Page 126 - 35_DS702_E_2014_Lightning_Protection_Guide

Conventional earthing impedance R st 160 n = 1 = 150 Ω n = 4 l 20 p 5 n = 20
Ω
Z
R A = 10 Ω
140
= 1 ... 4
n
n · l = 300 m
10
120
10
100
80
60
40 2 3 3 2 5 3
4 2
20 1
0
0 1 2 3 4 5 6 Time µs a
0.5 1 2 5 10
l
Z Characteristic impedance of the earthing conductor p Reduction factor
R A Earth resistance n Number of earth electrodes connected in parallel
n Number of earth electrodes connected in parallel a Average earth electrode distance
l Average earth electrode length l Average earth electrode length
Figure 5.5.13 Conventional earthing impedance R st of single-arm or Figure 5.5.14 Reduction factor p for calculating the total earth
multiple-arm surface earth electrodes of equal length resistance R A of earth rods connected in parallel

As can be seen from this diagram, it is more favourable to Earth rods connected in parallel
install a radial earth electrode than a single arm for the same To keep interactions within acceptable limits, the distances be-
length. tween the single earth electrodes for earth rods connected in
parallel should not be less than the drive-in depth.
Foundation earth electrodes If the single earth electrodes are roughly arranged in a circle
The earth resistance of a metal conductor in a concrete foun- and have approximately the same length, the earth resistance
dation can be calculated as an approximation using the for- can be calculated as follows:
mula for hemispherical earth electrodes: R
R = A'
R = E A p
A
d
where R A' is the average earth resistance of the single earth
where d is the diameter of the analogous hemisphere having electrodes. The reduction factor p as a function of the length
the same volume as the foundation: of the earth electrodes, the distance between the single earth
electrodes and the number of earth electrodes can be taken
d = 1.57 3 V from Figure 5.5.14.

V Volume of the foundation in m 3 Combination of strip earth electrodes and earth
rods
When calculating the earth resistance, it must be observed If earth rods provide a sufficient earth resistance, for example
that the foundation earth electrode is only effective if the con- in case of deep water carrying layers in sandy soil, the earth
crete body contacts the surrounding ground over a large area. rod should be installed as close as possible to the object to
Water-repellent, insulating shielding significantly increases the be protected. If a long supply line is required, it is advisable to
earth resistance or insulates the conductor in the foundation install a multiple radial earth electrode in parallel to the line to
(see 5.5.2). reduce the resistance as the current rises.

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