Application Note

3 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.
Checking Connection Resistance
Leading Up to the Electrode
Before testing the electrode, start by checking its
connection to the facility bonding system. Most
Fall-of-Potential testers have the ability to measure
2-pole, low ohms and are perfect for the job. You
should see less than 1 ohm:
•
At the main bonding jumper
•
Between the main bonding jumper and the
ground electrode conductor
•
Between the ground electrode conductor and
the ground electrode
•
Along any other intermediate connection
between the main bonding jumper and the
ground electrode
The Fall-of-Potential Method
The Fall-of-Potential method is the “traditional”
method for testing electrode resistance. The proce-
dure is specified in the IEEE-81 standard “Guide for
Measuring Earth Resistivity, Ground Impedance and
Earth Surface Potentials of a Ground System.” In it’s
basic form, it works well for small electrode systems
like one or two ground rods. We will also describe
the Tagg Slope Technique which can help you draw
accurate conclusions about larger systems.
Remember: for this method, the ground elec-
trode must be disconnected from the building
electrical service.
How it works
The Fall-of-Potential method connects to the
earth at three places. It is often called the “three-
pole method.” You may want to use a fourth lead
for precise measurements on low-impedance
electrodes, but for our initial discussions we will
consider three leads.
The connections are made to:
•
E/C1 – the ground electrode being tested
•
S/P2 – A voltage (potential) measurement stake
driven into the earth some distance away from
the electrode. Sometimes called the potential
auxiliary electrode
•
H/C2 – A current stake driven into the earth a
further distance away. Sometimes called the
current auxiliary electrode
Figure 3 shows this schematically and Figure 4
shows the three connections made using a typical
ground tester.
The ground tester injects an alternating current
into the earth between the electrode under test
(E) and the current stake (C2). The ground tester
measures the voltage drop between the P2 stake
and E. It then uses ohms law to calculate the
resistance between P2 and E.
To perform the test you position the C2 stake at
some distance from the electrode under test. Then,
keeping the C2 stake fixed, you move the P2 stake
along the line between E and C2, measuring the
impedance along the way.
The tricky part comes in determining where to
drive the stakes to get a true reading of the resis-
tance between the electrode and the earth.
At what point does the dirt surrounding the
electrode stop being a contributor of resistance
and become the vast earth? Remember that we
are not interested in the resistance between the
electrode and our stakes. We are trying to measure
the resistance that a fault current would see as it
passes through the mass of the earth.
The current probe generates a voltage between
itself and the electrode under test. Close to the
electrode, the voltage is low and becomes zero
when the P stake and electrode are in contact.
Measured
Resistance
Distance of P2 from E
Current
Spike
Potential
Spike
Electrode
Under test
Electrode/Earth
Impedance
I
V
E
P2
C2
d1 d2
C2
R
H
R
E
P2 C1&P1
V
I
Figure 3: 3-point measurement
Figure 4: A plot of measured impedances versus position of the
potential stake allows us to see the earth impedance