Earthing question - TNC in Bulgaria - DIY

[ivantheterrible]

Hi all,

For clarity, I'm not a qualified electrician, but have a reasonable amount of experience as I've done plenty of wiring (signed off by a proper sparky) as a farm manager in the UK.
I now have a house in Bulgaria which we bought some five years ago and have been slowly modernising during any holidays. We have now moved over full time and I want to rewire as the wiring here is ancient and of very very dubious quality. Anyone who's seen old Bulgarian house wiring will know the sort of thing. Flat two core wire fixed with nails between cores, two or three china and wire fuses for the whole system and lights that piggy-backed off of the two pin sockets. The sockets themselves are either not earthed in some cases or share a joined negative / earth back to the fuse board. I originally put in a small CU to separate some of stand alone items such as the washing machine and cooker but had to follow the same combined earth neutral as most of the wire is two core. I now wish to put in a decent CU and new three core wiring throughout, making the system a TN-C-S one. I don't trust the old wiring as the house is 100+ years old and some of the wire is at probably 50 going by the old cloth style insulation tape used in many places. I'm worried that if one of the earth / neural cores break, items will become live, so it needs doing ASAP.

So, to my questions -
Starting at the meter box outside of the property, I'm not sure of the integrity of the earth system itself as there appears to be no earth rod at our post (we are a fair way from the transformer or other properties, so I would have expected one. To add to that, it looks like there should be an earth rod as the connection is there, but it goes nowhere - (see the pictures) Apparently, they sometimes going "missing". Would it be a sensible idea to install my own at this post or within my property, or would I risk creating an earth loop? Bear in mind our nearest neighbour is nearly a km away and the transformer even further. Of course, I could check with the supplier, but have so far found that asking them anything seems pretty fruitless.
Then, on the rewiring. I intend to stick with a radial circuit rather than ring, as I'm pretty sure that's against the reg's here. If so, can I combine both TN-C and TN-C-S on separate radials as I go along or will this cause issues? I intend to do the whole lot, but it may take a while, hence the combination until all is done.
Sorry, for the multiple questions, and thanks for any advice.

 

[pc1966]

I'm not sure who on this forum would know much about eastern European electrics, maybe @Lucien Nunes is the best bet.

Almost certainly an earth rod is a good idea, as is making it TN-C-S by separating out the N & E within the installation.

Since EU and rest of world sockets are unfused, the supply breaker has to be 16A, maybe 20A max, so there is no real point in making a ring circuit. That is done in the UK as a cost effective way of allowing a 32A supply for the sockets, thus increasing the number that is sensible to allow through greater diversity of loads (though we don't actually limit it), while keeping cable size and cost down and adding an element of CPC (earth) redundancy. But it depends on fused plugs as the UK (and a few other places) have to make sense.

 

[Rockingit]

I'm scheduled to be in Bulgaria in a couple of weeks (Sofia) - may/not have a few spare hours in the afternoon if so - is that of any use to you??

 

[ivantheterrible]

I'm scheduled to be in Bulgaria in a couple of weeks (Sofia) - may/not have a few spare hours in the afternoon if so - is that of any use to you??

Thanks for the offer but we're just about as far as it's possible to be from Sofia whilst still being in Bulgaria.

 

[Lucien Nunes]

The more of the neutrals and earths you can separate and the nearer the origin you can do so, the better. There's no harm doing that alongside existing combined neutral/earths so long as it is clear what's what.

Adding a rod makes sense, it's not going to have a low enough Ra to prevent all risk from touch voltage in the event of an open supply PEN but it's better than not having one.

TN-C is scary when unfamiliar but has been quite widespread and still recognised and used until recently in locations with high technical standards. A well-executed 1990s TN-C installation in Finland for example might not be quite so alarming as a Soviet-era one in BG. The devil is in the detail.

 

[Marvo]

I still come across TN-C installations living in Africa and even being familiar with the arrangement it still gives me the heebiejeebies.

If none of yor neighbours have TN-C-S / PME'd installations the danger of just you having one is that your earth rod could end up carrying large currents under certain supply fault conditions.

