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I am currently learning about grounding in electrical installations and there is something that intuitively doesn't really make sense to me. It is related to the resistance measured between two grounded electrodes.

Suppose we have a large area, where the specific resistance of the earth is constant. We now install a grounding electrode. The grounding resistance of this electrode depends mainly on the surface area and depth of the electrode, as well as the specific resistance of the earth itself.

As I Understand, there will be an area of influence around each electrode, inside which the resistance will depend on the distance from the electrode. After moving far enough away from the electrode, the ground around it will act as an extremely high number of parallel resistors, effectively making the change in resistance 0 as one moves further and further away.

This means, that if we plant a second identical grounding electrode directly next to the first one, we will measure some resistance. Moving the electrode further away will increase the measured resistance, but only up to a certain point, at which the measured resistance remains constant. This is how the grounding resistance of electrical installations is even able to be measured.

What intuitively does not make sense to me, is that according to the above explanation, even when moving the two electrodes a substantial amount from each other, the resistance should remain nearly constant. It kind of makes sense for a distance of 10 or 20 meters. But a kilometer? 10? 100? Over very large distances it seems weird that the resistance should not change.

Is there something I'm missing, or does the resistance measured between the two electrodes actually stay nearly constant even over large distances?

(All of this assuming a nearly constant specific resistance of the earth. Obviously when considering two separate pieces of land separated by some structure there wouldn't be a connection)

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  • \$\begingroup\$ specific resistance = resistivity. Try this, a 2D version (surface ohms per square): esdjournal.com/techpapr/ohmmtr/…. \$\endgroup\$
    – Andy aka
    Commented Jul 1, 2022 at 17:17
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    \$\begingroup\$ There's a difference between 'being far enough away that changes are not worth worrying about as far as code is concerned', and 'constant'. Don't believe the people that tells you it varies up to x distance, and then becomes constant. It varies all the way out. Your intuition is correct. Technicians know more about making an installation compliant to code, than they do about physics. \$\endgroup\$
    – Neil_UK
    Commented Jul 1, 2022 at 17:48

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It never becomes 'constant' but it EFFECTIVELY becomes constant. Without using math, the relationship between distance (call it the 'horizontal axis') and resistance (call that the 'vertical' axis) in the curve below...

enter image description here

Clearly this is not a resistance vs. distance curve but the SHAPE is right (it's just the first representative curve I found on the net...)

The key characteristic is how the curve starts to flatten at the top. It will become nearly flat but never (in theory) quite totally flat. It will always be rising, but by less and less as you get further away.

So in theory it always rises, but at some point the curve stops changing so far as it practically matters.

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  • \$\begingroup\$ Makes sense. Does that mean, however, that e.g. putting one electrode on one side of the black forest and the other on the other side, effectively gives (almost) the same resistance as placing them 20m from each other? (assuming that at 20m, the point at which the curve is nearly constant has been reached already) - this is the part that doesn't really make sense to me: applying the theoretical explanation to an extreme practical edge case \$\endgroup\$
    – zomnombom
    Commented Jul 1, 2022 at 22:53

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