Application Note

5 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.
If you have resistance readings at the 20 %,
40 % and 60 % points between E and C2, then you
can apply the procedure to the data you’ve already
taken.
Calculate the slope coefficient (μ) using three
resistance measurements from 20 %, 40 % and
60 % of the distance from the electrode under test
to the C2 current stake.
Then go to the table in the back of this application
note and look up the P2/C2 ratio that corresponds
to your μ. This will tell you where to look on your
graph to ascertain the earth resistance. For the
sample data in Figure 6:
If we go to the table, for μ = 0.71 the corresponding
P2/C2 percentage is 59.6 %. So the approximate
earth resistance would be measured at
(59.6 % X 300 feet), or at 178 feet. This is very
close to our 60 % point at 180 feet, where we
read 6.8 ohms. So it would be safe to say the earth
resistance for the electrode under test is roughly
7 ohms.
The 62 % Rule
You may be able to use a shortcut if your test meets
the following criteria:
•
You are testing a simple electrode (not a large
grid or plate)
•
You can place the current stake 100 feet or more
from the electrode under test
•
The soil is uniform
Under these conditions you can place the current
stake 100 feet or more from the electrode under test.
Place the potential stake at 62 % of the distance
between the current stake and the electrode under
test and take a measurement. As a check, take two
more measurements: one with the potential probe
3 feet closer to the electrode under test, and one 3
feet farther away (see Figure 5). If you are on the flat
portion of the fall-of-potential curve then the read-
ings should be roughly the same and you can record
the first reading as your resistance.
( R
60 %
– R
40 %
)
( R
40 %
– R
20 %
)
μ =
( 6.8 – 5.8 )
( 5.8 – 4.4 )
μ =
= 0.71
Distance From Electrode Under Test Resistance
C2 P2 P2/C2 R
(feet) (feet) (ohms)
300 30 10% 3.7
300 60 20% 4.4
300 90 30% 5.3
300 120 40% 5.8
300 150 50% 6.5
300 180 60% 6.8
300 210 70% 7.0
300 240 80% 7.7
300 270 90% 8.8
Fall-of-Potential Plot
0.0
2.0
4.0
6.0
8.0
10.0
0 50 100 150 200 250 300
P2 Distance from Electrode Under Test (feet)
R (Ohms)
Current
Spike
Potential
Spike
Electrode
Under test
d
62 % d
E
P2
C2
Figure 5: Stake positions for the 62 % rule.
The Tagg Slope Technique
Large electrodes or grounding systems require
some special consideration. If you’ve plotted
resistance readings for nine different P2 locations
and there is no clear flattening on your graph,
then the Tagg Slope Technique (also called the
slope method) can help establish the earth imped-
ance. Figure 6 shows an example dataset for
which there is no obvious flat section. This curve
is characteristic of a test in which the current and
potential probes never get outside the influence of
the electrode under test. There can be a number
of reasons for a curve like this:
•
For electrode systems that cover large areas it
may be difficult to place stakes far enough away
•
You may not be able to place the C1 stake at the
center of the electrode
•
The area you have to place stakes may be limited
Figure 6: Earth impedance can be found from this curve by using
the Tagg Slope Technique