I'd suggest you might consider making it a TT installation (with RCD protection) by putting in a convincing earth rod, at least 4 or 5 meters deep and even 2 or 3 separate rods linked together with a wire the same size as your incoming supply or larger. Also if you've got a concrete slab with rebar or a steel outbuilding then connect to that as well. If you've got a bore hole with a steel pipe or even a wire boundary fence with steel poles or posts that's also fair game to include, the way to get a good earth is to be a sneaky barstard and take advantage of what's there.

The way we do it here is low tech using plenty of local labour. Dig the first meter by hand so you know you're not going to hit a supply cable or water main then use 2.4 meter long by 25mm rods that thread together knocked in with a 2-man slide hammer that weighs about 20 kilos. When the first rod is hammered in we use a brass threaded coupling to attach the second rod on top of it and sometimes even a third rod if the ground allows.

 

[pc1966]

@Marvo has excellent advice on earth rods, make sure you know there are no cables, pipes, sewer, etc, before driving them in! Also deeper is usually better so they are in permanently damp earth, however, your local soil characteristic really will determine what is practical.

However, I would say not to TT the installation unless you have fully separated E & N within the installation otherwise under open-PEN fault conditions you could find some "earthed" metal inside your home is live (at 230V or vaugely in that region) on the TN-C side while other metal that is TT will be on true Earth potential of 0V. Nasty!

If you keep it TN-C-S then in the UK at least the earth bonds to extraneous conductive parts such as service pipes and earth rods should be at least 10mm copper (or equivalent conductivity in other metals, 16mm Al or 80mm Fe, etc) to carry the high and persistent fault currents that open-PEN faults give rise to.

 

[pc1966]

Bear in mind our nearest neighbour is nearly a km away and the transformer even further.

Here you are unlikely to have other's current on an open-PEN fault, only your own installation.

I don't know how much lightning you get in that area but you might want to consider fitting the much more expensive type 1+2 SPD for overhead feeds, for example:

Type 1+2 Lightning Current and Surge Arrester (KM1+2-25-1+1 SC) - Kingsmill Industries

System: Single Phase TT/2 Pole Nominal Voltage: 230V AC

kingsmillindustries.com

That is suitable (and specified for) TT use but equally can be used on TN-S/TN-C-S, so it would allow any future decision on TT'ing the installation to go ahead without needing replacement.

 

[ivantheterrible]

@Marvo has excellent advice on earth rods, make sure you know there are no cables, pipes, sewer, etc, before driving them in! Also deeper is usually better so they are in permanently damp earth, however, your local soil characteristic really will determine what is practical.

However, I would say not to TT the installation unless you have fully separated E & N within the installation otherwise under open-PEN fault conditions you could find some "earthed" metal inside your home is live (at 230V or vaugely in that region) on the TN-C side while other metal that is TT will be on true Earth potential of 0V. Nasty!

If you keep it TN-C-S then in the UK at least the earth bonds to extraneous conductive parts such as service pipes and earth rods should be at least 10mm copper (or equivalent conductivity in other metals, 16mm Al or 80mm Fe, etc) to carry the high and persistent fault currents that open-PEN faults give rise to.

Thanks, that makes a lot of sense.

 

[ivantheterrible]

Thanks, that makes a lot of sense.

Hi again,
I'm now starting to think that it's actually a TT system I have, but frighteningly I can't find any earthing source on the property! I've included some pictures ... One and two, show the two single core cables coming in from overhead. The meter box is outside the property and was pictured earlier in this thread. Pictures three and four show the source cables coming into the fuse box, where the neutral is horribly combined with all the neutral lines coming in from plugs and lights ... These are piggy backed off of two main branches. Picture five shows the worst of the worst. Most connections are better than this, but it shows what I'm dealing with!

PS .... It looks like the pictures have uploaded in the reverse order

Any thoughts?

 

[pc1966]

Hi again,
I'm now starting to think that it's actually a TT system I have, but frighteningly I can't find any earthing source on the property! I've included some pictures ... One and two, show the two single core cables coming in from overhead. The

I one of the overhead lines (or the PEN after the supply meter) goes to the box/earth as it looks in your photo, then it is TN-C supply.

meter box is outside the property and was pictured earlier in this thread. Pictures three and four show the source cables coming into the fuse box, where the neutral is horribly combined with all the neutral lines coming in from plugs and lights ... These are piggy backed off of two main branches. Picture five shows the worst of the worst. Most connections are better than this, but it shows what I'm dealing with!

