Ground Resistivity - Amprobe GP-2A Manual De Uso

Fig. 7: Earth resistance measurement – large-sized earth rods

10.3 Ground resistivity

This test aims at analyzing the resistivity value of the ground in order to define the type of rods
to be used when designing the installation. For the resistivity test correct or not correct values
do not exist. The various values measured by positioning the rods at growing distances "a" must
be quoted in a graph. According to the resulting curve, suitable rods will be chosen. As the test
result can be affected by metal parts buried such as pipes, cables or other rods etc., it is advisable
in case of doubts to take a second measurement positioning the rods at an equal distance "a",
but rotating their axis by 90°.
Fig. 8: Measurement of ground resistivity
The resistivity value is given by the following relation: r E = 2 p a R where:
r E = ground resistivity
a = distance between probes [m]
R = resistance measured by the instrument [Ω]
26