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.

 

[Mike Johnson]

Yes the readings are from the Rod to Earth with the lead disconnected from the Panel using a dedicated tester, there are also certain requirements for putting in multiple earth rods.

 

[davesparks]

Yes the readings are from the Rod to Earth with the lead disconnected from the Panel using a dedicated tester, there are also certain requirements for putting in multiple earth rods.

The tester doesn't have to be a designated earth electrode tester, it can be a multi-function tester which has no single designated purpose.
Not all test methods require the rods to be disconnected.

Bear in mind we are discussing American regs here so UK rules don't apply,

 

[Mike Johnson]

Oh all right then, does not have to be a dedicated tester, but does have to have the right settings, the only way to get a correct reading of the earth rod resistance is to disconnect it from the panel, use the voltage and potential extra rods at 25M and 50% and connect it up to the correct type of reading tester, take readings at that setting, then move the centre rod 2M either side and take two more readings, then find the average of each reading and this will be the true earth resistance of the earth rod, and that does not make a jot of difference if it's in the USA the UK or mainland Europe, but does make a difference if its' been raining.

And bear in mind that on my first post I was comparing the requirements in France.

 

[davesparks]

Oh all right then, does not have to be a dedicated tester, but does have to have the right settings, the only way to get a correct reading of the earth rod resistance is to disconnect it from the panel, use the voltage and potential extra rods at 25M and 50% and connect it up to the correct type of reading tester, take readings at that setting, then move the centre rod 2M either side and take two more readings, then find the average of each reading and this will be the true earth resistance of the earth rod, and that does not make a jot of difference if it's in the USA the UK or mainland Europe, but does make a difference if its' been raining.

And bear in mind that on my first post I was comparing the requirements in France.

That is not the only possible method of testing, and I believe the method you have described is incirrect for a 3 wire fall of potential test, but I would need to get the books out to confirm.

 

[Mike Johnson]

Yes so would I, but I think it's pretty close, the post was just off the top of my head and could be slightly off.

 

[LastManOnline]

Interesting discussion compared to France where nearly all systems are TT, requirement for earth rod is mandatory and must be below 100ohms, preferable below 10ohms if a surge arrestor is fitted, I have never seen anything below 8ohms, but then never tried to get a reading lower than that as it satisfies the requirements at 10ohms.

Came across this thread by accident. Was curious to know the standard type of earth rod(4 ft.) used in France and how many would typically need to be sunk in order to meet the 100 ohm requirement?

 

[Mike Johnson]

That all accords on the ground conditions ,if as in my case you have clay below a stable layer then a 1m rod may do, but to be on the safe side at least 2m, in my case I got a resistance reading of under 100Ω which was treated as suspect there is a very big problem sinking more than one rod within the sphere of influence between each rod and connecting them together.

 

[LastManOnline]

That all accords on the ground conditions ,if as in my case you have clay below a stable layer then a 1m rod may do, but to be on the safe side at least 2m in my case I got a resistance reading of under 100Ω which was treated as suspect.

Would 100 ohm on a 2mt rod E regarded as average (expected) in your locality?

 

[Mike Johnson]

In my locality yes, but is unusual as I am on clay and not as the rest of the Dodogne on Granite, very hard to put a good earth rod into Granite therefor the use of copper mats come into play.

 

[LastManOnline]

In my locality yes, but is unusual as I am on clay and not as the rest of the Dodogne on Granite, very hard to put a good earth rod into Granite therefor the use of copper mats come into play.

Below 10 ohms for a surge arrestor is a new one for me. But then we dont really see TT here. You can't help but wonder how many issues would be solved if we all had a TNS supply system

 

[LastManOnline]

Cookie the panel is probably 200 amps and the wire size should be 2/0 copper. Aluminum wiring would be 4/0 aluminum. We install 2 ground rods regardless of the resistance to ground. I have never read 25 ohms from the panel to the rod. It’s always less than 1 ohm. The # 6 wire is correct and if the conduit is metal yes it has to be bonded to the panel by means of a grounding bushing

Hi Megawatt. Am curious about your comment that you "never read 25 ohms from the panel to the rod. It's always less than 1 ohm".
2 questions
1) Does that means that YOU take fault loop impedence tests even though as Cookie mentioned earlier you are not obliged to do that if you have sunk 2 rods?
2) Am I correct to conclude that you always get a reading of less than one ohm every time you sink 2 rods? ie you have never had a situation where you found it necessary to sink a third rod in order to achieve a reading of less than 1 ohm?

 

[pc1966]

Below 10 ohms for a surge arrestor is a new one for me.

It is commonly seen in lighting protection guides. Not quite sure why that specific value though. For example, page 57 of this document:

https://www.erico.com/catalog/literature/E907W-WWEN.pdf

 

[davesparks]

Hi Megawatt. Am curious about your comment that you "never read 25 ohms from the panel to the rod. It's always less than 1 ohm".
2 questions
1) Does that means that YOU take fault loop impedence tests even though as Cookie mentioned earlier you are not obliged to do that if you have sunk 2 rods?
2) Am I correct to conclude that you always get a reading of less than one ohm every time you sink 2 rods? ie you have never had a situation where you found it necessary to sink a third rod in order to achieve a reading of less than 1 ohm?

Read his post again, he has never had a reading over 1ohm from panel to rod, that's the resistance of the copper or aluminium conductor between the panel and the rod. He's not talking about the rod's resistance to Earth.

 

[LastManOnline]

Read his post again, he has never had a reading over 1ohm from panel to rod, that's the resistance of the copper or aluminium conductor between the panel and the rod. He's not talking about the rod's resistance to Earth.

?

It is commonly seen in lighting protection guides. Not quite sure why that specific value though. For example, page 57 of this document:

https://www.erico.com/catalog/literature/E907W-WWEN.pdf

Good read that. Thanks for posting it.