PS .... It looks like the pictures have uploaded in the reverse order

Any thoughts?

I'm guessing the externally accessible breaker on the meter box isolates the lot?

If so, please check it does! Really, check properly.

Then rip out the internal fusebox and start fresh! If much of the internal wiring is TN-C then you can't use RCDs[*] but you can at least start with a new CU that you populate with MCBs while TN-C and then change later to RCBOs as you convert internal circuits. The old wires can be extended to the new CU to begin with and as you replace circuit the proper cable brought in. I guess Bulgaria as its own T&E equivalent, probably with same-size CPC, but anything would be better!

[*] RCD on TN-C are prohibited in the UK. They don't work well as any CPC to CPC/Earth contact is likely to trip them, and you are also disconnecting the CPC along with N, which opens up other "interesting" fault situations that you might not want to encounter.

Edit: And do add an earth rod. If TN-C / TN-C-S it will help a lot, if you go TT eventually it is essential.

 

[ivantheterrible]

I one of the overhead lines (or the PEN after the supply meter) goes to the box/earth as it looks in your photo, then it is TN-C supply.

I'm guessing the externally accessible breaker on the meter box isolates the lot?

If so, please check it does! Really, check properly.

Then rip out the internal fusebox and start fresh! If much of the internal wiring is TN-C then you can't use RCDs[*] but you can at least start with a new CU that you populate with MCBs while TN-C and then change later to RCBOs as you convert internal circuits. The old wires can be extended to the new CU to begin with and as you replace circuit the proper cable brought in. I guess Bulgaria as its own T&E equivalent, probably with same-size CPC, but anything would be better!

[*] RCD on TN-C are prohibited in the UK. They don't work well as any CPC to CPC/Earth contact is likely to trip them, and you are also disconnecting the CPC along with N, which opens up other "interesting" fault situations that you might not want to encounter.

Edit: And do add an earth rod. If TN-C / TN-C-S it will help a lot, if you go TT eventually it is essential.

Thanks, I've actually found what I think must have been the earth source (See new picture below) .... but it's been cut off? I'll replace this with a new earth (Earth rod or maybe use the metal water piping as I know we have a long run "of over 200' uninterrupted" and no nearby neighbours to cause feedback issues?)

So it looks like I have a TT supply but with TN-C internal wiring? I've bought a typical UK type populated CU over from the UK, which I'll fit in the next few days. But; I'll split it to connect the TNC wiring on one side through MCB's with co-joined earth / neutrals, and on the other TT + Earth (still co-joined to incoming N) but with RCD protection as I slowly swap over the wiring from two core (L + NE) to three core (L+N+E).
Does that sound a sensible plan? Yes, I should get a professional in I know, but over here, it's the wild west! Many "electricians" are very dodgy and considering what I've seen so far, I'd prefer to trust myself until I can get it checked by someone I'd trust, which I'll do as soon as I can.
I'll post pictures as I go, so that you can either shoot me down in flames or give me more excellent critical appraisal / advice .... Thanks

 

[pc1966]

This PDF has a useful background on the earthing systems used for UK domestic supplies:

https://tinyurl.com/2hjutwaw

 

[pc1966]

Thanks, I've actually found what I think must have been the earth source (See new picture below) .... but it's been cut off? I'll replace this with a new earth (Earth rod or maybe use the metal water piping as I know we have a long run "of over 200' uninterrupted" and no nearby neighbours to cause feedback issues?)

All "extraneous conductive parts" should be bonded to your earth anyway, this means metallic service pipes or steelworks that are in contact with the true Earth. This applies to any supply system as you want to avoid finding an appliance with metalwork on one CPC system being at a different voltage to something else you can touch on another.

In the UK at least you are not allowed to rely on a water pipe as a means of earthing, basically because it can be changed at any time by a plumber or the water supply company and it will likely be replaced by plastic (or plastic joint) so it may become un usable and by folks who know little of, and have no requirement to, deal with electrical aspects.

So while you might have a fantastic earth due to a long length of private metal pipe, you probably should add a dedicated rod and that would be the basis for designing your system if TT. Basically on a TT system you measure the rod Ra, if that is low enough to be stable (UK regs below 200 ohms) that can then be used to select means of fault disconnection. almost always a RCD (or RBCO) as you are very, VERY unlikely to have a rod able to trip a 32A MCB or whatever.

