Application Note

4 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.
Close to the electrode, the potential probe is said
to be within the influence of the electrode. Close
to the current probe the voltage is almost the full
voltage output by the tester. But somewhere in the
middle, something interesting happens.
As we move from the influence of the electrodes
and into the mass of the earth, the test current no
longer causes significant change in potential. If
you plot a series of measurements, moving the
potential stake away from the electrode under test,
and towards the current stake you will notice a
flattening of the curve. An ideal curve is shown
in Figure 3 (see previous page).
The flattest part
of the curve is where we read the earth resis-
tance. In reality, the curve never goes entirely flat
but reaches a very gentle slope where changes in
resistance are small.
The extent of the influence of the electrode
depends on its depth and it area. Deeper electrodes
require that the current stake be driven farther
away (see Table 1). For large ground rings, grids or
arrays the influence of the electrode may extend for
hundreds of feet. Table 2 gives suggested starting
points for current and potential stake placement.
Table 1: Approximate Distance to Auxiliary Stakes using
the 62 % Rule (in feet)
Depth of Electrode
under Test (E)
Distance from E to
Potential Stake (P2)
Distance from E to
Current Stake (C2)
6 50 82
8 62 100
20
81 131
30
100 161
Table 2: Approximate Distance to Auxiliary Stakes for
Electrode Arrays (in feet)
Widest Dimension
(Diagonal, diameter
or Straight-line) of
Electrode Array under
Test (E)
Distance from E to
Potential Stake (P2)
Distance from E to
Current Stake (C2)
65 100 165
80 165 265
100 230 330
165 330 560
230 430
655
Measurement Tips
• Bring a good, long tape measure.
• Finding the horizontal part of the curve
will require at least 5, but more likely 7
or 9 measurements.
•
It’s a good idea to take three of your
resistance readings with the P2 stake
at 20 %, 40 % and 60 % of the distance
between E and C2. This will allow you
to use the Tagg Slope Technique.
• When placing the stakes make sure
the current stake, the potential stake
and the electrode under test form a
straight line.
• If you get a very high impedance
measurement or over-range, try pour-
ing some water around the test stakes
to improve their contact to earth. This
isn’t cheating since our intention is
not to measure the resistance of our
stakes, but to measure the resistance
of the electrode.
• Keep the potential and current leads
separated to avoid signal coupling
between the two.
• At a new construction site, you may
want to take multiple sets of measure-
ments. Resistance may drop over time
as the earth settles
Because of the possibility of interaction between an
electrode rings, grids or arrays, and the measurement
stakes you should not take shortcuts – plot the Fall-
of-Potential graph to be sure you are getting accurate
results.
In testing a bonded array of electrodes the
combined resistance of the array will be less than
the lowest reading you measure for any individual
electrode. If, for example, you have two 8-foot rods
spaced more than 8 feet apart you can be confident
that the combined resistance will be substantially
less for the combined system.
The three-wire measurement will deliver good
results if you use a short C1 lead, or if you don’t
mind having a fraction of an ohm of lead resistance
in your reading. For ground resistance measure-
ments over 10 ohms, the effect of the resistance
of the C1 lead will be small. But for very precise
measurements, especially at low resistances, a
four-wire tester allows you add a fourth lead
to eliminate the contribution of the C1 lead. By
running a separate potential lead (P1) to the elec-
trode under test you can take the drop along the C1
current lead out of the measurement.