earthing calculation

[Sahib]

NEC just does not cover the kind of substations where earthing would matter any.

NEC purports to cover all kinds of substation. See below.

250.191 Grounding System at Alternating-Current
Substations. For ac substations, the grounding system shall
be in accordance with Part III of Article 250.

May be the modern earth fault relay is so sensitive that suitable sensitivity may be chosen to operate at an earth fault current no matter how low it may be. But for substation with moderately sensitive earth fault relay, low ground resistance may still be required for the operation of ground fault relay.

 

[Sahib]

Flat conductors are for lightning protection. They would fuse open in the presence of a long duration fault, and are not meant to clear one. Even so, the only place I have seen flat conductors is at radio tower installations. All the subs depend on the concrete slab and the conductors bond the steel to the grid in the concrete and are large, round and stranded.

Flat conductor readily dissipates high frequency energy of lightning; the ground rod dissipates the low frequency energy and so their combination is more effective than either in lightning protection.

For a ground rod? How do you get water to the bottom of a 40 foot long ground rod? Or even an 8 foot long one. Also, it's the conductivity of the entire sphere of influence that matters, not just the surface of the ground rod.

It is not water alone that is effective in reducing the ground resistance. Presence of soluble salts in the soil is also required. For that purpose, one method is :a small pit of 1' depth may be dug around the rod and common salt may be filled in. Adding water to it will make it diffuse into ground thereby gradually lowering the ground resistance.

 

[K8MHZ]

Flat conductor readily dissipates high frequency energy of lightning; the ground rod dissipates the low frequency energy and so their combination is more effective than either in lightning protection.

It is not water alone that is effective in reducing the ground resistance. Presence of soluble salts in the soil is also required. For that purpose, one method is :a small pit of 1' depth may be dug around the rod and common salt may be filled in. Adding water to it will make it diffuse into ground thereby gradually lowering the ground resistance.

The NEC does not allow that type of ground rod installation. If chemicals are to be used, they must be part of an electrolytic electrode listed for the purpose.

The NEC requires not only that at least 8 feet of a rod electrode are in direct contact with the soil, but they must be driven, not installed in a hole full of salt.

See 250.53 (G) and 250.52(A)(6).

 

[Sahib]

The NEC does not allow that type of ground rod installation. If chemicals are to be used, they must be part of an electrolytic electrode listed for the purpose.

You are correct. I failed to take into account the corrosive nature of the salt on the electrode and what the code says about that.

The NEC requires not only that at least 8 feet of a rod electrode are in direct contact with the soil, but they must be driven, not installed in a hole full of salt.
See 250.53 (G) and 250.52(A)(6).

The code prohibited method mentioned in my previous post, though successfully followed in other countries such as India, only requires a 1' pit around the ground rod, already driven, for salt filling.

 

[K8MHZ]

You are correct. I failed to take into account the corrosive nature of the salt on the electrode and what the code says about that.

The code prohibited method mentioned in my previous post, though successfully followed in other countries such as India, only requires a 1' pit around the ground rod, already driven, for salt filling.

Please remember that this is an NEC forum, so pretty much all of our answers will be based upon the NEC in one manner or another.

Now, outside the NEC, there are just plain physics involved. You mentioned salt being corrosive, you are correct. It also absorbs water and will draw moisture from the soil it is in contact with making that soil less conductive. True electrolytic electrodes are surrounded by a special soil. The salt is inside a pipe with holes in it. The pipe is the electrode, there is no rod.

In the US, we can't just make our own electrolytic ground. It has to be listed for the purpose. In your country you may be able to make your own. The basics of their construction can be found on the Internet.

 

[mivey]

I calculated the Rmat resistance using 1/0[95 sqr.mm] stranded bare copper 0.5 m depth [only 80 m total length] and 15 driven rods of 12 m [4 ft.] length and 17 mm dia and I get 3.94 ohm.[calculation according IEEE 80/2000]

I get 3.94 ohms using a 3x3 mesh (4 conductor runs in each direction, 7.33m x 6m spacing) of #1/0 CU buried 0.5m deep with 3/4" x 32 ft rods at each intersection (16 rods)

A two-layer model would probably work better because the resistivity could be much lower at the lower depths.