So it looks like I have a TT supply but with TN-C internal wiring?

No, if the CPC is linked to the supply neutral by a wire it is TN.

If those wires are separate at source it is TN-S, if there is only one wire (N & CPC) then it is TN-C

It is only if your CPC system is only linked to the supply through the Earth itself by means of a rod, buried foundations, etc, that you have a TT system.

I've bought a typical UK type populated CU over from the UK, which I'll fit in the next few days. But; I'll split it to connect the TNC wiring on one side through MCB's with co-joined earth / neutrals, and on the other TT + Earth (still co-joined to incoming N) but with RCD protection as I slowly swap over the wiring from two core (L + NE) to three core (L+N+E).
Does that sound a sensible plan?

I'm not sure I quite understand what you are proposing. More worryingly, I'm not sure you understand it either!

Trying to have a mixed earthing system seems to me to be a recipe for disaster if something goes wrong on the supply side, then TN-C objects go live and you could be touching that along with a true Earthed TT object from another circuit.

If it were me (who knows a bit about what I am doing) I would try to make it a bit like a UK TN-C-S arrangement where you treat the supply meter box as TN-C, then you split for TN-S (as the DNO does here) along with additional earthing and bonding of water pipes on the CPC from that point. Within your CU I would put existing TN-C circuits NE on N bar & L from MCBs, and migrate as many as you can, as quickly as practical, to 3-wire TN-S using L & N off RCBOs and the CPC to the E bar.

Yes, I should get a professional in I know, but over here, it's the wild west! Many "electricians" are very dodgy and considering what I've seen so far, I'd prefer to trust myself until I can get it checked by someone I'd trust, which I'll do as soon as I can.
I'll post pictures as I go, so that you can either shoot me down in flames or give me more excellent critical appraisal / advice .... Thanks

On this forum we cannot condone attempting work beyond your capabilities as this appears to be. Replacing a CU really is a job for a professional electrician, none of us want to hear of a fatal accident as a result.

However from what you currently have, and if that is representative of "professional" work in many cases, I can see you are between a rock and a hard place. In your situation it might be worth looking at attending a courses to learn a bit more and get some advice, or trying to find local help if they are better than whoever did this. At the very least you need to have some test tools to check stuff before you switch it, and to check it really is dead before you try to work on anything (i.e. safe isolation). A proper voltage tester / continuity tester is a critical thing to have for safe isolation. Also some half-decent socket tester, etc.

 

[pc1966]

If you really want to be TT earthed, then once all circuits are migrated to TN-S (3 wire) and all are on RCBOs (or otherwise RCD protected) you could unlink your CPC from the supply PEN and rely on your rod.

There can be reasons for wanting TT if a broken PEN is likely and you might be in contact with the Earth and your CPC system, but the down-side is you become dependant on the RCD aspect for fault disconnection and they are more complicated than MCB's thermal-magnetic trips.

Edit: More on TT earth considerations:

TT earthing considerations - Electrical

tinyurl.com

 

[ivantheterrible]

All "extraneous conductive parts" should be bonded to your earth anyway, this means metallic service pipes or steelworks that are in contact with the true Earth. This applies to any supply system as you want to avoid finding an appliance with metalwork on one CPC system being at a different voltage to something else you can touch on another.

In the UK at least you are not allowed to rely on a water pipe as a means of earthing, basically because it can be changed at any time by a plumber or the water supply company and it will likely be replaced by plastic (or plastic joint) so it may become un usable and by folks who know little of, and have no requirement to, deal with electrical aspects.

So while you might have a fantastic earth due to a long length of private metal pipe, you probably should add a dedicated rod and that would be the basis for designing your system if TT. Basically on a TT system you measure the rod Ra, if that is low enough to be stable (UK regs below 200 ohms) that can then be used to select means of fault disconnection. almost always a RCD (or RBCO) as you are very, VERY unlikely to have a rod able to trip a 32A MCB or whatever.


No, if the CPC is linked to the supply neutral by a wire it is TN.

If those wires are separate at source it is TN-S, if there is only one wire (N & CPC) then it is TN-C

It is only if your CPC system is only linked to the supply through the Earth itself by means of a rod, buried foundations, etc, that you have a TT system.


