone to glance through anyway, with its many diagrams. It was written in England by a Swedish engineer who worked there for a few years on electrification projects.


Overhead TN-C-S was, to the best of my knowledge, the original use of 'PME' in the UK. It was studied and trialled by ERA (electricity research association) in the late 1930s. Voltage-operated ELCBs with TT ('normal earthing') were also discussed. Both were seen as options for safety in rural supply, if one didn't want a further conductor.  Fact-finding missions (probably not by that name) were made to German utilities and factories. The German utilities had done some chopping and changing between systems. As was mentioned in a thread here some time ago, they tend in Germany even now to want a particular LV network to have one or the other type of customer earthing, not mixed TT/TN*. Concerns with 'PME' in the UK were not just the needless-to-say ones, but also the expectation of some normal load currents straying through the ground ... concern for the sake of telephones and telegraphs. On the factory side, one of the 1930s works mentions a German case with one voltage-operated ELCB per machine-tool, where operators were seen rigidly following the instruction of pressing to test before starting their work.


TN-C-S in urban (cabled) networks was - as others have mentioned - presumably post-WWII, when new cable designs were used, the sheaths of earlier cables having provided a free extra conductor. Trusty lead-sheathed cables, with their sheath and possible armour in contact with soil (any covering being water-permeable) also gave good earthing of the network's neutral-point by forming basically a network of earthing electrodes. The change to plastic-sheathed cables has changed the earth-potential rise on LV networks in the event of faults from the primary side of the transformer, which I know has concerned some countries. Dangers coming in from the higher voltages are a situation for which TT typically has an advantage over even TN-S, as it makes the installation earthing independent - at least until insulation breakdown or surge-protection operation.  Thinking the other way (about how TN-S isn't always the ideal in every way), safety to 230 V appliances against getting an extreme voltage is an advantage of TN-C-S with multiple earthing in contrast to TN-S.

 

I sympathise with the point about the depth of understanding in the old days. The main concerns of earthing methods have been discussed and analysed for over a century. Situations have changed (and so has terminology!), so old works aren't necessarily very useful in their conclusions; but there's a lot of old explanation that people nowadays who come fresh to the problems would find useful. There are several good old works I'd like to share, including the ERA ones, but can't as they're still in copyright, or at least the official scans are claimed to be. Anyone with IET library credits to be spent could look here for the 1937 and 1941 ERA papers. More impressive is the effort by Gosland (1950) to do a quantitative comparison of all the dangers of all the options. Inevitably it is built on a raft of assumptions, but it has some good discussion and plots. I'm sure I've pointed it out here before, sometime, mentioning the author's response to a questioner who noted the shakiness of assumed numbers ("... 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.").  Doubtless very good analyses of many of the issues were done in Germany or its neighbours, well before the fact-finding studies of the 1930s ERA reports.  One advantage in the old days was that people had to choose their analyses carefully and do them by hand, instead of putting a very detailed test-system into a simulation program and hoping to get a general answer out. Understanding was needed from the start.


Regarding the mentioned foundation-earthing in Germany (or separate electrodes): this can be of some use against a PEN-fault in any case, since domestic loads most of the time are small. But it's particularly useful with 3-phase supply, where the typical neutral current is only a few amps instead of tens. Major loads such as cookers, water heaters etc traditionally don't even have a neutral connection in countries that have 3-phase installations as normal. Then the neutral really is just a balancer for a few small loads, meaning that a super-good earthing isn't necessary to control its voltage if the supply neutral gets a fault. In the UK situation it wouldn't take many loads together to reach a dangerous neutral voltage when the current has to pass through a modest electrode and normal soil.

 

ProMbrooke:

I remember on another forum someone posting a TN-S overhead earthing supply from the 40s or somewhere about there. I feel like back then people understood electrical theory to a great depth.

