2
“WATTS CURRENT” TECHNICAL BULLETIN
Issue 09
Summer 2016
Featured Product:
S
oil resistivity is a key factor for determining what the resistance of a grounding
electrode will be, to what depth it must be driven in order to obtain low earth
resistance, and even the type of grounding system that should be designed for
this location and facility.
The resistivity of the soil varies widely throughout the world and changes
seasonally. Resistivity is strongly affected by the content of electrolytes in the
soil, its moisture content, and even its temperature. Soil resistivity measurements
are therefore critical for designing and constructing a grounding system
appropriate for your site and requirements.
This article describes a simple way to measure soil resistivity known as the
Wenner method. Our test instrument will be the AEMC
®
Ground Tester Model
6472. This instrument automatically calculates and displays soil resistivity, with
no manual calculations necessary.
The Model 6472 also features advanced capabilities such as test currents up
to 250 milliamps for testing soils with high resistivity, automatic test frequency
selection for testing in high EMI environments, and the ability to connect to a
computer via AEMC’s DataView
®
data analysis software.
Soil Resistivity vs Resistance
In this article, we refer to both soil resistivity and soil
resistance. To avoid possible confusion, it’s important to
have a basic understanding of what these terms mean
and how they differ. Simply put, resistance (represented
by the letter
R
) is a property of a specific physical
item of a definite size and shape; while resistivity
(represented by the Greek letter
ρ
), is a general
property of a material irrespective of its shape or size.
For example, a 500-foot length of 10-gauge copper
wire has a specific resistance, measureable in
ohms (
Ω
); while copper as a material possesses
a general resistivity, expressed in units such as
ohm-centimeters (
Ω
cm) or ohm-meters (
Ω
m).
Changing the length or gauge of the wire
changes its resistance, while the resistivity of
the copper from which it is made remains the
same.
Performing a Wenner Soil Resistivity
Test with the AEMC
®
Model 6472
