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+ρ
(%)
E
30
20
10
Jan.
May
-10
March
July
-20
-30
Soil resistivity ρE in as influenced by season
Figure 8
without taking precipitation into consideration
(earth spike depth < 1.5 m)
The following table shows typical soil resistivity for several types of soil.
Soil Resistivity
Type of Soil
ρ
[Ωm]
E
Wet bog soil
Arable fields, loam and clay,
moist gravel
Moist, sandy soil
Dry sandy soil,
dry gravel
Rocky ground
Bedrock
Soil Resistivity ρ
Table 1
for Various Types of Soil
E
GMC-I Messtechnik GmbH
Sept.
Nov.
8 ...
60
20 ...
300
200 ...
600
200 ...
2000
300 ...
8000
4
...
10
10
10
5.6.2 Calculating Dissemination Resistance
The following table includes formulas for calculating dissemination
resistance for various types of earth electrodes.
These rule-of-thumb formulas are entirely adequate for practical
application.
Number
Earth Electrode Type
Earth strip (crow's foot
1
earth electrode)
2
Earth rod
3
Ring earth electrode
4
Mesh earth electrode
5
Earth plate
Hemispherical
6
earth electrode
Table 2
Formulas for the Calculation of Dissemination Resistance R
Types of Earth Electrodes
R
= Dissemination resistance (Ω)
A
ρ
= Soil resistivity (Ωm)
E
I = Earth electrode length (m)
D = Ring earth electrode diameter, diameter of the equivalent circular area of a
mesh earth electrode or diameter of a hemispherical earth electrode (m)
F = Surface area (square meters) enclosed by a ring or a mesh earth electrode
a = Edge length (m) of a rectangular earth plate, a is defined as follows for rectan-
gular earth plates: √ b x c, where b and c are the two sides of the rectangle.
J = Volume (cubic meters) of an individual foundation
Formula
Auxiliary Information
⋅
2 ρ
E
----------
—
R
=
A
I
ρ E
--- -
R
=
—
A
I
⋅
2 ρ
E
----------
R A
=
D = 1.13 ·
3D
⋅
2 ρ
E
----------
D = 1.13 ·
R
=
A
2D
⋅
2 ρ
E
---------- -
R A
=
—
⋅
4,5 a
ρ
E
------- -
D = 1.57 ·
R A
=
⋅
π D
for Various
A
2
F
2
F
3
J
23
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