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From this thread How the current actually flows in a circuit?, I know that between 2 points in a conducting wire, the potential difference isn't necessary to maintain the current between them (if we suppose that the wire has nearly zero resistence).

I got a beautiful schema from @Bob D about the wiring of AC current from distribution to the household appliance.

Pic

I wonder whether there is any current flow from the hot wire down to earth (through B -> A -> E -> F -> Earth -> back to source (point K) or not.

The usual answer that I get from my research is that "No, there is not because there is no potential difference between the neutral wire and the Earth, so no current flow through the earthing wire EF)". However, I can't understand it because as stated above, potential difference isn't necessary in this case to maintain the current starting from the hot wire (suppose that the earthing wire has nearly-zero resistance). Therefore, the answer can't satisfy me.

@Bob D did his best to help me understand this, one of his explanations is that

"the current path for the load current is in red as that is the lowest resistance path. Some current may flow through ground but it would generally be small".

However, I still can't understand. As I understand, the current is huge until point B, and as there is no resistence until point F, the current remains huge and decreases as it goes through earth (up to point K). If I understand correctly, there is indeed a huge current from the neutral wire down to earth. It is limited only when passing through the earth. Therefore, if I stupidly touch the neutral wire, there should be indeed a huge current flowing through my body, which contradicts what has been said about the effect of touching the neutral wire, for example here Why don't we get a shock touching neutral wire?

So my ultimate question is: Is there any current supposed to flow down to earth in the earthing wire EF ? how big/small is it (if there is any) and how could we this quantification ? Thanks!

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  • $\begingroup$ The "no potential difference required for current flow" only applies to perfect conductors in ideal circuits. All real current flow (excepting weird effects in superconductors) requires a potential difference. In an idealized circuit we assume that the conductor has zero resistance and no voltage drop because the resistance and the required voltage drop are small enough to be negligible. $\endgroup$
    – Malcolm
    Commented Mar 23 at 21:47
  • $\begingroup$ The grounding of the neutral wire is at K H G; the neutral wire is not touching the casing to A D E F. There is an ammeter at E F, and if some substantial current flows at E F, it will trip the circuit and disconnect everything. $\endgroup$ Commented Mar 24 at 2:19

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The purpose of a grounding rod is to supply a low resistance path to earth in case of, for example a short circuit in some apparatus. Various electrical codes have different requirements for bonding to ground. Generally a good ground contact should have about 5-10 $\Omega$ resistance to ground. ( IEC (IEC/BS EN 62561-2:2012)) The max allowed by the NEC is 25$\Omega$. link To accomplish this you have to hammer an 8ft copper rod (or similar) into the ground. The earth is not a good conductor.
Electric supply companies use a hot and a neutral wire to supply electricity. If they could use the ground as a return path, they could save half the cost of their wiring. They are always looking for ways to save money, but they don’t do this. Why? Because a return path through the earth has much too high a resistance.

Wires do have an appreciable resistance; this is why circuits that carry larger currents must have larger diameter wires. In domestic wiring applications a 15A circuit requires 14 gauge wire, 20A: 12 gauge and 30A: 10 gauge. Say you try to run a heavy piece of electrical equipment with an amp draw of 15A, if you have 100ft of cable you need at least 10 gauge wire or else you will have an appreciable voltage drop at your equipment and it will overheat and cut out on thermal overload. (A 14gauge extension cord would drop about 8V) This is also why lights may dim or flicker when you turn on something like a wetvac. 14gauge wire has about 2.6 $\Omega$ resistance per 1000 ft, 10 gauge has less than half that about 1$\Omega$/1000ft.

That being said, the resistance of the human body internally is about 300$\Omega$ (because it’s wet and salty) . Most of the resistance is at the skin which may be 10000$\Omega$, when dry. Reference The purpose of the grounding rod is to have a much lower resistance than you and provide an easier path to ground. That is why if you touch the neutral bar (which I don’t suggest you do) you will not get a shock.

The electric cable that attaches a house to its electric supply is usually 4 gauge copper. This has a resistance of about 0.25 $\Omega$/1000ft. The wire in your drawing between A and K probably has a resistance of about 0.025ohm. If there were 100A going through it the potential difference between K and A would be at most 2.5V.

But that is not the case! Domestic power as you can see in the diagram has a midpoint connection on the transformer. This means that the two poles which are split in the distribution panel are out of phase. This is done to reduce power loss in the neutral line. If one half of the panel is delivering 20A and the other 15A, then there is only 5A travelling back down the neutral line, and not 35A. Being out of phase the currents partially cancel.

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I wonder whether there is any current flow from the hot wire down to earth (through B -> A -> E -> F -> Earth -> back to source (point K) or not.

The usual answer that I get from my research is that "No, there is not because there is no potential difference between the neutral wire and the Earth, so no current flow through the earthing wire EF)". However, I can't understand it because as stated above, potential difference isn't necessary in this case to maintain the current starting from the hot wire (suppose that the earthing wire has nearly-zero resistance). Therefore, the answer can't satisfy me.

Potential difference is necessary in this case, but not from E to F. The potential difference would need to be between F and G.

Whatever current goes from E to F must also go from F to G. Although the earthing wire from E to F has negligible resistance, the earth from F to G does have non-negligible resistance. To get back to K any current would have to pass through that resistance, and that would require a potential difference between F and G.

But F and G are short circuited through H, K, and E. So there cannot be a potential difference between F and G, and therefore there cannot be a current from F to G and therefore there cannot be a current from E to F.

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  • $\begingroup$ Hello, thanks. But if I look again at the picture of the question of my thread physics.stackexchange.com/questions/807064/…, it is the same situation with the point A and B: only potential difference from the source, no potential between them, but there is a current between them, which contradicts what you've said in your answer. Could you please have a look at this ? Many thanks! $\endgroup$ Commented Mar 23 at 21:27
  • $\begingroup$ @InTheSearchForKnowledge that does not contradict what I said in my answer. There is no resistance between A and B in the other question. There is resistance between F and G in this question. They are physically different situations. $\endgroup$
    – Dale
    Commented Mar 23 at 23:21
  • $\begingroup$ Hello, but you are comparing AB to FG but not EF. FG in this question should be equivalent to AC in the other question, right ? $\endgroup$ Commented Mar 24 at 9:34
  • $\begingroup$ @InTheSearchForKnowledge I revised the answer based on your comments. Let me know if that helps $\endgroup$
    – Dale
    Commented Mar 24 at 17:16
  • $\begingroup$ Hello, thanks! Can you please explain why "F and G are short circuited through H, K, and E" as I don't see any short circuit here ? $\endgroup$ Commented Mar 24 at 18:26

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