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

[LastManOnline]

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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.