 

[mivey]

Isn't 12-m almost 40-ft in length? That's one hell of a ground rod!

We go deeper than that sometimes!

 

[mivey]

Generally soil resistivity is measured in ohms per cm which is a volume of soil, not just in meters which is linear, placing rods closer then twice their length will cause a low gain over installing just over this, this is because you want each rod to be out of the sphere of influence of the other rods

For uniform soil, but for multi-layer soil penetration (usually the case with long rods) the sphere is not a function of the total length.

If this is for other electrical purposes, keep in mind current does not want to go to earth, it wants to return to source from which it came, most times this is the last transformer it was fed from, except the SWER system mention above that actually uses the earth as one of the conductors of a circuit earth plays no part in the operation of electrical systems, most of these kind of requirements are built around old myths that keep on circling around in the electrical world.

Probably also because MV & HV systems can use the earth as a conductor at times and people confuse the two systems.

 

[mivey]

Regardless, every substation I have ever seen does not rely on ground rods for earthing.

Every substation I work on does (MV & HV).

The grounding at a sub station is more focused on lighting protection than line to ground faults. As such, they are overkill for the utility voltage and current capacity just by design. I have seen electrode conductors in the 750 MCM range on sub stations. Those sure aren't going to a ground rod.

We also consider the grounding during faults.

 

[mivey]

As has been stated in countless (well I did not count them anyway) threads here and times in Mike Holt's materials, an earth ground (even as low as 4 ohms) is not an effective fault clearing path at low voltages. But for MV or higher it can be. That is point 1 for low impedance earth connections at a substation.
Lightning protection is important when dealing with long runs of open transmission wires. Point 2.
And given the possibility of current leaks and even high voltages from via electrostatic or electromagnetic induction, the best possible equipotential grid is valuable. The earth resistance is not the most important part of that, but it helps. Point 3.

All good points.

 

[petersonra]

NEC purports to cover all kinds of substation. See below.

You have to understand what the code actually says to understand what it means. Yes there are a few substations that are covered by the code. But utility owned substations are specifically excluded from the code.

In any case, 250.191 (grounding of substations) refers you back to chapter 3 of article 250 that starts at 250.50 which is just the standard grounding arrangements. Nothing special at all.

May be the modern earth fault relay is so sensitive that suitable sensitivity may be chosen to operate at an earth fault current no matter how low it may be. But for substation with moderately sensitive earth fault relay, low ground resistance may still be required for the operation of ground fault relay.

Are you saying that utilities in the US routinely use sensitive GF equipment on their substations?

There are even a very few electrical systems left in the US that use the earth as one of its conductors. Not common, but I think there are still some around.

 

[mivey]

There are even a very few electrical systems left in the US that use the earth as one of its conductors. Not common, but I think there are still some around.

Not as the sole conductor, but the unbalanced currents on a multi-grounded system can be about 50% neutral conductor and 50% earth about halfway down a feeder. Near a fault to Earth the Earth% is higher of course.

 

[mivey]

Are you saying that utilities in the US routinely use sensitive GF equipment on their substations?

Yes.

 

[Sahib]

Yes.

Only static relays; no electromagnetic relays.

 

[kwired]

Please remember that this is an NEC forum, so pretty much all of our answers will be based upon the NEC in one manner or another.

Now, outside the NEC, there are just plain physics involved. You mentioned salt being corrosive, you are correct. It also absorbs water and will draw moisture from the soil it is in contact with making that soil less conductive. True electrolytic electrodes are surrounded by a special soil. The salt is inside a pipe with holes in it. The pipe is the electrode, there is no rod.

In the US, we can't just make our own electrolytic ground. It has to be listed for the purpose. In your country you may be able to make your own. The basics of their construction can be found on the Internet.

Not saying that there are not substations where NEC applies, but there are many more where it doesn't apply. Most of them probably have little regulation as far as codes related to electrical safety. They probably do follow NESC or other standards though if anything to at least be able to say they followed a recognized standard should a lawsuit come up for any reason.

