Approximate Evaluation Of Intentional Rods' Contribution - Amprobe GP-2A Manual De Uso

The measuring method allows defining the specific resistivity of a ground layer up to the depth
corresponding approximately to the distance "a" between the rods. If you increase the distance
"a" you can reach deeper ground layers and check the ground homogeneity. After several
measurements you can trace a profile according to which the most suitable rod is chosen.
Fig. 9: Measurement of ground resistivity
Curve 1: as decreases only in depth, it's advisable to use a very deep rod
Curve 2: as E decreases only until the depth a, it's not useful to increase the depth of the rod
beyond a
Curve 3: the ground resistivity is quite constant, so increasing depth does not make E decrease,
therefore a ring rod must be used

10.3.1 Approximate evaluation of intentional rods' contribution

The resistance of a rod Rd can be calculated with the following formulas ( = average resistivity of
the ground).
a) resistance of a vertical rod
Rd = r / L
where L = length of the element touching the ground
b) resistance of an horizontal rod
Rd = 2r / L
where L = length of the element touching the ground
c) resistance of linked elements
The resistance of a complex system made of more elements in parallel is always higher than the
resistance, which could result from a simple calculation of single elements in parallel, especially
if those elements are close to each other and therefore interactive. For this reason, in case of a
linked system the following formula is quicker and more effective than the calculation of the
single horizontal and vertical elements:
Rd = r / 4r
where r = radius of the circle which circumscribes the link
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