HT instruments M71 - Manual (Page 26)

M71
EN – 25
Method for medium and large sized earth rods
This procedure is based on the volt ampere metric method as well, however it is mainly
used whenever it is difficult to place an auxiliary current rod at a distance equal to fivefold
the diagonal of the area limiting the earth installation. Place the current probe at a distance
equal to the diagonal of the earth installation. To make sure that the voltage probe is
placed outside the area affected by the rod under test, take several measurements, firstly
placing the voltage probe at half way between the rod and the current probe, later moving
the probe to both the earth rod and the current probe.
Fig. 8: Earth resistance measurement for medium and large sized earth rods
The measuring method allows to define the specific resistance till the depth corresponding
approximately to the distance “a” between two rods. If you increase the distance “a” you
can detect 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 selected.
Use several rods in parallel
and moisten the
surrounding ground if the
instrument does not
manage to supply the
current necessary to
perform the test due to a
high ground resistivity.