Amprobe DMIII-Multitest Power-Quality-Recorder - Manual (Page 91)

AMPROBE
DMIII MULTITEST
EN-89
16.5. Ground Resistivity Measurement
PURPOSE OF THE TEST
This test analyzes the resistivity value of the ground in order to define the type of rods to
use.
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. The test result can be affected by metal parts
buried underground (e.g. 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
B
la
ck
Blue
B
lu
e
Green
G
re
e
n
Red
R
e
d
a
a a a
a
a
90°
The resistivity value is calculated with the following formula:

=2

aR
Where:

= specific resistivity of the ground
a= distance between the rods (m)
R= resistance measured by the instrument (

)
2° measurement:
compared to the previous
measurement the rods are
rotated by 90°.
1° measurement:
the rods are positioned
at a distance “a”