Hi guys. To those in countries with TNC-S. What are your requirements regarding installation of earth rods for domestic installs?

[LastManOnline]

Hi Guys. Firstly, am glad to join the group. Secondly I am very curious to hear from those who have TNC-S as their electrical supply I have 2 questions (1) Are you required to install an earth rod (stake or mesh) on the homeowners property? and (2) what are your regulations regarding its resistance? Many thanks in advance.

 

[ElectroChem]

Australia uses a TNC-S variant we call Multiple Earthed Neutral (MEN).

We are required to install an earth electrode at each installation, tied to Supply Neutral at the main switchboard.

The resistance from the main earth bar to the earth electrode or any equipotential bonding point is required to be <0.5ohms.
Circuit CPC resistance is based on fault loop impedance, basically the PEC has to coordinate with the active to allow disconnection inside required time.

 

[LastManOnline]

Australia uses a TNC-S variant we call Multiple Earthed Neutral (MEN).

We are required to install an earth electrode at each installation, tied to Supply Neutral at the main switchboard.

The resistance from the main earth bar to the earth electrode or any equipotential bonding point is required to be <0.5ohms.
Circuit CPC resistance is based on fault loop impedance, basically the PEC has to coordinate with the active to allow disconnection inside required time.

Thank you. And the actual resistance of the earth rod itself? Does it provide an effective return path in the event of a broken neutral on the DSO, s side?

 

[ElectroChem]

Thank you. And the actual resistance of the earth rod itself? Does it provide an effective return path in the event of a broken neutral on the DSO, s side?

The resistance to general mass of earth is generally not required to be measured, the stake just has to be pounded ~1m in.
Some installations like substations have ground resistivity measurements in their construction standards.
The electrode provides enough return to trip RCDs if there is a broken neutral, but not always breakers. We have had electrocutions from broken neutrals causing touch voltage on metal piping. One of the reasons our AS3000:2018 standard now mandates all circuits have RCD protection on any new house or new works in an old house.

 

[LastManOnline]

The resistance to general mass of earth is generally not required to be measured, the stake just has to be pounded ~1m in.
Some installations like substations have ground resistivity measurements in their construction standards.
The electrode provides enough return to trip RCDs if there is a broken neutral, but not always breakers. We have had electrocutions from broken neutrals causing touch voltage on metal piping. One of the reasons our AS3000:2018 standard now mandates all circuits have RCD protection on any new house or new works in an old house.

Thanks again electrochem. You appear to have a very similar situation to ourselves here in Ireland. I have questioned my own regulatory body about this one (no satisfactory reply so far) because in reality if the DSO, s Neutral breaks, then bonded metalwork in the home becomes live. Any person touching this is not protected by rcd, s. If you draw out the fault path, bizarre as it sounds, the rcd won't detect an imbalance.That raises the question of what role the earth rod is, designed to play?

 

[ElectroChem]

Thanks again electrochem. You appear to have a very similar situation to ourselves here in Ireland. I have questioned my own regulatory body about this one (no satisfactory reply so far) because in reality if the DSO, s Neutral breaks, then bonded metalwork in the home becomes live. Any person touching this is not protected by rcd, s. If you draw out the fault path, bizarre as it sounds, the rcd won't detect an imbalance.That raises the question of what role the earth rod is, designed to play?

I think you'll find the fault path will still cause an imbalance in the RCD. Any current path except out on the active and back on the neutral tail from the RCD will be detected. The broken DSO neutral is upstream of the RCD, so should not affect it.

 

[Risteard]

The Earth rod in a neutralised installation is there as another Earth reference for the DSO's (ESB) PEN conductor. It is deemed to comply if it is 16mm at 1.2m depth (5/8ths inch and 4ft). So other than for larger installations what RECI are telling you is correct under the Rules.

Now I fully realise that in rocky areas of Donegal etc. this could give you about 600 Ohms on the spike electrode.

Can't think what the soil's like off-hand around Castlegregory or North Kerry.

 

[R-fur]

If the supply neutral breaks then all the bonded metal in a house will become live, but as it is all bonded together there is minimal shock risk. It is when you take a supply out of the equipotential zone ie an extension into the garden that you introduce a hazard.

 

[LastManOnline]

I think you'll find the fault path will still cause an imbalance in the RCD. Any current path except out on the active and back on the neutral tail from the RCD will be detected. The broken DSO neutral is upstream of the RCD, so should not affect it.

