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.

 

[Cookie]

After the main disconnect neutral and protective earth wires are separate (code wise) with only a few exceptions most of which have been eliminated.

I say code wise, because there are TONS of existing installations where sub-panels in the same building are 3 wire or 4 wire instead of 4 wire or 5 wire. This was never allowed by code even 100 years back, however before the rise of the internet there were many misconceptions on what code allowed or did not allow.

Further, before the 1997 code you could use the neutral conductor to ground large appliances- and prior to 2008 subpanels in separate buildings which created the misconception it was ok for most everything else.

 

[Cookie]

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.

No MV neutral? Certainly your MV cable must be screened.

In the US we have two types of systems mains as follows:

5-10% of the nation is fed from 3 phase 3 wire MV distribution. Outside of California this is typically due to legacy reasons.

90-95% is via a 4 wire multi gorunded wye. There is a 4th MV neutral which is grounded a minimum of 4 times per mile, at every transformer and at splice joint. 3/4 of the time the transformer is Y grounded primary, 1/4 the times delta or ungrounded wye primary.

 

[R-fur]

3.3kV and up wards we only use three wires, but the source transformers have an earth connection, either to the star point if it is a star winding or via an earthing transformer if it is delta. Cable screens are connected to earth.

 

[Julie.]

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:


View attachment 54561


View attachment 54562


View attachment 54563

Open delta made sense to us, however in the UK we wouldn't use it, we may have two transformers across two phases, but the secondaries would be treated as two different supplies.

You do mention another aspect that was alien to us, and that was connecting a distribution transformer between phase and earth/neutral - absolutely not in the UK, all distribution transformers are between phases (single) or all three in delta.

Although primary transformers have a star (wye) winding, the star is earthed and not used. That way if there is an earth current, it can only be from a fault, so easily detected.

Florida power and light most certainly had three phase transformers (3 primary windings) where two phases came from one ohl, and the third came from another ohl.

 

[Cookie]

Open delta made sense to us, however in the UK we wouldn't use it, we may have two transformers across two phases, but the secondaries would be treated as two different supplies.

You do mention another aspect that was alien to us, and that was connecting a distribution transformer between phase and earth/neutral - absolutely not in the UK, all distribution transformers are between phases (single) or all three in delta.

Although primary transformers have a star (wye) winding, the star is earthed and not used. That way if there is an earth current, it can only be from a fault, so easily detected.

Florida power and light most certainly had three phase transformers (3 primary windings) where two phases came from one ohl, and the third came from another ohl.

Honestly, its a bit alien to me in that I don't agree with it. But thats how its done and few question it. This paper however does though it could be written a bit better:

http://www.ecs.csun.edu/~bruno/MultiGroundedNeutralFinal_4-17-7.pdf

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.

 

[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.