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Concrete
Boggy soil, turf
Farmland, loam
Humid sandy soil
Dry sandy soil
Rocky soil
Gravel
Lime
River / lake water
Sea water
0.1 1 10 100 1000 10000 ρ E in Ωm

Figure 5.5.4 Earth resistivity ρ E in case of different types of soil

about 1.5 m, the maximum deviation of the earth resistivity the earth resistivity in Figure 5.5.5, the earth resistance R A of
from the average value is about ± 30 % (Figure 5.5.5). an earth-termination system measured on a particular day can
be converted to the maximum value to be expected.
If earth electrodes (particularly earth rods) are buried deeper,
the fluctuation is only ± 10 %. From the sine-shaped curve of Measurement
The earth resistivity ρ E is determined using an earthing meas-
+ ρ E in % burial depth < 1.5 m uring bridge with four clamps (four-conductor method / four-
pole measuring method) which operates according to the zero
30 burial depth > 1.5 m method.
20 Figure 5.5.6 shows the measurement arrangement of this
10 Jun Jul Aug Sep Oct Nov measuring method named after WENNER. The measurement
0 is carried out from a fixed central point M which is kept for
Jan Feb Mar Apr May Dec
10 all subsequent measurements. Four measuring probes (30 to
20 50 cm long earth spikes) are driven into the soil along a line
30 a – a' pegged out in the ground. The earth resistivity ρ E of the
-- ρ E in % ground can be determined from the measured resistance R:
Figure 5.5.5 Earth resistivity ρ E as a function of the time of year
without precipitation effects (burial depth of the earth E = 2 e R
electrode < 1.5 m)
R Measured resistance in Ω
e e e e Probe spacing in m
ρ E Average earth resistivity in Ωm up to a depth accord-
ing to the probe spacing e
By increasing the probe spacing e and retuning the earthing
a M a’
measuring bridge, the curve of the earth resistivity ρ E can be
determined as a function of the depth.

measuring Calculation of earth resistances
device Table 5.5.1 shows the formulas for calculating the earth re-
sistances of commonly used earth electrodes. In practice, these
Figure 5.5.6 Determination of the earth resistivity ρ E by means of a approximate formulas are quite sufficient. The exact calcula-
four-terminal measuring method (WENNER method) tion formulas are specified in the following sections.

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