HT instruments MACROTEST 5035 - Manual (Page 78)

MACROTEST 5035
EN - 77
11.10. EARTH RESISTIVITY MEASUREMENT
This test aims at analysing the resistivity value of the ground in order to define the type of
rods to be used.
EQUIPMENT PARTS TO BE TESTED
For the resistivity test admissible 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), in case of doubts take a second
measurement positioning the rods at an equal distance "
a", but rotating their axis by 90°.
Black
Bl
ac
k
B
lu
e
Blu
e
Green
G
re
en
Re
d
R
ed
a
a a a
a
a
90°
2
90°.
compared to the previous measurement
the rods are rotated by
2° Measurement:
The resistivity value is calculated with the following formula:
ρ=2πaR
where:
ρ= specific resistivity of the ground
a= distance between the rods (m)
R= resistance measured by the instrument (
Ω)
The measuring method allows to define the specific resistance 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
ρ measurements, at growing distances “a”, you can trace a profile like the following
ones, according to which the most suitable rod is chosen.