Earth Rods in TN-C-S and TN-S

[Cookie]

Why aren't earth rods required on a TN-C-S or TN-S supply? In the US earth rods are mandatory for both. Two earth rods if the first one is over 25 ohms.

Here is how its typically done:

View: https://youtu.be/4vxYXzNtE9A?t=3459



#6 copper (13.3mm2) is run from the neutral bar of the first disconnect after the meter socket, down in conduit and connected to earth rods at least 6 feet apart. In this case he also bonds the conduit coming from the meter. Service looks to be 225amps in what is probably 4/0 (107.2mm2) copper.

 

[DPG]

Why aren't earth rods required on a TN-C-S or TN-S supply? In the US earth rods are mandatory for both. Two earth rods if the first one is over 25 ohms.

Here is how its typically done:

View: https://youtu.be/4vxYXzNtE9A?t=3459



#6 copper (13.3mm2) is run from the neutral bar of the first disconnect after the meter socket, down in conduit and connected to earth rods at least 6 feet apart. In this case he also bonds the conduit coming from the meter. Service looks to be 225amps in what is probably 4/0 (107.2mm2) copper.

I'm sure in a recent thread one of our American members said earth rods weren't routinely tested when they were installed.

 

[Wilko]

Why aren't earth rods required on a TN-C-S or TN-S supply? In the US earth rods are mandatory for both.

Hi - the local earth rod is not mandatory in UK BS7671 for TNS and TNCS as the UK local network operator may be relied upon. It’s not contrary to regulation to add local rods to these earthing types and in some cases it may be necessary.

 

[Julie.]

What is your typical Ze in a tn-c-s or tn-s in the US?

 

[Cookie]

What is your typical Ze in a tn-c-s or tn-s in the US?

No idea, I've never measured it but I'd guess very low. Not uncommon for the transformer to be only 20 feet away and a 1/0 AL URD or AL drop.

Here are our typical perspective fault currents:

https://www.alabamapower.com/content/dam/alabamapower/Business/Services%20by%20Industry/Architects%20&%20Engineers/A-E-Fault-Currents-Tables-FINAL-8-2003.pdf

How do they compare with the UK?

Hi - the local earth rod is not mandatory in UK BS7671 for TNS and TNCS as the UK local network operator may be relied upon. It’s not contrary to regulation to add local rods to these earthing types and in some cases it may be necessary.

What do you mean by may be relied upon?

 

[R-fur]

As above, plus in the UK you never earth the neutral in a domestic situation. In a customers installation the neutral is only earthed if it fed from a private transformer or generator.

 

[Cookie]

As above, plus in the UK you never earth the neutral in a domestic situation. In a customers installation the neutral is only earthed if it fed from a private transformer or generator.

What do you mean by never earth the neutral? When the neutral comes in from the utility it must be earthed. Also, up until recently you could run a 3 wire feed to a detached building (provided there was no phone and plumbing between them) The neutral must also be earthed at the remote building:

Code now requires a 4 wire system in all installations, but again you MUST have a ground rod at both the service and remote building:

What do you think? How does it compare?

 

[Lucien Nunes]

The ESQCR (Electricity Safety, Quality and Continuity Regulations 2002) require the supplier to provide a neutral that is solidly connected to earth. Any N-E interconnection at the customer's premises (e.g. on TN-C-S) must made by the network operator in the service terminal box and sealed from interference by the customer. The earth (if provided) and neutral leave the service terminal box separately and the customer may not earth the neutral themselves.

What I do not understand about the NEC on this is that if you have a poor connection, say in the meter socket (note: we don't have meter sockets, meters have integral screw terminals), there is a likelihood of significant installation neutral current running to ground at the rod, resulting in possibly a high voltage gradient from the rod and loss of an equipotential zone. The customer might, or might not do something about that diligently. In the UK, a broken CNE is unlikely because there is only one connection, made by the DNO with heavy duty terminals, which it can break, and if it does the DNO are obliged to attend as an emergency to rectify it.

 

[Cookie]

The ESQCR (Electricity Safety, Quality and Continuity Regulations 2002) require the supplier to provide a neutral that is solidly connected to earth. Any N-E interconnection at the customer's premises (e.g. on TN-C-S) must made by the network operator in the service terminal box and sealed from interference by the customer. The earth (if provided) and neutral leave the service terminal box separately and the customer may not earth the neutral themselves.

That doesn't make sense to me though, because as soon as you bond to water pipes, gas pipes, ect you are also grounding down the neutral because said pipes are in contact with the earth (soil) before they enter the premises.

What I do not understand about the NEC on this is that if you have a poor connection, say in the meter socket (note: we don't have meter sockets, meters have integral screw terminals), there is a likelihood of significant installation neutral current running to ground at the rod, resulting in possibly a high voltage gradient from the rod and loss of an equipotential zone. The customer might, or might not do something about that diligently. In the UK, a broken CNE is unlikely because there is only one connection, made by the DNO with heavy duty terminals, which it can break, and if it does the DNO are obliged to attend as an emergency to rectify it.

Well, the city metal water piping system (or even phone lines) might mask that giving no indication of a problem. In the US water company workers typically carry a set of jumpers when working with sections of pipe or water meters.

However if no low Z path exists back to the transformer yes the ground rod will become live as well as anything else connected to the building's grounding and bonding system. In that case voltage will also go up and down with line to neutral connected loads giving indication to the building occupants.
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I'm sure in a recent thread one of our American members said earth rods weren't routinely tested when they were installed.

Unless called for in the specs ground rods are rarely tested.

