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Ground Resistance

ebee
It used to be fag packet type approximation that when measuring the earth impedance that as a very very much rule of thumb starter for ten sort of thing.

approx 80% of the total figure between two fart away points might be made up of the area around the local rod/plate/tapes etc and the other 20% between that area and all the area between them up to the area surrounding the other earthing point.

What would be a sort of max in ohms between two extremely far points example two countries ? say England and New Zealand or Oz?
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    If you put two rods each 1 m long and 1 cm diameter 2 cm apart, the resistance is predominantly that of the earth between them i.e. a 2 cm length of 200 cm² CSA. Then there are parallel paths, as it were, all the way around, but they are significantly longer.


    Now put the rods 1 m apart. The resistance of the bit between them is 50 times as much, but the closer parallel paths are not much longer, so the overall effect is that the resistance is not 50 times higher.


    The further apart you get, the CSA of all the parallel paths comes to dominate over their length. I suspect that there comes a point where getting further apart makes very little or no difference.


    Now think of a TT installation and it's EFLI. We think of the resistance of the two rods - local and transformer. We do not worry much about their distance apart.


    The Earth is 40,000 km round, so if you go to the other side, the CSA at its widest point is 127 x 106 km² = 127 x 1018 mm²; so if you take a slice there, it is going to have pretty negligible resistance.


    Don't forget the huge amount of iron in the Earth's core.


    My calculus is not good enough to work it out properly, but my guess would be of the order of 1,000 Ω.
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    If you put two rods each 1 m long and 1 cm diameter 2 cm apart, the resistance is predominantly that of the earth between them i.e. a 2 cm length of 200 cm² CSA. Then there are parallel paths, as it were, all the way around, but they are significantly longer.


    Now put the rods 1 m apart. The resistance of the bit between them is 50 times as much, but the closer parallel paths are not much longer, so the overall effect is that the resistance is not 50 times higher.


    The further apart you get, the CSA of all the parallel paths comes to dominate over their length. I suspect that there comes a point where getting further apart makes very little or no difference.


    Now think of a TT installation and it's EFLI. We think of the resistance of the two rods - local and transformer. We do not worry much about their distance apart.


    The Earth is 40,000 km round, so if you go to the other side, the CSA at its widest point is 127 x 106 km² = 127 x 1018 mm²; so if you take a slice there, it is going to have pretty negligible resistance.


    Don't forget the huge amount of iron in the Earth's core.


    My calculus is not good enough to work it out properly, but my guess would be of the order of 1,000 Ω.
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