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I'm a 1st year EE student and i'm trying to get my head around earthing systems at the moment and could do with some help.

The first thing that i'm struggling to understand is how current establishes itself through the earth in the case of a Single Wire Ground Return system.

enter image description here

In the schematic above, i understand that the high potential top wire will allow electrons to be 'shuffled' back and to toward a place of lower potential, i.e. ground. I'm having a hard time explaining however, how a force exists between the ground below the load on the right, and the ground connected to the transformer winding. In a conductor within a copper wire for example i understand how the electrons are 'pushed' and 'pulled' in a specific direction, but how do the two grounded points 'know' that the other exists, and therefore that there is a complete circuit, without a wire?

My next question is most likely answerable from the first, but in an AC system, why is it the case that current requires a return path to the transformer as opposed to simply ending with the ground after the load?

Thanks for any help - sorry if the diagram is not so realistic!

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  • $\begingroup$ There are many types and purposes for grounding that depend on whether you are talking about grounding of the electrical system outside of buildings, grounding of electrical systems within buildings, and grounding of electrical equipment within buildings to name a few. E.g., what is the transformer in you diagram. Is it an electrical utility transformer? A transformer in equipment? $\endgroup$ – Bob D Feb 7 at 20:55
  • $\begingroup$ It's more of a theoretical point...how does the current know how to get back to its source. Does it receive a signal somehow? $\endgroup$ – Nick Cory Feb 7 at 21:03
  • $\begingroup$ There needs to be a potential difference between the two arrows and a conductive path for current to flow. The earth itself often serves that purpose for system grounds. $\endgroup$ – Bob D Feb 7 at 21:27
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Current flows through the earth for this sort of power transmission configuration exactly if it was a wire.

See wikipedia

The resistance of the ground is non infinite, it matters and it can be measured.

Note that this usage of the ground as a conductor is quite different from the usage of earth ground as equipment ground for safety in, for example, residential electrical wiring (third prong in outlet). I won’t go into that further here.

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  • $\begingroup$ Thank you. In the SWER system, I'm trying to get my head around how the current actually knows how to travel back to the source on the atomic/electron level $\endgroup$ – Nick Cory Feb 7 at 21:05
  • $\begingroup$ Consider a light bulb plugged into the wall. I can ask the same question. How does the current “know” to travel back to the source? Instead of a return wire imagine the return wire is a big sheet of metal. these situations are analogous. There’s nothing special about swer. If you don’t understand the light bulb either then that is a little bit of a different question. $\endgroup$ – jgerber Feb 7 at 21:10
  • $\begingroup$ I do understand that point. But in that scenario the whole sheet of metal would become charged. Not just a path along it surely. Whereas the whole ground cannot become charged? $\endgroup$ – Nick Cory Feb 8 at 21:20
  • $\begingroup$ The metal will not become charged. It will stay neutral but current density will flow through it. The density will be non uniform if the sheet is really really big. The current density will be highest in a direct line between the two terminals but low far from the contact points. $\endgroup$ – jgerber Feb 8 at 21:44
  • $\begingroup$ The non uniform distribution is set up because the voltage (supplied by the power source) creates electric fields with spatial patterns related to the location of the conductors. The fields are strongest close to the conductors so we see the largest currents there. For the power poles the current density will be highest in a direct line between poles but then weaken as you move away from the poles. $\endgroup$ – jgerber Feb 8 at 21:47

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