 

[R-fur]

Cookie the way things are done in the US is very different from the UK. It is always the suppliers responsibility the earth his neutral, they then present three terminals to the customer Live Neutral and Earth. Or in a TT supply just Live and Neutral. A neutral earth link in the customers installation would be seen as a fault. I believe in the US it is normal to have a transformer feeding only a couple of houses, in the UK the norm is to have a 1000kVa transformer feeding a couple of hundred houses. Also your range of voltages is very weird to us (120, 208, 240. 277, 480 etc) in the UK it is 230/400 or HV. The extensive LV networks operated by the supply company's are MEN ie multiple earthed neutral. The neutral is earthed at every joint, also neutrals of adjacent transformers are interlinked. This results in a system where the neutral is tied down to earth in a very reliable way, not just a spike at your house. In a TT system the supplier still earths his neutral but only supplies a Live and Neutral to the customer. The customer then has to supply his own earth connection for his earths only (not the neutral) so that any fault current flows to earth and back to the suppliers neutral/earth.

 

[Julie.]

So the real question is, why do you need additional earthing rods in the US, why aren't your network suppliers providing proper earthed supplies?



There are lots of differences between different countries, and this results in major differences, for example in florida I know if they have a hv/mv supply via ohl which is only say red and yellow phases, if they need to install a three phase supply to someone, they will fit a three phase transformer, and pick up a blue phase from any other ohl in the vicinity!

To us, that is pure no-no

Fpl didn't see it as anything unusual - I guess it is common over the whole of the US.

As a result, all the isolation and earthing processes we were used to were totally different and had to be re-written in line before we could issue our own permits to work!

 

[Cookie]

Cookie the way things are done in the US is very different from the UK. It is always the suppliers responsibility the earth his neutral, they then present three terminals to the customer Live Neutral and Earth. Or in a TT supply just Live and Neutral. A neutral earth link in the customers installation would be seen as a fault. I believe in the US it is normal to have a transformer feeding only a couple of houses, in the UK the norm is to have a 1000kVa transformer feeding a couple of hundred houses. Also your range of voltages is very weird to us (120, 208, 240. 277, 480 etc) in the UK it is 230/400 or HV. The extensive LV networks operated by the supply company's are MEN ie multiple earthed neutral. The neutral is earthed at every joint, also neutrals of adjacent transformers are interlinked. This results in a system where the neutral is tied down to earth in a very reliable way, not just a spike at your house. In a TT system the supplier still earths his neutral but only supplies a Live and Neutral to the customer. The customer then has to supply his own earth connection for his earths only (not the neutral) so that any fault current flows to earth and back to the suppliers neutral/earth.

But why would it be seen as a fault? I can't picture an extra ground rod as being an issue.

Correct- typically one transformer supplies 5-7 homes, a 3 phase bank 1-3 businesses and large businesses typically get their own transformer.

Regarding earthing of the LV utility neutral that is done at many points in the US.

 

[Cookie]

So the real question is, why do you need additional earthing rods in the US, why aren't your network suppliers providing proper earthed supplies?



There are lots of differences between different countries, and this results in major differences, for example in florida I know if they have a hv/mv supply via ohl which is only say red and yellow phases, if they need to install a three phase supply to someone, they will fit a three phase transformer, and pick up a blue phase from any other ohl in the vicinity!

To us, that is pure no-no

Fpl didn't see it as anything unusual - I guess it is common over the whole of the US.

As a result, all the isolation and earthing processes we were used to were totally different and had to be re-written in line before we could issue our own permits to work!

Never head that one before. I think what is being described is an open delta configuration. The primary takes two MV phases connected in open wye grounded, and the secondary is connected in delta giving 240 volts open delta, 120/240 open delta or 240 volts corner grounded open delta.

Here is an example:

 

[Lucien Nunes]

What do you think? How does it compare?

In a conventional installation e.g. domestic, combined neutral/earth conductors within the customer's wiring, e.g. a feeder (aka submain) to a sub-panel, have been prohibited for a long time and would be considered a serious fault now if found (I've never seen one in 30 years). All our TN installations are more like your second drawing, except that the main panel has separate neutral and earth bars like the sub-panel, and the link is made in the service terminals so it cannot accidentally be disconnected by anyone other than the DNO.

The rod at the main panel is optional and not usually necessary on a TN system, although the equipotential bonding (note UK meaning, different to USA) to any metallic building frame or services is mandatory as per your drawing and is either connected within the panel or the customer's separate main earthing terminal bar. I.e. we bond, but do not need to earth, a TN installation. The ESQCR is statutory legislation; the DNO are required by law to ensure that the neutral is solidly and sufficiently earthed and that any earth they provide is maintained at a low enough impedance not to need a local rod.

The rod at the sub panel in the remote building is optional if the earth conductor in the submain is large enough and meets the criteria. Alternatively, any or all panels can be made TT, using only the rod(s) if that is superior or mandatory (e.g. when supplying outlets in a marina) or if there is no earthing facility offered with the supply.

But why would it be seen as a fault? I can't picture an extra ground rod as being an issue.

.
It's not, if connected to the customer's main earth terminal or panel earth. But it must never be connected directly to neutral within the customer's installation, as the DNO have exclusive responsibility for earthing the neutral to ensure it is done correctly and falls under their legal obligation to maintain that connection.

 

[R-fur]

There is no open delta or high leg delta in the UK, only three phase delta star transformers or single phase transformers, there is some split phase but it is unusual and old. Our HV is three wire with no neutral.
Our laws/regs insist that the supply company earth their neutral and the customer does not, except as I said if it is a private transformer or generator.