 

[JoeStillman]

For high voltage substations, the NESC applies.

The objective with a substation grounding grid is to limit the step and touch potentials - the voltage a person is exposed to while a fault is flowing to ground. I checked the figures in the OP and came up with a very similar total resistance using a spreadsheet I developed from IEEE 80. My cacls show you would need to lower the soil resistance to about 160 ohm-meters to get to 4 ohms. You should be able to treat the soil and get the resistance down into that range.

The maximum allowable resistance is a function of the available ground-fault current. With approximately 6 ohms total resistance, your step voltages are within tolerances up to about 12,000 amps of available fault current. With a 4-ohm grid, the tolerable ground fault is about 18,000 amps. If you know the availble fault current, you may be able to use the IEEE 80 methods to show that your system is adequzte at 5.95 ohms.

 

[kwired]

For high voltage substations, the NESC applies.

Is it truly adopted as the law like the NEC often is? Or does the industry just follow the NESC in general?

I can easily see them not having any law that says they must follow a particular standard, who is making sure they follow such laws if they are there? Most people that are required to follow the NEC also have an AHJ that does make sure they are following it to some extent, usually through permits and inspection processes.

Only time it would come up for most power companies would be in a lawsuit where someone was trying to put them at fault and using violation of such codes to help win their case. Otherwise it becomes a situation of policing themselves by striving to follow some well known standard for the very purpose of reducing liability, if most of the industry agrees with an installation practice and has that practice published in a standard, it would be hard to claim they did something wrong.

The POCO around my neck of the woods have no such regulations that I am aware of yet they still are interested in safety as well as have the need to protect themselves from liabilities and choose to follow the NESC for at least the majority of what they do.

 

[GoldDigger]

The POCO around my neck of the woods have no such regulations that I am aware of yet they still are interested in safety as well as have the need to protect themselves from liabilities and choose to follow the NESC for at least the majority of what they do.

NEC is enforced directly by building codes and indirectly by OSHA.
POCO would still have to satisfy OSHA with their own safety program even if they are are not separately required to follow NESC.

 

[mivey]

Only static relays; no electromagnetic relays.

Both. But digital relays are replacing the EM relays.

 

[hurk27]

For high voltage substations, the NESC applies.

The objective with a substation grounding grid is to limit the step and touch potentials - the voltage a person is exposed to while a fault is flowing to ground. I checked the figures in the OP and came up with a very similar total resistance using a spreadsheet I developed from IEEE 80. My cacls show you would need to lower the soil resistance to about 160 ohm-meters to get to 4 ohms. You should be able to treat the soil and get the resistance down into that range.

The maximum allowable resistance is a function of the available ground-fault current. With approximately 6 ohms total resistance, your step voltages are within tolerances up to about 12,000 amps of available fault current. With a 4-ohm grid, the tolerable ground fault is about 18,000 amps. If you know the availble fault current, you may be able to use the IEEE 80 methods to show that your system is adequzte at 5.95 ohms.

I'm kind of confused with your math, at 6 ohms a fault current using only earth as a return path of 12KA would require a voltage drop of 72K volts, At the 3' shell mark you would have 75% of this voltage that a person could be in contact with or 54KV, in no way could you achieve a low enough impedance with earth to prevent a safe voltage drop, this is why a equal potential grid in a substation must be bonded to the return path of it's source by a low impedance path, at 12ka this would require that this low impedance path to be at least .005 ohms or less which would give you a 60 volt touch/step potential between the grid and earth until the GF relay cleared the circuit, adding another load of 4 ohms to this 60 volts would not drop it any further at least noticeable if you consider .005 volts off the 60 volts much with 12ka behind the 60 volts.

Again as I said before earth would not play much of a roll in protection against touch/step potential.

We forget that the idea behind the EPG is not to provide a lower impedance to earth, but to bring any person who might be in the area of the fault to the same voltage of the source of the fault, bird on a wire theory, when you reach the edge of the grid you have the same problem and is why they taper the grid around a substation deeper once it leaves the fenced in area, depending upon the available source voltage will determines how far the EPG tapers outside of the fenced in area.