earth impedance

[karl riley]

I am starting a new thread as an offshoot of the grounding electrode thread. Brian John, you mentioned that there would be problems if earth resistance were very low. So here is the info I am going by at present. I am open to correction.

The resistance of earth is reported by university researchers to be negligible; practically speaking, zero. This is because the cross-sectional area is unlimited.

When we measure earth impedance we are just measuring the impedance of the electrode/earth interface. Once it gets into the earth, further impedance is negligible.

This is the only way the SWER systems work for long distance primary distribution lines using the earth as the only return path to remote locations. They have to keep the transformer grounding electrodes to low values (I think it is 5 ohms). Not as efficient as three phase with neutral, but workable. They do it to save in cost of installation.

Brian, I agree that there would be problems if our grounding electrode impedances were very low, since neutral would be coursing all over the neighborhood, not just in the water pipes. I had a thread on this but got little discussion.

So does anyone have any experimental data to either confirm or deny this earth impedance info? It was a surprise to me.

Karl

 

[c-h]

Re: earth impedance

I don't have any experimental data. I will just repeat what I have been told or have read.

Anyway, you are quite correct. The resistance of the earth is very low. The only figure I have seen is 0.05 ohms/km and then as "Allow for 0.05 ohm/km when calculating voltage drop on SWER systems". Despite the low value, it poses a limitation on how long you can make SWER lines. The current will reach very deep down in the earth, hundreds or thousands of meters. The depth depends on the resistivity of the soil, which varies widely with different soils. There is some reactance from the separation of the current carrying paths (yes, I just made that term up ) when the earth serves as a conductor.

Like you say, the bulk of the earth resistance is from the earth electrode/earth interface. This has prompted some reasearch into ways of reducing this resistance. This has revealed one interesting fact: The resistance drops quickly with depth. Drive an 8 ft rod and you have a poor conductor. Drive an 80 ft rod and you have a good conductor. Drive it 800 ft and you have a superconductor (Please, don't take that last one literally.)

I have been trying to figure out ways of reducing the resistance in the interface between earth electrode and earth. My latest idea is drilling a deep well and filling it with conductive concrete...

Not as efficient as three phase with neutral, but workable. They do it to save in cost of installation.

What is your definition of efficiency? If you can achieve the same result at less cost, I would say it's more efficient.

 

[charlie b]

Re: earth impedance

Lets look at our use of terms here, and more will become clear. The resistance of ?earth? may be low, but only if by that term you mean ?planet Earth.? By contrast, the resistance of dirt is high.

I?m no longer have access to a laboratory, but I would offer the following conjecture to anyone who wishes to take on an experiment. Take a 10 foot section of 3? PVC, and fill it with dirt. Compact it in as tightly as you can. You can even experiment with different types of soil and different amounts of moisture in the dirt. For each sample, connect a ohmmeter?s probe into the dirt at each end of the pipe, and measure the resistance. My conjecture is that the answer will always be on the order of hundreds of ohms, and will likely be much higher.

Would this conjecture seem to be inconsistent with the notion that the resistance of earth is low? Not at all! Look at it this way: Connect a 1000 ohm resistor to a 9 volt battery. What current do you get? Next connect, in parallel to the 1000 ohm resistor, a set of 10,000,000 other resistors, each being the same 1000 ohms. What happened to the current? Sending current through planet Earth is like connecting a battery to a vast great number of parallel resistors. As C-H has already said, ?The current will reach very deep down in the earth, hundreds or thousands of meters.? You could look at it another way also: resistance is proportional to length and inversely proportional to area. If the current path has a diameter of ?hundreds or thousands of meters,? then the area is huge, and the resistance will be insignificant.

 

[brian john]

Re: earth impedance

http://www.groundperfect.com/deepframe1.htm


I haven't read all of the above link, so I do not nrcessarly endorse the page. But if you care read away.

 

[dereckbc]

Re: earth impedance

Brian John, thanks for posting the Web Site. I read that article back in 95 when developing standards for MCI.

Although I do agree deep earth provides low resistance when measured by a three-point measurement, you have to consider the frequency of interest. My point here is if you have to drive a rod 40 feet, and lets say only the last 10 feet are in good conductive soil, that leaves you with a 30-foot conductor. If the frequency of interest is say 1 Mhz, like lightning, the impedance of the bonding jumpers, GEC, and 30-foot of rod negate even a 1 ohm GE.

IMO the best GE is a Ufer if designed correctly. I am not referring to the concrete encased electrode cited in 250. If the UFER is employed GEC and bonding jumpers lengths are shortened considerable minimizing the high frequency impedance.

 

[pierre]

Re: earth impedance

Dereck
If you are not refering to the NEC Ufer, exactly what are you refering to? Please explain, as this is very interesting.

