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I am on my 4th year on Electrical Engineering education. Over the years the concept of earth in electric circuits bugged me. And now, again, I came across with something which I cannot understand.

Take a look at this ancient-patent. It belongs to Amos Emerson Dolbear, an eminent man of science: https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US350299.pdf

Here, between rows 14 and 19, he talks about "[...]a ground the potential of which is considerably above the normal[...]" and again he writes "[...]a ground the potential of which is considerably below the normal[...]".

Now,

i) What does he mean by "normal" potential?

ii) How can you make a buried terminal to have different potential than the ground? Terminal is buried, therefore terminal's potential has to equal be equal to the earth potential (which is accepted as zero potential, right? -Who accepts this? Why they accept this? Is there an evidence regarding the level of potential of the soil/earth?). The only sort-of-explanation I can come up with is that he does not reduce the ground resistance considerably. He keeps the resistance between the buried terminal and the earth, hence this allows two different potential levels on the different sides of the earth resistance.

iii) If we accept the icky-explanation above, how does a current flow into the earth? Is not earth, which contains sand, soil, rocks, minerals and mines, organic waste, an insulator? Certainly, it should not be a conductor.

I know I burst out the questions all over, but I am very ethusiastic to understand this. Thanks for your attention. -- imp0nderablef1uid

Edit: To make the question more clear, I summarize the question as follows:

i) how can you get different potential than earth potential on a terminal buried into the ground, and, ii) how can this work, i.e. how can currents flow into what seems like an awful conductor?

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  • \$\begingroup\$ If you measure the voltage on a terminal buried in ground at different places on the terminal, you will find the potential difference to be very close to zero, unless there is a fairly sizable current flowing through the terminal. But if you measure the Voltage between two different terminals buried in different locations, you may find a substantial voltage difference due to current flowing in the dirt beneath your feet. \$\endgroup\$
    – user57037
    Commented Sep 13, 2015 at 4:41
  • \$\begingroup\$ @mkeith How can the electric current flow in the dirt? Since the terminal ends in the ground, there is an open circuit. Therefore no current should flow. \$\endgroup\$
    – user86330
    Commented Sep 13, 2015 at 5:36
  • \$\begingroup\$ Stack exchange says that this question is put on hold, even though I gave a summary of what I ask. Is not it clear what I am asking? \$\endgroup\$
    – user86330
    Commented Sep 13, 2015 at 6:26
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    \$\begingroup\$ Current flows in dirt readily. Could that be the source of all your misunderstanding? You did not realize that dirt is capable of conducting electricity? The range of conductivity varies, but generally, if there is green vegetation around, the soil will be somewhat conductive. When it gets very dry, sometimes the conductivity is very low. I use electric fences with one grounded terminal to keep goats inside a paddock. When it gets very dry, the shock given by the fence becomes measurably and noticeably weaker. \$\endgroup\$
    – user57037
    Commented Sep 14, 2015 at 18:30
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    \$\begingroup\$ This topic can become kind of lengthy. Normally, when you are looking at a schematic, it is assumed that "ground" has the same potential everywhere and is considered to be 0V by convention. This may not be connected to earth ground. But consider the case of a PCB. Why does current flow on the ground plane? The only way it can flow is if there is a non-zero electric field. But we still call it a ground plane. Most of the time, the electric field in the plane is very small, and potential differences are not important. If they are important, we have to include them in our model. \$\endgroup\$
    – user57037
    Commented Sep 14, 2015 at 21:28

3 Answers 3

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By convention, the mass of the Earth is at a potential of 0 volts.

Your assumption is incorrect - the ground is not an insulator. Soil contains water and dissolved mineral salts and so conducts electricity. Wet soil with organic material can be a relatively good conductor, but dry stony or sandy soil has a much higher resistivity.

Once you start treating the Earth as a giant spherical resistor, it all makes more sense. If you hammer a ground stake into the Earth, well away from any other electrics, you get a good 0V reference point.

However, if you start passing current through the ground, then the voltage rises at one terminal, and falls at the other.

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  • \$\begingroup\$ Thanks. This really cleared the subject a bit. I think you have a point by saying that the Earth is a conductor because of minerals dissolved in underground water. Edit: Can you comment on how can you pass a current in the ground? \$\endgroup\$
    – user86330
    Commented Sep 15, 2015 at 9:24
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    \$\begingroup\$ You pass a current through the ground by hammering in two stakes and applying a voltage across them. The ions dissolved in the water in the soil act as the charge carriers, instead of electrons in a metal wire. \$\endgroup\$
    – Simon B
    Commented Sep 15, 2015 at 11:29
  • \$\begingroup\$ Thank you very much. Now I have a clearer understanding of the matter. \$\endgroup\$
    – user86330
    Commented Sep 15, 2015 at 12:54
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    \$\begingroup\$ It is not impossible (and is done today in rural places) to have your power company provide just one phase and return current is through the earth, wiki \$\endgroup\$
    – crasic
    Commented Sep 15, 2015 at 16:23
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Hi when electricity is generated the three phases are connected a certain way eg one end of each coil is connected together this forms a "star" connection.This is where the neutral or return cable is fixed to and ...where the earth cable is connected to. The other three ends of the coils are "live" and are at120 degrees apart.So in an electrical installation all earth faults flow back to the generator causing high currents and tripping overloads/ melting fuses? The neutral cable does the same. Don't think of the earth/ground as the dirt you stand on but the route the current takes back to the generator .... Hope this helps

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  • \$\begingroup\$ Thanks for your effort. But in the patent, Dolbear says, on row 27, "[...]C is a wire leading into ground[...]", therefore earth/ground is actually the dirt I stand on. Still, the questions remains: i) how can you get different potential than earth potential on a terminal buried into the ground, and, ii) how can this work, i.e. how can currents flow into what seems like an awful conductor? \$\endgroup\$
    – user86330
    Commented Sep 12, 2015 at 20:43
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Its possible with sending currents into soil, like submarine communication stations do, like Nikola Tesa did, ...like every storm does.
Earth is a good conductor.

If you place two rods in soil and you connect the source, the current will flow trough soil. Then you can place another pair of rods and measure the potential, this technique is used to measure soil conductivity. The Kirchoff rule V=R*I. In case of lightning, you have to stand with both legs close each to other, because the voltage between two legs is enough to kill you, the same rule applies if a powerline falls down, you can walk away using small steps, only.
NT used an inductor and air capacitor to push the AC current trough soil, forming a standing wave. He was the first scientists that has studied about telluric currents.

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  • \$\begingroup\$ Yes, I read all about Nikola Tesla, and his work. But this is not the explanation. You accept my question as a "fact". I desire to understand the reason currents go into the earth. I mean, how can you explain this using kirchoff's circuit laws? Or any other physical law? \$\endgroup\$
    – user86330
    Commented Sep 12, 2015 at 19:41
  • \$\begingroup\$ ehi Marko thanks for your answer, why don't you try to expand it a bit since OP question is way wider? \$\endgroup\$
    – Vladimir Cravero
    Commented Sep 12, 2015 at 19:45
  • \$\begingroup\$ Whoa! I didn't know there is a special name for those currents: telluric currents. Thanks for pointing out. By using this term in the search engines and libraries I find many articles on the subject. \$\endgroup\$
    – user86330
    Commented Sep 16, 2015 at 19:08

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