I'm not sure I quite understand what you are proposing. More worryingly, I'm not sure you understand it either!

Trying to have a mixed earthing system seems to me to be a recipe for disaster if something goes wrong on the supply side, then TN-C objects go live and you could be touching that along with a true Earthed TT object from another circuit.

If it were me (who knows a bit about what I am doing) I would try to make it a bit like a UK TN-C-S arrangement where you treat the supply meter box as TN-C, then you split for TN-S (as the DNO does here) along with additional earthing and bonding of water pipes on the CPC from that point. Within your CU I would put existing TN-C circuits NE on N bar & L from MCBs, and migrate as many as you can, as quickly as practical, to 3-wire TN-S using L & N off RCBOs and the CPC to the E bar.

On this forum we cannot condone attempting work beyond your capabilities as this appears to be. Replacing a CU really is a job for a professional electrician, none of us want to hear of a fatal accident as a result.

However from what you currently have, and if that is representative of "professional" work in many cases, I can see you are between a rock and a hard place. In your situation it might be worth looking at attending a courses to learn a bit more and get some advice, or trying to find local help if they are better than whoever did this. At the very least you need to have some test tools to check stuff before you switch it, and to check it really is dead before you try to work on anything (i.e. safe isolation). A proper voltage tester / continuity tester is a critical thing to have for safe isolation. Also some half-decent socket tester, etc.

Thanks
The one thing I did do when we bought the house was to go around bonding things.

 

[ivantheterrible]

All "extraneous conductive parts" should be bonded to your earth anyway, this means metallic service pipes or steelworks that are in contact with the true Earth. This applies to any supply system as you want to avoid finding an appliance with metalwork on one CPC system being at a different voltage to something else you can touch on another.

In the UK at least you are not allowed to rely on a water pipe as a means of earthing, basically because it can be changed at any time by a plumber or the water supply company and it will likely be replaced by plastic (or plastic joint) so it may become un usable and by folks who know little of, and have no requirement to, deal with electrical aspects.

So while you might have a fantastic earth due to a long length of private metal pipe, you probably should add a dedicated rod and that would be the basis for designing your system if TT. Basically on a TT system you measure the rod Ra, if that is low enough to be stable (UK regs below 200 ohms) that can then be used to select means of fault disconnection. almost always a RCD (or RBCO) as you are very, VERY unlikely to have a rod able to trip a 32A MCB or whatever.


No, if the CPC is linked to the supply neutral by a wire it is TN.

If those wires are separate at source it is TN-S, if there is only one wire (N & CPC) then it is TN-C

It is only if your CPC system is only linked to the supply through the Earth itself by means of a rod, buried foundations, etc, that you have a TT system.


I'm not sure I quite understand what you are proposing. More worryingly, I'm not sure you understand it either!

Trying to have a mixed earthing system seems to me to be a recipe for disaster if something goes wrong on the supply side, then TN-C objects go live and you could be touching that along with a true Earthed TT object from another circuit.

If it were me (who knows a bit about what I am doing) I would try to make it a bit like a UK TN-C-S arrangement where you treat the supply meter box as TN-C, then you split for TN-S (as the DNO does here) along with additional earthing and bonding of water pipes on the CPC from that point. Within your CU I would put existing TN-C circuits NE on N bar & L from MCBs, and migrate as many as you can, as quickly as practical, to 3-wire TN-S using L & N off RCBOs and the CPC to the E bar.

On this forum we cannot condone attempting work beyond your capabilities as this appears to be. Replacing a CU really is a job for a professional electrician, none of us want to hear of a fatal accident as a result.

However from what you currently have, and if that is representative of "professional" work in many cases, I can see you are between a rock and a hard place. In your situation it might be worth looking at attending a courses to learn a bit more and get some advice, or trying to find local help if they are better than whoever did this. At the very least you need to have some test tools to check stuff before you switch it, and to check it really is dead before you try to work on anything (i.e. safe isolation). A proper voltage tester / continuity tester is a critical thing to have for safe isolation. Also some half-decent socket tester, etc.

