2

I wonder how the earth (real earth, not earth wire) works as a conductor in Single Wire Earth Return system, or in case of a fault in regular grounded systems.

  1. How far can electricity flow in earth, i.e. how distant may the user be from the power source, is it about the same as for regular wires?
  2. What happens if we connect to the earth two AC power sources that are out of phase, can such a system work? If yes, would it work if we did so with a wire (connecting two power sources out of phase)?
  3. I recall reading somewhere that Earth is so massive that the electricity flowing in it doesn't change its charge (or something like that). Is it true? If yes, what does it mean?
  4. Is there any other difference between electric flowing in earth vs. flowing in wires which I didn't ask for above?
George Lee
  • 129
  • 4
  • Does this answer your question? [Why is earth used for ground? Literally earth?](https://electronics.stackexchange.com/questions/145240/why-is-earth-used-for-ground-literally-earth) – Eugene Sh. Feb 07 '22 at 15:24
  • @EugeneSh. that question is inappropriate to this question. You need to look up SWER: http://www.stonepower.se/Images/SWER.pdf – Andy aka Feb 07 '22 at 15:28
  • 1
    OK, retracted. Will let the comment be for reference. – Eugene Sh. Feb 07 '22 at 15:29
  • The main difference would probably be that soil (especially dry) has much higher resistance than a typical wire. – user4574 Feb 07 '22 at 17:19

3 Answers3

3

  1. As long there is a difference in charge between two points, the charges will travel any distance needed to equalize the difference to zero.

  2. Connect (non floating) AC Sources to Ground: each source is short (zero voltage, max current,...), Connect two AC sources with different phases: short circuit with the superposed waveform.

  3. Assume earth is a huge capacitor, the few power plants of the humans do not really change its amount of charge - so far. Edit: as others stated already, there is a steady interchange of charge between the surface of earth and the atmosphere. And these transfers of charge are a lot larger than any human made impact (so far)

  4. Is there a difference between water flowing in pipes or soil? No. And the same applies to electrical charge. Electrical charge always is the same - at least in the universe as we know it. It's simply different amounts of positive and negative particles in matter. Edit: Of course the current in soil a a complex thing: kind of soil, moisture level, what kind of matter is in the soil, metals, minerals,... but in the end, normal laws of physics apply. The current will flow along the shortest path with the lowest resistance.

schnedan
  • 2,538
  • 7
  • 16
  • "Connect (non floating) AC Sources to Ground: each source is short (zero voltage, max current,...), Connect two AC sources with different phases: short circuit with the superposed waveform" — What's the difference between these two scenarios? What do you mean with the first scenario? – George Lee Feb 07 '22 at 19:44
  • I assume the AC sources to be referenced to Earth (non floating). In that case connecting the source to earth will close the circuit, current flows and the source is shorted. in case 2 I assume just 2 sources with connected reference, and the sources also connected. then each source is connected via the inner resistance of the other source... Anyway if you just qould provide a sketch of your scenario, we don't need to assume. – schnedan Feb 08 '22 at 08:55
  • When I say "connecting power source to earth" I mean exactly what you call "referenced to earth", because in my understanding, the only difference between hot and neutral is whether it is grounded. Please correct me if wrong. Sorry for using confusing wording, I'm not familiar which wording are usually used. – George Lee Feb 08 '22 at 14:42
  • So here is [Wikipedia](https://en.wikipedia.org/wiki/Single-wire_earth_return) answering part of my question: "If more capacity is needed, a second SWER line can be run on the same poles to provide two SWER lines 180 degrees out of phase" I just wonder how it works, and if it would work in a wire just as it works in the earth. – George Lee Feb 08 '22 at 14:46
  • OK,... in case of 2(3) phases with 180° (120°) offset, both the generator coils (or three) are connected to ground/earth on one end. The other ends are connected the overland wires (normally passing a transformer to level up voltage). On your site its the same. all loads are connected to one, two or three phases at one end and to ground on the other. The currents of all phases superpose. Ideally at the neutral point, the sum of the currents is zero. But depending on the earth resistance the neutral point at your site and in the power plant might differ-then a current will flow in the soil – schnedan Feb 08 '22 at 15:20
  • Your case is this https://en.wikipedia.org/wiki/Earthing_system#/media/File:TT-earthing-EN.svg without the N wire... so any compensating current flows between the earthing symbols – schnedan Feb 08 '22 at 15:23
  • https://de.wikipedia.org/wiki/Datei:3-phase_flow.gif shows you a three phase generator + a three phase load, the earthing points are where the 3 coils/loads are interconnected. these points are virtually zero (in theory touching them should not kill you:-) ). if you interconnect these points with a wire or through earth, in a perfect world no current will flow in this wire (both potentials are equal = no current) – schnedan Feb 08 '22 at 15:29
  • Are you saying that no current flows through earth? This is the opposite of what you said in the answer (and of what is obviously true). – George Lee Feb 09 '22 at 14:24
  • if the system would be perfectly symmetrical and the overland wires would have no resistance... yes, then no current would need to flow through the earth. But in reality a compensating current flows. it compensates for any imperfection of the system. Plus this is only true for a multiphase system. in a single wire setup earth is the only return-path which exist. So, in a single wire setup all the current must flow through earth. in a multiphase system, if all phases have a symmetrical load, the current through earth would be very little compared to the energy transported. – schnedan Feb 09 '22 at 14:43
  • In reality, the current would be anything between zero and maximum – schnedan Feb 09 '22 at 14:44
0

Earth is made of mostly insulation=dielectric (e.g. water has Dk=80) and conductive particles (salts, metals). Combined makes it poor for DC but good for AC dissipating by diffusion of AC currents and lightning is AC. Dielectrics have capacitance between conductors, so the earth is a "mesh" of both and varies greatly with moisture and soil.