Thanks

The Earth rod in a neutralised installation is there as another Earth reference for the DSO's (ESB) PEN conductor. It is deemed to comply if it is 16mm at 1.2m depth (5/8ths inch and 4ft). So other than for larger installations what RECI are telling you is correct under the Rules.

Now I fully realise that in rocky areas of Donegal etc. this could give you about 600 Ohms on the spike electrode.

Can't think what the soil's like off-hand around Castlegregory or North Kerry.

Thank you guys for your responses.

Risteard.I have spoken with RECI and the ECSSA and both maintain that the earth electrode is a "back-up". My point is a backup for what? Check what happens when the ESB, s neutral breaks? If you draw out the fault path of someone touching bonded metalwork that has become live you will see that the rcd, s won't trip. Hence I don't see how the earth electrode compensates in any way by providing an alternative fault path.

Hi ElectroChem,
You are correct in your conclusion regarding a standard line to earth fault. Any imbalance between "active" (live I assume") and N will operate the rcd. But the fault path that occurs during a broken neutral when someone touches live metalwork won't cause an imbalance in the rcd. The reason been that the bonded metalwork (now live) is connected to the incoming Neutral (now broken and hence live) upstream of the rcd

 

[Risteard]

Risteard.I have spoken with RECI and the ECSSA and both maintain that the earth electrode is a "back-up". My point is a backup for what? Check what happens when the ESB, s neutral breaks? If you draw out the fault path of someone touching bonded metalwork that has become live you will see that the rcd, s won't trip. Hence I don't see how the earth electrode compensates in any way by providing an alternative fault path.

This is why I mentioned an Earth reference. It's not really a backup, but one of the electrodes tying the ESB PEN conductor to Earth to try to minimise the voltage on it with respect to Earth.

Now it could be demonstrated with calculations that with any sort of appreciable load it will rise unless we can ensure a very low impedance indeed. Industrial etc. installations do have specific Rules about measuring and verifying this, but as I mentioned for domestic and small commercial etc. installations then it is a deemed to satisfy the Rules type situation whereby you have to sink a galvanised 16mm (5/8") rod 1.2m (4') as per ET101:2008 (4th Edition).

I'm still working through I.S. 10101:2020 (5th Edition). The bits about electrodes seems to be spread through the Standard more. But there is a transition period in place anyway.

 

[LastManOnline]

If the supply neutral breaks then all the bonded metal in a house will become live, but as it is all bonded together there is minimal shock risk. It is when you take a supply out of the equipotential zone ie an extension into the garden that you introduce a hazard.

Hi R-fur,
All valid points.Its interesting to have your input as you in the UK (I assume) usually don't install earth electrodes for TNC-S installations. Which is an interesting angle because while we in Ireland, Aus, NZ are now discussing the purpose of the earth electrode in a TNC-S system, we could just as well ask "How is, it that in the UK, they seem to manage just fine without them"?

 

[Risteard]

"How is, it that in the UK, they seem to manage just fine without them"?

I would caution against that view. The UK is flying in the face of what the rest of the world does in this regard, and it is likely that they will at some point require electrodes on TN-C-S systems. In fact this was proposed in the runup to the 18th Edition, but the requirement was withdrawn before final publication.

 

[R-fur]

I don know what regulations your supply company's have to work to but in the UK there are requirements to earth the neutral at many points on the network. I worked for a local supply company for over 20 years and I can only think of one time where there was an issue. Having said that I think a spike at the customers premises is a good idea. I think the reason it was not included in the latest regs was down to difficulties in defining which properties require a spike (what do you do in flats, tenements, multi stories etc)

 

[LastManOnline]

This is why I mentioned an Earth reference. It's not really a backup, but one of the electrodes tying the ESB PEN conductor to Earth to try to minimise the voltage on it with respect to Earth.

Now it could be demonstrated with calculations that with any sort of appreciable load it will rise unless we can ensure a very low impedance indeed. Industrial etc. installations do have specific Rules about measuring and verifying this, but as I mentioned for domestic and small commercial etc. installations then it is a deemed to satisfy the Rules type situation whereby you have to sink a galvanised 16mm (5/8") rod 1.2m (4') as per ET101:2008 (4th Edition).

I'm still working through I.S. 10101:2020 (5th Edition). The bits about electrodes seems to be spread through the Standard more. But there is a transition period in place anyway.

