Application Note

6
What are the methods of earth ground testing?
There are four types of earth ground testing
methods available:
• Soil Resistivity (using stakes)
• Fall-of-Potential (using stakes)
• Selective (using 1 clamp and stakes)
• Stakeless (using 2 clamps only)
Soil resistivity
measurement
Why determine the soil resistivity?
Soil Resistivity is most necessary when deter-
mining the design of the grounding system for
new installations (green field applications) to
meet your ground resistance requirements.
Ideally, you would find a location with the
lowest possible resistance. But as we discussed
before, poor soil conditions can be overcome
with more elaborate grounding systems.
The soil composition, moisture content, and
temperature all impact the soil resistivity. Soil
is rarely homogenous and the resistivity of the
soil will vary geographically and at different
soil depths. Moisture content changes season-
ally, varies according to the nature of the sub
layers of earth, and the depth of the permanent
water table. Since soil and water are generally
more stable at deeper strata, it is recommended
that the ground rods be placed as deep as
possible into the earth, at the water table if
possible. Also, ground rods should be installed
where there is a stable temperature, i.e. below
the frost line.
For a grounding system to be effective, it
should be designed to withstand the worst
possible conditions.
How do I calculate soil resistivity?
The measuring procedure described below
uses the universally accepted Wenner method
developed by Dr. Frank Wenner of the US Bureau
of Standards in 1915. (F. Wenner, A Method
of Measuring Earth Resistivity; Bull, National
Bureau of Standards, Bull 12(4) 258, p. 478-
496; 1915/16.)
The formula is as follows:
r = 2 π A R
(r = the average soil resistivity to depth A in ohm—cm)
π = 3.1416
A = the distance between the electrodes in cm
R = the measured resistance value in ohms from
the test instrument
Note: Divide ohm—centimeters by 100 to
convert to ohm—meters. Just watch
your units.
Example: You have decided to install
three meter long ground rods as part of
your grounding system. To measure the
soil resistivity at a depth of three meters,
we discussed a spacing between the test
electrodes of nine meters.
To measure the soil resistivity start the
Fluke 1625-2 and read the resistance
value in ohms. In this case assume the
resistance reading is 100 ohms. So, in
this case we know:
A = 9 meters, and
R = 100 ohms
Then the soil resistivity would equal:
r = 2 x π x A x R
r = 2 x 3.1416 x 9 meters x 100 ohms
r = 5655 Ωm