Thanks, I appreciate your advice.
One of the first things I did when we bought the house was to go around bonding things as best I could. (water pipes, metal sinks ect). Basically, the electrics here are @hite, and I've had a few shocks, zaps or whatever you want to call non fatal electrical tinglings?
I have a reasonable understanding, but as stated I'm no professional. I do have the basics like a multimeter to check if something is live, but none of the expensive kit like earth testing meters ect.
The thing here is, that all of the sockets ect are supplied with two core cable. Neutral and earth are combined from the fuse box on. I'm planning to totally rewire and whilst I originally thought t was a TN-C supply due to the N+E linking at the fuse box, it seems there's no earth other than what might be supplied at the transformer which is over a Km away (The source cables are single core L+N and no not neutral sheaved with earth btw). So I really suspect I need a more local earth? I understand the UK situation with water pipes, but chances are mine won't be changed for years and if they were I'd certainly know as we have a two acre plot in a very rural location. I have an earth rod, but it's a single 5/8th one I bought out from the UK to service a shed rather than a whole house, so the water pipe, though not unacceptable in the UK seems a better option here in rural BG?
Whilst the split box sounds weird, I may have not explained it very well. I can't put the circuits that have combined E+N through an RCB protected system, but the circuits that I rewire as TT will require it. As it stands at the moment, all the wiring goes through a couple of old fashioned ceramic fuses and then into 16a MCB's. That's fine to protect cables, but does nothing to protect me! My thinking is to migrate separate circuits as I rewire across to the RCD protected side as I go.
I know that confusing two earthing systems is not great, but short ripping the whole lot out in one go (I wish!) I'm not sure it can be helped?
I'm a rubbish at explaining my thinking as the wiring is in this house.
PS .... I totally understand nobody wants to hear that some --- has zapped himself to death because of what's been said on here. Thanks

 

[pc1966]

Thanks, I appreciate your advice.
One of the first things I did when we bought the house was to go around bonding things as best I could. (water pipes, metal sinks ect). Basically, the electrics here are @hite, and I've had a few shocks, zaps or whatever you want to call non fatal electrical tinglings?

Bonding exposed metal objects (those with any sort of electrical function, and service pipes) together is a very good start and was common practice in the UK before RCD became cheap and ubiquitous.

I have a reasonable understanding, but as stated I'm no professional. I do have the basics like a multimeter to check if something is live, but none of the expensive kit like earth testing meters ect.

A multimeter is better than nothing, but make damn sure it is at least CAT-III rated to 300V so it won't explode in your face if you make a mistake like trying to measure volts on amps range, etc.

However, multimeters are frowned upon for safely and reliably detecting if something is live because they can do so many jobs, and some of those might also have a fuse that can be blown and you don't realise it. A simple mistake that can be fatal is checking for AC volts on the DC range and seeing only a fraction of what is there. Conversely, they are often around 1M ohm impedance so they can should voltages due to capacitive coupling that are not significant.

That is what the simple voltage testers are worth getting. they do one basic job reliably, and draw 2-3mA to light the LEDs so don't show the "phantom voltages" that multimeters report. As they can be had for £10 on Screw fix it is worth getting one and using as your main test. But always check your tester works before (and ideally after) checking something is not live. A quick check on a known good socket or similar (some show 12V so even car battery will do) should light it up and then your next test is better proven.

The thing here is, that all of the sockets ect are supplied with two core cable. Neutral and earth are combined from the fuse box on. I'm planning to totally rewire and whilst I originally thought t was a TN-C supply due to the N+E linking at the fuse box, it seems there's no earth other than what might be supplied at the transformer which is over a Km away

That is your earth! And that is why TN-C installations (supply) normally have multiple earths added at various points for reliability.

(The source cables are single core L+N and no not neutral sheaved with earth btw). So I really suspect I need a more local earth? I understand the UK situation with water pipes, but chances are mine won't be changed for years and if they were I'd certainly know as we have a two acre plot in a very rural location. I have an earth rod, but it's a single 5/8th one I bought out from the UK to service a shed rather than a whole house, so the water pipe, though not unacceptable in the UK seems a better option here in rural BG?

Use both. The pipe should be bonded anyway and likely will be VERY good, but another rod will not do any harm. Just be careful where it is driven in!

Whilst the split box sounds weird, I may have not explained it very well. I can't put the circuits that have combined E+N through an RCB protected system, but the circuits that I rewire as TT will require it. As it stands at the moment, all the wiring goes through a couple of old fashioned ceramic fuses and then into 16a MCB's. That's fine to protect cables, but does nothing to protect me! My thinking is to migrate separate circuits as I rewire across to the RCD protected side as I go.
I know that confusing two earthing systems is not great, but short ripping the whole lot out in one go (I wish!) I'm not sure it can be helped?