Also, lets say we do get a really low impedance to earth and in this instance the earth happens to be a very good conductor. What do you think will happen the the neutral current that normally travels back to the transformer via the grounded conductor?

Pierre

 

[charlie b]

Re: earth impedance

Nothing will happen. The grounded (i.e., neutral) and ground (i.e., GEC) are connected to each other only at the main panel. There is no complete path for current to travel through planet Earth, no matter how good a connection you have, and no matter how low the resistance of earth.

 

[dereckbc]

Re: earth impedance

Pierre, I do not want to hy-jack the thread to discuss UFER. If you will start a new thread or write me a PM, I will be happy to explain.

As for your second question, I disagree with Charlie. The current in the grounded conductor would decrease. The reason is assumming the transformer has the neutral grounded, and again at the service.

 

[c-h]

Re: earth impedance

I agree with Dereck on the current in the neutral. I would be very suprised if the transformer is not grounded. It's possible, but I find it unlikely. The transformer is required to be grounded in the European systems, with some exceptions. Even if it is not, the other premises served by the same transformer will ground the transformer through the neutral / grounded conductor. It's possible to omit all intentional connections between the neutral and earth but then you have a system with a ungrounded neutral.

Another option is to use a separate grounding conductor in the entire system, connected to the grounded conductor /neutral only at the transformer. This system was once standard in underground services in Great Britain, and has been revived in recent years in other countries for its merit of reducing EMF. (The new UG work I see here in Stockholm, Sweden, appears to have a separate grounding conductor.) With insulation monitoring to detect unintentional connections between the grounded and the grounding conductor, you could in theory reduce the current in the earth to a minimum.

 

[c-h]

Re: earth impedance

I might add that the different grounding arrangements have been the topic of newspaper articles and parliamentary debates in the past few years here. The magnetic field in Stockholm has been measured to an average of 0.4 microtesla, with peak values of 6 microtesla.

 

[charlie b]

Re: earth impedance

Dereck and C-H: I don?t think that I am envisioning the same physical setup as the two of you are envisioning. Perhaps you can describe the circuit, the complete circuit, through which current will flow, the circuit that includes one or more ground rods, and that also includes planet Earth.

But let me first assert that a facility might have a dozen transformers, each establishing a separately derived system, and therefore each having its neutral connected (via a GEC) to a grounding electrode, and thus to planet Earth. I further assert that there will be no current through planet Earth from the ground rod of any transformer to the ground rod of any other transformer. There is no complete path. Keep in mind that you would not connect the neutral of any transformer to the neutral of any other with a grounded (neutral) conductor. The neutrals will be electrically connected (bonded) via the grounding electrode system, which in turn is in contact with planet Earth in many places. But this arrangement forms only one leg of a possible path. There is no other leg.

Am I missing something here?

 

[bennie]

Re: earth impedance

Will someone please explain the difference between a premises delta wye power system and a utility delta wye distribution system?

A utility delta wye is also a MGN system, which pumps a lot of net current into the earth. Why doesn't the premises system also do this?

 

[iwire]

Re: earth impedance

This is all very interesting guys thanks.

Charlie at a simple temp service I might install at a job site, I bond the neutral and drive two rods.

The POCO bonds and grounds the Neutral at their end.

Wouldn't some of the neutral current from my temp service flow through the earth and back to the POCO transformer?

 

[karl riley]

Re: earth impedance

I just got back to check the responses to my question. Great responses!

Answer to what I meant by a SWER line being less efficient than a normal 3-phase with neutral, I meant that the power loss is more, hence it costs more to operate. The savings in installation is what they are after.

Charlie B., if the earth is in parallel with the service neutral, by way of the electrode at the service and the electrode at the transformer, then current obviously flows in both paths. If we could get the grounding electrode impedance down to negligible (I think a bad idea) then the current through the earth would rival the current in the neutral.

I know you understand parallel paths, so what were you talking about?

Now I am going to read that link and see what it says. Be back later.

Karl

 

[karl riley]

Re: earth impedance

I'm back already. Brian, that link perpetuates the misunderstanding about the functions of the grounding electrode. They say that the electrode is for safety from shock etc. I have noticed that companies which sell grounding systems perpetuate the myth for their own economic interests. It amounts to a scam. Of course, whoever wrote the link may just be ignora nt himself and think he has a good sales pitch.

I will go back and read the whole link for whatever actual data it may contain after I calm down. But Dereck has already brought out the fallacy of deep grounding when it comes to lightning.

Karl

 

[karl riley]

Re: earth impedance

Reply tp c-h: concerning average magnetic fields in Stockholm of 0.4 mT, which equals 4 mG; that would be highly unusual if you really mean average all over the city. One needs to state where the measurements were taken, so that we know what the average is of. On the street? At what height above ground? In buildings? Only under or above distribution lines?

