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

[Julie.]

....

Regarding FP&L do you have a picture? Never seen that done before but it is possible they are doing it- a million flavors for everything exist in the US lol.

No sorry, I think it was late 80's early 90's when we were installing some chp schemes in the US, when I last actually worked on it, no mobile phones then!

They were all 13.8kv generators, and nothing appeared odd at first - 13.8kv is a 3 wire system it appears, however it got back-fed via the 4.16kv system that is 4 wire, and cross connected different phases from different lines to give a three phase transformer supply, scared the "xxxxxxx xxxx" out of us.

Was back over with fhp probably about 15 years ago, and mentioned it to the engineers I was with and they said it was fairly common still.

 

[Cookie]

PM me the location if you remember it, I can check via Google Earth.

 

[Julie.]

Gosh, that was a long time ago, I remember there was a 82MVA in Bartow - a chemical plant, one a bit bigger at a rock crushing place near brooksvile, a small 20MVA in white spring - chemical company again, one about 60-70mva in a solid sewage plant St Petersburg, but they had a change a few years later and another in mulberry - think it was only around 42MVA - couple of years after we added a second unit, but I wasn't involved in that there was also one at fernando beach, I did some work on the first one, almost a copy of mulberry and would have liked to work on the upgrade in late 80's but ended on the design of connah's quay station - which didn't happen until around '93

Couldn't place them on a map though

Except connah's quay of course!

 

[Cookie]

Its alright- no compulsion

 

[Julie.]

Its alright- no compulsion

It's bringing back a few memories - good and bad!

Even dug out my old ieee red book to see if it was mentioned in there!

 

[Cookie]

You sir are well rounded

 

[pc1966]

From memory in the UK it used to be common to see a volt or two difference between the N and E of TN-S LV distribution, probably down to the N impedance and the imbalance between house's demands on the 3 phases.

In that situation if you linked N & E in the property wiring you could see large "earth loop" currents which, I would guess, could be a fire risk under extreme conditions. That has always been my understanding of why N & E must be treated as isolated for domestic supplies (as well as easier to insulation test, etc).

Of course if the DNO chooses to do so as a PME arrangement then one could assume it has been designed to be safe!

 

[Megawatt]

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.

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

 

[Mike Johnson]

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.

 

[davesparks]

I have never read 25 ohms from the panel to the rod. It’s always less than 1 ohm.

The 25 ohms would be from the rod to the actual earth, measured with a suitable earth electrode tester.
I can see why it is much easier to just install the two rods and not worry about trying to get a reading on the first rod then go back and install a second one if it's over 25 ohms.

 

[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