Application Note

8
What are the methods of earth ground testing?
Fall-of-Potential
measurement
The Fall-of-Potential test method is used to
measure the ability of an earth ground system or
an individual electrode to dissipate energy from
a site.
How does the Fall-of-Potential
test work?
First, the earth electrode of interest must be
disconnected from its connection to the site.
Second, the tester is connected to the earth
electrode. Then, for the 3-pole Fall-of-Potential
test, two earth stakes are placed in the soil in a
direct line—away from the earth electrode. Nor-
mally, spacing of 20 meters (65 feet) is sufficient.
For more detail on placing the stakes, see the
next section.
A known current is generated by the Fluke
1625-2 between the outer stake (auxiliary earth
stake) and the earth electrode, while the drop in
voltage potential is measured between the inner
earth stake and the earth electrode. Using Ohm’s
Law (V = IR), the tester automatically calculates
the resistance of the earth electrode.
Connect the ground tester as shown in
the picture. Press START and read out the R
E
(resistance) value. This is the actual value of
the ground electrode under test. If this ground
electrode is in parallel or series with other
ground rods, the R
E
value is the total value of
all resistances.
How do you place the stakes?
To achieve the highest degree of accuracy when
performing a 3–pole ground resistance test,
it is essential that the probe is placed outside
the sphere of influence of the ground electrode
under test and the auxiliary earth.
If you do not get outside the sphere of influ-
ence, the effective areas of resistance will
overlap and invalidate any measurements that
you are taking. The table is a guide for appro-
priately setting the probe (inner stake) and
auxiliary ground (outer stake).
To test the accuracy of the results and to
ensure that the ground stakes are outside the
spheres of influence, reposition the inner stake
(probe) 1 meter (3 feet) in either direction and
take a fresh measurement. If there is a signif-
icant change in the reading (30 %), you need
to increase the distance between the ground
rod under test, the inner stake (probe) and the
outer stake (auxiliary ground) until the measured
values remain fairly constant when repositioning
the inner stake (probe).
>20 m (65 ft) >20 m (65 ft)
H/C2
OFF
3
POLE
4
POLE
4
POLE
2
POLE
S/P2
ES/P1
E/C1
START
TEST
DISPLAY
MENU
CHANGE
ITEM
SELECT
1625-2
ADVANCED EARTH / GROUND TESTER GEO
E S H
AC
Rresistance
300kΩEarth/Ground Resistance 300kΩ
DC
Resistance 3kΩ
RA
R
—
R
~
2
POLE
3
POLE
3
POLE
4
POLE
Inner
stake
Outer
stake
Earth
electrode
Depth of
the ground
electrode
Distance
to the
inner stake
Distance
to the
outer stake
2 m 15 m 25 m
3 m 20 m 30 m
6 m 25 m 40 m
10 m 30 m 50 m