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.