Method For Small-Sized Earth Rods; Method For Large-Sized Earth Rods - Amprobe GP-2A Manual De Uso

10.2.2 Method for small-sized earth rods

Let a current stream between the earth rod under test and an auxiliary probe placed at a
distance equal to fivefold the diagonal of the area limiting the earth installation itself. Place the
voltage probe at approximately half way between the earth rod and the current probe, finally
measure voltage between both of them.
Fig. 6: Earth resistance measurement – small-sized earth rods

10.2.3 Method for 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 as well as the auxiliary rod, take several measurements,
firstly placing the voltage probe at half way between the installation and the auxiliary current
probe, later moving the probe to both the installation under test and the auxiliary current
probe. Such measurements shall give compatible results, any difference among measurement
values taken indicates that the voltage rod was driven within the influence area of the
installation under test or of the auxiliary current rod. Such measurements cannot be considered
as reliable.
In this instance it is necessary to further extend distance between the auxiliary current rod and
the rod under test, then repeat the whole procedure as above described.
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