Application Note

What are the methods of earth ground testing?
Testing individual ground electrode resistances of high
voltage transmission towers with overhead ground or static
wire, requires that these wires be disconnected. If a tower
has more than one ground at its base, these must also be
disconnected one by one and tested. However, the Fluke
1625-2 has an optional accessory, a 320 mm (12.7 in)
diameter clamp-on current transformer, which can measure
the individual resistances of each leg, without discon-
necting any ground leads or overhead static/ground wires.
Selective measurement
Selective testing is very similar to the
Fall-of-Potential testing, providing all the same
measurements, but in a much safer and easier
way. This is because with Selective testing, the
earth electrode of interest does not need to be
disconnected from its connection to the site! The
technician does not have to endanger himself
by disconnecting ground, nor endanger other
personnel or electrical equipment inside a non-
grounded structure.
Just as with the Fall-of-Potential test, two
earth stakes are placed in the soil in a direct
line, away from the earth electrode. Normally,
spacing of 20 meters (65 feet) is sufficient. The
tester is then connected to the earth electrode
of interest, with the advantage that the connec-
tion to the site doesn’t need to be disconnected.
Instead, a special clamp is placed around the
earth electrode, which eliminates the effects of
parallel resistances in a grounded system, so
only the earth electrode of interest is measured.
Just as before, 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. Only the current flowing through the
earth electrode of interest is measured using
the clamp. The generated current will also flow
through other parallel resistances, but only
the current through the clamp (i.e. the current
through the earth electrode of interest) is used to
calculate resistance (V = IR).
If the total resistance of the ground system
should be measured, then each earth electrode
resistance must be measured by placing the
clamp around each individual earth electrode.
Then the total resistance of the ground system
can be determined by calculation.
Connect the ground tester as
shown. Press START and read out
the R
E
value. This is the actual
resistance value of the ground
electrode under test.
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