In a typical building in the US, I see less than 0.2 mG (0.02mT) in most office or home spaces unless there are certain wiring errors or there is a power line very close or very high-current transmission.

So as not to divert this thread, you could start an EMF one if you wish to follow up on this.

Karl

 

[hurk27]

Re: earth impedance

Karl
This is the one reason I think we need the isolation between the primary side of the transformer to the secondary side. What I'm refering to is the bond from the MGN to the X/O at each transformer. without this connection we would loose a lot of stray current in the earth and we would also see a lot less current on the grounding electrode systems in buildings. I think this was a mistake in allowing this as both systems (primary/secondary) can be grounded seprately with a rod/water pipe ground at the service and the wire from the MGN to the bottom of the pole at the transformer with one at each pole. they are seprate systems why not keep them seprate?

 

[brian john]

Re: earth impedance

Hurk:

I have done a lot of ground current investigations in buildings and in all but one of the cases we were able to isolate the source of the current. In the unsolved case we did know the source of the current was from inside the building (story posted in another thread). With no facility load there was no ground current.

I do not say stray voltage/current is not an issue, just that I have never see evidence of this stray voltage/current in buildings I have investigated.

 

[charlie b]

Re: earth impedance

Originally posted by iwire:The POCO bonds and grounds the Neutral at their end. Wouldn't some of the neutral current from my temp service flow through the earth and back to the POCO transformer?

Originally posted by karl riley:Charlie B., if the earth is in parallel with the service neutral, by way of the electrode at the service and the electrode at the transformer, then current obviously flows in both paths. . . . I know you understand parallel paths, so what were you talking about?

I don?t get involved with the design of utility power transmission or distribution, so I missed the concept you both had been describing: the impact of the utility?s practice of grounding the primary neutral throughout the length of the line.

Start with the basic notion that every time an electron leaves the ?hot side? of any source, another electron must necessarily enter the source?s ?cold side.? Thus, as electrons travel down the utility?s distribution ?hot line,? other electrons are returning via the utility?s distribution neutral. But that neutral is tied to planet Earth at many places along the path. This places the neutral in parallel with planet Earth, and causes current to flow through the dirt. OK. You and I are now on the same page.

Now let?s put our transformer on a pad on a section of dirt that has current flowing through it, and then tie the neutral of our transformer to that same dirt. It seems to me that you are both speaking of current flowing between the utility?s system and our transformer, by virtue of their both being connected to the same dirt, and by virtue of a difference in potential between the ground rods of the two systems (that being the result of the voltage drop along the primary neutral that is in parallel with the dirt). I don?t think so. Here?s why:

Any current leaving our transformer via one phase conductor will have to be matched by the amount of current entering our transformer via either the other two phase conductors or our own neutral conductor. Kirchhoff?s Current Law would require this. The current that is flowing in the dirt, and that approaches our transformer?s ground rod, will continue past our ground rod without going up into our transformer. The driving force of that current through dirt is driving that current in the direction of the primary neutral current. OK, I?ll grant that any given electron moving through the dirt might go up our ground rod into our transformer. But if it does, then another electron that had left our transformer?s phase conductor will come to the center point of our transformer, and instead of going back through a coil, it will go out the ground rod and into the dirt. There is not net flow of current either into our transformer from the dirt, or vice versa.

Here?s two analogies that come to mind:

(1) Stand in a shallow, fast moving creek, and drink a cup of coffee. The water is moving past your feet, but that doesn?t mean that you will be drinking creek water.

(2) Let?s you and I go shopping at the same mall. I?ll select a $5 item from Store #1, and leave a $5 bill on the floor just outside the store. You select a $5 item from Store #2 right across the hall, and leave five $1 bills on the floor just outside the store. The clerk from Store #1 steps into the hall and collects the five $1 bills. The clerk from Store #2 steps into the hall and collects the $5 bill. There were two different paths for cash flow (i.e., electron flow). The two paths had the middle of the hall (the ground rod of our transformer) as one point in common. The right amount of money went to the right places, so that neither store clerk was short-changed. But that doesn?t mean that I paid for your purchase, nor that you paid for mine.

 

[Guest]

Re: earth impedance

From my perspective the earth is in the loop. The earth is part of the circuit. The earth can be two things:

1. Open circuit.
2. Closed circuit.

The earth has resistance, so it can't be an open circuit.

It's not open, so it must be closed.

The closed circuit resistance is not infinite.

The earth is a resister in parallel with the resistance of the PoCo neutral (because of transformer earthing).

It's not quite that simple because there are other parallel paths besides Point-A to PoCo, and PoCo to Point-A. There are also points B-through-Z.

There will be a flow of electrons through the earth that's instigated by the earth ground at Point-A, and the PoCo transformer earth grounds.

Going back to the shopping mall analogy the money did not flow in the conventional manner, but there was a flow of money.