How far can electricity flow in earth

  • It depends on the Capacitance and salts so it will spread like tree roots.
  • It diffuses rapidly so you would not likely feel lightning in water 100m away because of the volume of water relative to you. Similar for soil (ask an Earth Scientist how they model it, or I can show you). But if you would have a long conductor above the ground, a mile away, you could feel a zing.

What happens if we connect to the earth two AC power sources that are out of phase, can such a system work?

  • That would be a fault condition. Split-phase (or "out of phase") is neutral grounded at source. If isolated, it would still pose a differential fault.

I recall reading somewhere that Earth is so massive that the electricity flowing in it doesn't change its charge (or something like that). Is it true? If yes, what does it mean?

  • Charge is neutralized locally due to leakage resistance but given lightning strokes every second on average around the globe, it's only meaningful locally to change attractions to clouds momentarily. But with trans-Atlantic signals, there are considerable voltage differences and currents flowing.
  • At the end of the day, we universally define "ground" as 0V only where it is connected, even if it floating from protective earth (PE). Assumptions on frequency and conductor impedance will alter that assumption of ground. Even if the voltage might be different from one tower to the next, it is still a Protective Earth for local threats from Lighting or grid faults. So there is a distinction between PE and electronic 0V grounds and depends on their connections or isolation. In a PC, they are common and connected to the frame for EMI reasons.

Is there any other difference between electric flowing in earth vs. flowing in wires which I didn't ask for above?

No if you mean the atomic charge part, but at the macro level, yes, everything is different.

Is there anything I left out in the above answers? Maybe. Try this out.

  • The equivalent RLC network is quite different. Earth is massively parallel, so low L and very wet, so large C dielectric and also very high R unless wet with salt. Characteristic Impedance is defined as \$Zo=\sqrt{\dfrac{L}{C}}\$ so from what I said above, Earth is very low impedance AC for all frequencies whereas wires are only very low impedance for DC and low frequency in long ranges and can only be better if made like a copper ground plane.
George Lee
  • 129
  • 4
Tony Stewart EE75
  • 1
  • 3
  • 54
  • 182
  • When I say "connecting power source to earth" I mean exactly what you call "grounded at source", because in my understanding, the only difference between hot and neutral is whether it is grounded. Please correct me if wrong. Sorry for using confusing wording, I'm not familiar which wording are usually used. – George Lee Feb 08 '22 at 15:18
  • So here is [Wikipedia](https://en.wikipedia.org/wiki/Single-wire_earth_return) answering part of my question: "If more capacity is needed, a second SWER line can be run on the same poles to provide two SWER lines 180 degrees out of phase" I just wonder how it works, and if it would work in a wire just as it works in the earth. – George Lee Feb 08 '22 at 15:18
  • It is called split phase where one of the 3 primary phase transformed down to single phase 240 60Hz has a center tap earth grounded also called neutral. Thus L1 , L2, N feed to to multiple subscribers along with PE. – Tony Stewart EE75 Feb 09 '22 at 04:52
0

  1. There's no particular limit to how far the electricity can go.

  2. If you're only connecting the grounds, or even the neutrals, it doesn't cause any problems. You are giving all of them a common ground reference. If you're connecting different phases, it depends exactly what you're connecting and whether or not the generators are connected by some other route. But connecting the phases out of a generator to ground probably isn't a sensible thing to do anyway.

  3. For many purposes, you can consider the Earth as a big capacitor. Big things have capacitance even without another nearby conductor with an insulator between. This doesn't much matter if you're looking at current flow.

  4. Damp dirt makes for a rather high resistivity. But if your grounding rod is sufficiently big, then the Earth makes up for it by having a very large cross sectional area. Most of the resistance is where the grounding rod meets the earth.

Simon B
  • 18,609
  • 1
  • 29
  • 55
  • At point 2, note that I'm talking about a SWER system. It looks like the other answerers here say that it wouldn't work. – George Lee Feb 07 '22 at 19:54
  • @GeorgeLee Will edit to clarify. – Simon B Feb 07 '22 at 22:20
  • So here is [Wikipedia](https://en.wikipedia.org/wiki/Single-wire_earth_return) answering part of my question: "If more capacity is needed, a second SWER line can be run on the same poles to provide two SWER lines 180 degrees out of phase" I just wonder how it works, and if it would work in a wire just as it works in the earth. – George Lee Feb 08 '22 at 15:19
  • @GeorgeLee a common arrangement for domestic AC in North America is 3 wire, with 2 hots and a grounded neutral. The same would work for SWER, but at higher voltages, and using the Earth as the combined neutral and ground. – Simon B Feb 08 '22 at 17:02