Overhead TN-S:  I'm sure I remember references to this being used in the UK at some time. Just now, the only one I can find is in the work by Gosland, mentioned below: and there it's only an option, called "Direct Earthing to Separate Earth-Wire, Singly Earthed", without a claim that it was used in practice. I first thought it was mentioned in an old book, ~1929: Rural Electrification .. I don't see it there now, but the book should be fun for someone to glance through anyway, with its many diagrams. It was written in England by a Swedish engineer who worked there for a few years on electrification projects.


Overhead TN-C-S was, to the best of my knowledge, the original use of 'PME' in the UK. It was studied and trialled by ERA (electricity research association) in the late 1930s. Voltage-operated ELCBs with TT ('normal earthing') were also discussed. Both were seen as options for safety in rural supply, if one didn't want a further conductor.  Fact-finding missions (probably not by that name) were made to German utilities and factories. The German utilities had done some chopping and changing between systems. As was mentioned in a thread here some time ago, they tend in Germany even now to want a particular LV network to have one or the other type of customer earthing, not mixed TT/TN*. Concerns with 'PME' in the UK were not just the needless-to-say ones, but also the expectation of some normal load currents straying through the ground ... concern for the sake of telephones and telegraphs. On the factory side, one of the 1930s works mentions a German case with one voltage-operated ELCB per machine-tool, where operators were seen rigidly following the instruction of pressing to test before starting their work.


TN-C-S in urban (cabled) networks was - as others have mentioned - presumably post-WWII, when new cable designs were used, the sheaths of earlier cables having provided a free extra conductor. Trusty lead-sheathed cables, with their sheath and possible armour in contact with soil (any covering being water-permeable) also gave good earthing of the network's neutral-point by forming basically a network of earthing electrodes. The change to plastic-sheathed cables has changed the earth-potential rise on LV networks in the event of faults from the primary side of the transformer, which I know has concerned some countries. Dangers coming in from the higher voltages are a situation for which TT typically has an advantage over even TN-S, as it makes the installation earthing independent - at least until insulation breakdown or surge-protection operation.  Thinking the other way (about how TN-S isn't always the ideal in every way), safety to 230 V appliances against getting an extreme voltage is an advantage of TN-C-S with multiple earthing in contrast to TN-S.

 

I sympathise with the point about the depth of understanding in the old days. The main concerns of earthing methods have been discussed and analysed for over a century. Situations have changed (and so has terminology!), so old works aren't necessarily very useful in their conclusions; but there's a lot of old explanation that people nowadays who come fresh to the problems would find useful. There are several good old works I'd like to share, including the ERA ones, but can't as they're still in copyright, or at least the official scans are claimed to be. Anyone with IET library credits to be spent could look here for the 1937 and 1941 ERA papers. More impressive is the effort by Gosland (1950) to do a quantitative comparison of all the dangers of all the options. Inevitably it is built on a raft of assumptions, but it has some good discussion and plots. I'm sure I've pointed it out here before, sometime, mentioning the author's response to a questioner who noted the shakiness of assumed numbers ("... 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.").  Doubtless very good analyses of many of the issues were done in Germany or its neighbours, well before the fact-finding studies of the 1930s ERA reports.  One advantage in the old days was that people had to choose their analyses carefully and do them by hand, instead of putting a very detailed test-system into a simulation program and hoping to get a general answer out. Understanding was needed from the start.


Regarding the mentioned foundation-earthing in Germany (or separate electrodes): this can be of some use against a PEN-fault in any case, since domestic loads most of the time are small. But it's particularly useful with 3-phase supply, where the typical neutral current is only a few amps instead of tens. Major loads such as cookers, water heaters etc traditionally don't even have a neutral connection in countries that have 3-phase installations as normal. Then the neutral really is just a balancer for a few small loads, meaning that a super-good earthing isn't necessary to control its voltage if the supply neutral gets a fault. In the UK situation it wouldn't take many loads together to reach a dangerous neutral voltage when the current has to pass through a modest electrode and normal soil.