HT instruments MACROTEST G3 - Manual (Page 105)

MACROTESTG3 - COMBIG3
EN - 104
area of the ground system by reducing this distance to once the diagonal of the
ground system (see Fig. 38).
To confirm that the voltage probe is located outside the zone of influence of the system
under test and the auxiliary earth rod, it is necessary to perform several measurements by
initially placing the voltage probe at the midpoint between the system and the auxiliary
current rod, then moving the probe both to the system under consideration and to the
auxiliary current rod.
These measurements should provide compatible results, any significant differences
between the various measured values indicate that the voltage probe has been stuck
within the zone of influence of the system under test or the auxiliary current rod. Such
measurements cannot be considered as reliable. It is necessary to further extend the
distance between the auxiliary current rod and the rod under test, then repeat the whole
procedure as above described.
Fig. 38: Ground measurement for big-sized earth networks
Use multiple probes in parallel and wet the surrounding ground (see Fig. 38) if the
instrument is not able to supply the current required to perform the test due to a high
resistance of the ground.
12.12.2.
Ground resistivity measurement
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 measurement of resistivity, there
are no correct or incorrect values. The various values obtained using distances between