I think you hit the nail on the head with "unless we can ensure a very low impedence". This was the point I was eventually hoping to introduce. I did, nt mention it initially as I feel the whole topic of earthing is challenging enough and I wanted to take it one step at a time.
If we take the standard earth rod, as ElectroChem pointed out, you will never trip an mcb under fault conditions. An rcd yes. However imagine having an earth that had a low enough resistance (impedence) that we could achieve this?. We had that in urban areas 25 years, ago due to the metallic services supplying each home (water etc). We still occasionally come across older homes where the old metallic services have a resistance of less, than an ohm. In fact in some areas a broken neutral has gone unnoticed because the earthing supplies such an effective return path. We came across one last year when "Irish Water" installed a new water meter (plastic) in the water main, cutting through the copper pipe.Homeowner was then without power. This in turn exposed the broken neutral.
This leads me to think that if we are to supply an earth to a home it needs to be of low enough resistance to be able to offer an effective return path should the DSO, s neutral break. A single earth rod is simply not able to achieve this.

 

[Risteard]

You need to think of it more in terms of minimising touch voltages than to ensure operation of any protective devices.

 

[R-fur]

As per Risteard you are basically bonding local earth to your earth bar to minimise danger.

 

[davesparks]

we could just as well ask "How is, it that in the UK, they seem to manage just fine without them"?

We don't manage just fine without them and we are very much behind the rest of the world on this point.
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The Earth rod in a neutralised installation is there as another Earth reference for the DSO's (ESB) PEN conductor. It is deemed to comply if it is 16mm at 1.2m depth (5/8ths inch and 4ft). So other than for larger installations what RECI are telling you is correct under the Rules.

Only 4' deep? That seems a bit too short to me, 5/8" at 8' long generally will give a far more stable connection due to the depth.
At 4' the vast majority of the rod will be affected by seasonal variations and not be so stable.

 

[LastManOnline]

We don't manage just fine without them and we are very much behind the rest of the world on this point.
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Only 4' deep? That seems a bit too short to me, 5/8" at 8' long generally will give a far more stable connection due to the depth.
At 4' the vast majority of the rod will be affected by seasonal variations and not be so stable.

Hi Dave,
Thanks for your comment. Could you elaborate on why you feel as you do? Specifically what advantages, do you see in having a, rod installed?

 

[Risteard]

Only 4' deep? That seems a bit too short to me, 5/8" at 8' long generally will give a far more stable connection due to the depth.
At 4' the vast majority of the rod will be affected by seasonal variations and not be so stable.

I agree, but for domestic installations that's the standard. I suppose it's a cost/benefit type thing. Galvanised rods are used and these aren't extendable - at least the ones commonly used here. For larger installations an Earth nest or the like may be required.

 

[davesparks]

Hi Dave,
Thanks for your comment. Could you elaborate on why you feel as you do? Specifically what advantages, do you see in having a, rod installed?

Because broken PEN faults on the DNO system can and do happen, and when they do they can be incredibly dangerous.

The advantage I see, as long as earth electrodes are properly installed in accordance with a well written regulation, is that the danger under a broken PEN fault will be greatly reduced.
Initially when the regulation comes in to effect there will be little difference but, after a few years, and as more and more installations have earth electrodes installed the overall effect will be to create a network where the dangers under PEN fault conditions are reduced.

If the regulations were to require an earth electrode system of <20ohm Ra connected to the first consumers earth terminal after the point of connection to the DNO's PME supply (this kind of wording should avoid anyone trying to connect earth rods to a 10th storey flat) then once multiple installations have them connected the combined effect could easily be <2 ohms.
So for the majority of PEN faults which affect more than one installation we end up with a very low resistance path to earth connected and minimised danger.

Of course then we will need a means of alerting people to the fact that the supply neutral has failed.
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I agree, but for domestic installations that's the standard. I suppose it's a cost/benefit type thing. Galvanised rods are used and these aren't extendable - at least the ones commonly used here. For larger installations an Earth nest or the like may be required.

I've never used a galvanised rod, only the copper bonded steel rods available in 4' and 8' lengths with threaded ends for screwed couplings.

 

[LastManOnline]

Because broken PEN faults on the DNO system can and do happen, and when they do they can be incredibly dangerous.

The advantage I see, as long as earth electrodes are properly installed in accordance with a well written regulation, is that the danger under a broken PEN fault will be greatly reduced.
Initially when the regulation comes in to effect there will be little difference but, after a few years, and as more and more installations have earth electrodes installed the overall effect will be to create a network where the dangers under PEN fault conditions are reduced.