It seems you are either confused by the two types, or are going to try and mix-and-match within one installation which is a very bad idea.

Going from TN-C to TN-C-S is a fairly simple step, as the CU using MCBs is fine for the N+E of existing circuits and what you are doing it having a known safe and reliable N-E link before the CU (UK practice [*]) and as peossible you separate out N+E of final circuits so you can put those N & L on RCD/RCBO protection.

To go TT safely you must have all circuits CPCs isolated from the N, otherwise it has additional risks you don't currently have.

[*] In the USA they link N&E inside the first/main panel/CU as they often have "two-wire" supplies like you see, though in practice 3 wires as 120-0-120V. Sub-panels are then separate N & E routes so 3/4 wire.

I'm a rubbish at explaining my thinking as the wiring is in this house.
PS .... I totally understand nobody wants to hear that some --- has zapped himself to death because of what's been said on here. Thanks

Above all take care and never assume anything, verify! And get help/advice as far as you possibly can.

 

[Lucien Nunes]

the source cables coming into the fuse box, where the neutral is horribly combined with all the neutral lines coming in from plugs and lights

OK. This terminal stud is really what defines the setup and it is a pretty standard feature of Soviet-era installations. If one of the two incoming single-core cables is connected to it, that cable (the supply neutral) is deliberately being used to earth the fuse box chassis and therefore hopefully anything else that is supposed to be earthed. That makes it a TN-C or TN-C-S system, or at least it should.

From that point onwards, loads and circuits might have their earths connected three different ways:
a) Using a 2-core cable with one combined core from that stud serving both neutral and earth: TN-C
b) Using a 3-core cable with two separate cores from that stud serving neutral and earth: TN-C-S
c) Not connected back to that stud, but to a separate rod: TT

Mixing circuits (a) and (b) on the same system isn't itself dangerous. One wants as many (b) as possible because they can be RCD protected and are inherently safer than (a) even without RCD. Mixing circuits (c) with either (a) or (b) is dangerous in itself, before anything is even switched on, because of the risk of the rod earth being at a different potential to the incoming CNE.

If you trawl pictures of Soviet panels on the WWW, you will find plenty of examples of the CNE terminal stud or bar with umpteen cables attached, looking insecure and often on the brink of failure through overheating or just falling off. Paradoxically, it's the one point at which you really don't want a bad connection on TN-C systems as loss of continuity causes supposedly earthed bits to become live.

If any of the above is not 100% clear and obvious, I'd suggest some further detailed study in the principles and practice of earthing before tackling any alterations. One could make things worse rather than better.

 

[ivantheterrible]

OK. This terminal stud is really what defines the setup and it is a pretty standard feature of Soviet-era installations. If one of the two incoming single-core cables is connected to it, that cable (the supply neutral) is deliberately being used to earth the fuse box chassis and therefore hopefully anything else that is supposed to be earthed. That makes it a TN-C or TN-C-S system, or at least it should.

From that point onwards, loads and circuits might have their earths connected three different ways:
a) Using a 2-core cable with one combined core from that stud serving both neutral and earth: TN-C
b) Using a 3-core cable with two separate cores from that stud serving neutral and earth: TN-C-S
c) Not connected back to that stud, but to a separate rod: TT

Mixing circuits (a) and (b) on the same system isn't itself dangerous. One wants as many (b) as possible because they can be RCD protected and are inherently safer than (a) even without RCD. Mixing circuits (c) with either (a) or (b) is dangerous in itself, before anything is even switched on, because of the risk of the rod earth being at a different potential to the incoming CNE.

If you trawl pictures of Soviet panels on the WWW, you will find plenty of examples of the CNE terminal stud or bar with umpteen cables attached, looking insecure and often on the brink of failure through overheating or just falling off. Paradoxically, it's the one point at which you really don't want a bad connection on TN-C systems as loss of continuity causes supposedly earthed bits to become live.

If any of the above is not 100% clear and obvious, I'd suggest some further detailed study in the principles and practice of earthing before tackling any alterations. One could make things worse rather than better.

Thanks, I get what you are saying and it makes sense.