HT instruments MACROTEST G3 - Manual (Page 106)

MACROTESTG3 - COMBIG3
EN - 105
increasing "d" rods should be reported in a graph from which, according to the curve
obtained, it is possible to determine the type of rods to use. As the test result can be
affected by metal parts buried such as pipes, cables or other rods etc., it is advisable to
take a second measurement positioning the rods at an equal distance "d", but rotating
their axis by 90° (see Fig. 39).
Fig. 39: Ground resistivity measurement
The resistivity value is given by the following relation: 
E
= 2  d R where:

E
= specific ground resistivity
d = distance between the probes [m]
R = resistance measured by the instrument [ ]
The measuring method allows defining the specific resistivity of a ground layer up to the
depth corresponding approximately to the distance “d” between the two rods. If you
increase the distance “d”, 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.
Curve 1: as 
E
decreases only in depth, its
advisable to use a very deep rod
Curve 2: as
E
decreases only until the depth
d it is not useful to increase the
depth of the rods 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.
Fig. 40: Ground resistivity measurement
Approximate evaluation of intentional rods' contribution
The resistance of an Rd rod can be calculated with the following formulas (
 r = average
resistivity of the ground).
d