If the regulations were to require an earth electrode system of <20ohm Ra connected to the first consumers earth terminal after the point of connection to the DNO's PME supply (this kind of wording should avoid anyone trying to connect earth rods to a 10th storey flat) then once multiple installations have them connected the combined effect could easily be <2 ohms.
So for the majority of PEN faults which affect more than one installation we end up with a very low resistance path to earth connected and minimised danger.

Of course then we will need a means of alerting people to the fact that the supply neutral has failed.
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I've never used a galvanised rod, only the copper bonded steel rods available in 4' and 8' lengths with threaded ends for screwed couplings.

I fully agree. Having a low, resistance earth electrode network is the key to dealing with the dangers of broken neutrals in a TNCS system. A low resistance return path will prevent bonded metalwork from becoming live. Those kinds of low resistance values would require not one but several rods or a mesh. It's, interesting to look back at ElectroChems comment about how people have been electrocuted in Australia under broken neutral situations DESPITE the earth rod been Installed and this is because the resistance is way to high. We have had similar issues here. So it really is, nt a question of should we or should, nt install an electrode. The question in my view is why are we not installing an earth electrode with a resistance low enough to deal with the issues raised by broken neutrals

 

[davesparks]

The question in my view is why are we not installing an earth electrode with a resistance low enough to deal with the issues raised by broken neutrals

Because the regulations don't require it and the majority of installers will always work to the minimum standard rather than the best they can achieve.

This is one thing the USA do better than us, they install an electrode system made up of 2x 8' earth rods as standard (they only need one if it is less than 25ohms to earth but they don't usually test anything and just install 2 regardless)

 

[LastManOnline]

Because broken PEN faults on the DNO system can and do happen, and when they do they can be incredibly dangerous.

The advantage I see, as long as earth electrodes are properly installed in accordance with a well written regulation, is that the danger under a broken PEN fault will be greatly reduced.
Initially when the regulation comes in to effect there will be little difference but, after a few years, and as more and more installations have earth electrodes installed the overall effect will be to create a network where the dangers under PEN fault conditions are reduced.

If the regulations were to require an earth electrode system of <20ohm Ra connected to the first consumers earth terminal after the point of connection to the DNO's PME supply (this kind of wording should avoid anyone trying to connect earth rods to a 10th storey flat) then once multiple installations have them connected the combined effect could easily be <2 ohms.
So for the majority of PEN faults which affect more than one installation we end up with a very low resistance path to earth connected and minimised danger.

Of course then we will need a means of alerting people to the fact that the supply neutral has failed.
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I've never used a galvanised rod, only the copper bonded steel rods available in 4' and 8' lengths with threaded ends for screwed couplings.

Hi Dave,
Just saw that I did, nt reply to you on this one. I think your summary is 90% good, but requires one adjustment. Your thought that multiple installations installing individual earth rods will lower the overall resistance and make a more effective return path under broken PEN circumstances.True if the broken Pen is some distance from your home, but if it happens outside your home then ADS is entirely dependent on the resistance of YOUR electrode.

Anyway, I have appreciated all the comments, and it's been interesting to note how the role of the earth electrode still seems to be a matter of debate and to some degree even controversy

I thought I,d leave the final word on the matter to John Ward, whose views I, ve come to respect,

Regarding the common belief that the earth rod is a backup for a broken PEN he states.."If you thought your rcd, s will operate when the DSO, s neutral breaks, you would be WRONG.." (Utube video)

Regarding the earth electrode in a TNC-S system... " to have any real effect the electrode would need a VERY low resistance, well under 10 ohms. In practice that will be next to impossible with a single rod.. "

 

[davesparks]

True if the broken Pen is some distance from your home, but if it happens outside your home then ADS is entirely dependent on the resistance of YOUR electrode.


Regarding the earth electrode in a TNC-S system... " to have any real effect the electrode would need a VERY low resistance, well under 10 ohms. In practice that will be next to impossible with a single rod.. "

Yes but the chances of a broken PEN occurring at all are very small, the chances of it occurring in such a location that it affects only a single installation are a vanishingly small fraction of the chances of it happening in the first place.
You cannot guard against absolutely every fault, but you can guard against the vast majority, and the fact that a fault could occur affecting just one installation is not a valid reason to not protect against faults which could affect multiple installations.


The simple answer to your last comment is to install more than one rod if a single rod is not sufficient, just look at the USA where they generally install 2x 8' rods minimum for a house service, and I think they space them at least 16' if I remember correctly (their code allows a single rod if it has a suitably low Ra, but they never do any testing so they just install 2 rods).

