Earthing Calculations

http://alt.dehn.de/pdf/ds/eb/DS162e.pdf

http://admin.copperalliance.eu/docs/librariesprovider5/power-quality-and-utilisation-guide/651-earthing-systems-basic-constructional-aspects.pdf?sfvrsn=6&sfvrsn=6

Me thinks they react differently so the calculation has to be different.

Different shapes, different formulas. How not?

The single electrode formula is calculated starting with a differential equation

See [for instance]:

http://admin.copperalliance.eu/docs/librariesprovider5/power-quality-and-utilisation-guide/631-earthing-systems-fundamentals-of-calculation.pdf?sfvrsn=6&sfvrsn=6

or IEEE 80/1986 [not IEEE 80/2000].

But the area ground resistance started from round plate formula-also single electrode-and further it was developed experimentally.

Single conductor earths or grounding rods or strips perform quite differently from grids. The use of a grid is normally only to retain a ground in a given area to maintain the same potential. While they will provide some grounding, it is much more limited. The same way a vertical ground rod will provide much more grounding than the same rod placed horizontally in the ground. The reason if you study the theory in depth is that the rod, strip or grid is making contact with only a small portion of the earth which really has poor surface conduction. The radiant lines of energy go out quite a ways and the resultant resistance can be quite low. One test that shows the difference in how the conduction works in the earth is to drive 3 ground rods (set A) 1 foot apart and 3 more ground rods (set B) 10 feet apart. Measure the resistance of one single rod, then compare that to set A and you will find that A is not much improvement, but compare it with B and you will find a really, really large improvement. From this you should deduce how a grid and a strip or vertical rod should all behave differently and have different formulas.

All that being said, the earth is not a homogeneous item and varies a lot, even in the same location, so formulas are only a starting point, and a proper ground test is essential to providing a good earth or ground connection.