 

[LastManOnline]

Yes but the chances of a broken PEN occurring at all are very small, the chances of it occurring in such a location that it affects only a single installation are a vanishingly small fraction of the chances of it happening in the first place.
You cannot guard against absolutely every fault, but you can guard against the vast majority, and the fact that a fault could occur affecting just one installation is not a valid reason to not protect against faults which could affect multiple installations.


The simple answer to your last comment is to install more than one rod if a single rod is not sufficient, just look at the USA where they generally install 2x 8' rods minimum for a house service, and I think they space them at least 16' if I remember correctly (their code allows a single rod if it has a suitably low Ra, but they never do any testing so they just install 2 rods).

Yes the chances are small but as you said in one of your previous posts "They can and do happen.. and are incredibly dangerous".

You may have misunderstood what I am proposing. I am critical of the current earth electrode arrangement not because I don't think they should, nt be ther. I think they are in fact vital. It's just that the current arrangement of a 4 foot rod is not fit for purpose.

The American system is certainly better, (two 8ft rods) but how much better?. They don't test so they have no idea if the arrangement functions.

 

[7029 dave]

My option with a TNCS, is that an earth mat or nest should be linked to every such earthing arrangement.

 

[davesparks]

My option with a TNCS, is that an earth mat or nest should be linked to every such earthing arrangement.

Whilst it would be a very good earth, it would be very hard to implement in any existing building, and incredibly expensive for small installations such as houses.
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You may have misunderstood what I am proposing. I am critical of the current earth electrode arrangement not because I don't think they should, nt be ther. I think they are in fact vital. It's just that the current arrangement of a 4 foot rod is not fit for purpose.

The American system is certainly better, (two 8ft rods) but how much better?. They don't test so they have no idea if the arrangement functions.

I agree that a single 4' rod is not fit for purpose. Even a single 8' rod would be a big improvement when you consider the first 2' to 3' can be affected by seasonal variations quite significantly.

The problem we would have is that if you make the requirements too unwieldy more and more people will ignore them for the smaller installations, the requirements need to be realistically practical as well as improving safety.

I believe that for all new builds and extensions a requirement to connect an earth to the reinfircing in the foundations would help the situation greatly, however I think this would require something put into the building regulations to ensure that it happens. Electrician's often don't get involved until after the foundations have been poured, and builders/groundworkers won't be in any rush to help out even if the electrician has spoken to them in time, it would need to be in the building regs and be part of the LABC inspection of the foundations.

 

[7029 dave]

Whilst it would be a very good earth, it would be very hard to implement in any existing building, and incredibly expensive for small installations such as houses.

Yes I agree the cost would be expensive, but we should start somewhere, maybe with new builds for starters.
The dangers of a broken PEN can be catastrophic.

 

[LastManOnline]

My option with a TNCS, is that an earth mat or nest should be linked to every such earthing arrangement.

Agreed. That would be the ideal solution
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Yes I agree the cost would be expensive, but we should start somewhere, maybe with new builds for starters.
The dangers of a broken PEN can be catastrophic.

Agreed. This is not
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Agreed. That would be the ideal solution
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Agreed. This is not an issue that can be ignored indefinately

 

[LastManOnline]

Whilst it would be a very good earth, it would be very hard to implement in any existing building, and incredibly expensive for small installations such as houses.
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I agree that a single 4' rod is not fit for purpose. Even a single 8' rod would be a big improvement when you consider the first 2' to 3' can be affected by seasonal variations quite significantly.

The problem we would have is that if you make the requirements too unwieldy more and more people will ignore them for the smaller installations, the requirements need to be realistically practical as well as improving safety.

I believe that for all new builds and extensions a requirement to connect an earth to the reinfircing in the foundations would help the situation greatly, however I think this would require something put into the building regulations to ensure that it happens. Electrician's often don't get involved until after the foundations have been poured, and builders/groundworkers won't be in any rush to help out even if the electrician has spoken to them in time, it would need to be in the building regs and be part of the LABC inspection of the foundations.

Agreed. The solutions have to be realistic. This issue was, nt caused overnight and certainly wo t be solve overnight. I think first step needs, to be an industry wide recoqnition of the issue and then a realistic balanced approach to solving it. For instance, as previously mentioned, underground TNCS supplies are much less at risk. I would suggest beginning with looking at measures first in areas with overhead supplies which tend to pose the greatest risks.