Page 129 - 35_DS702_E_2014_Lightning_Protection_Guide

l 1 (m)
area under
80 consideration A 1 r
70
60 class of LPS I circular area A 2
mean radius r
50
40 class of LPS II
30
20 In case of ring or foundation
10 class of LPS III-IV A = A 1 = A 2 earth electrodes, the mean
A
0 r = π radius r of the area enclosed
0 500 1000 1500 2000 2500 3000 by the earth electrode must
r ≥ l 1 not be less than l 1 .
ρ E (Ωm)
Figure 5.5.1.1 Minimum lengths of earth electrodes Figure 5.5.1.2 Type B earth electrode – Determination of the mean
radius – Sample calculation

dial earth electrodes (surface earth electrodes), which must be
connected to a down conductor. 12 m
A type A earth electrode arrangement require at least two area under
earth electrodes. A single earth electrode is sufficient for indi- consideration A 1 5 m r
vidually positioned air-termination rods or masts.
A minimum earth electrode length of 5 m is required for class 12 m 5 m circular area A 2
of LPS III and IV. For class of LPS I and II the length of the 7 m mean radius r
earth electrode is defined as a function of the soil resistivity.
Figure 5.5.1.1 shows the minimum earth electrode length I 1 .
The minimum length of each earth electrode is: 7 m Example: Residential building,
class of LPS III, l 1 = 5 m
I 1 x 0.5 For vertical or inclined earth electrodes
A 1 = 109 m 2
I 1 For radial earth electrodes A = A 1 = A 2
r = A r = 109 m 2 No additional
The values determined apply to each single earth electrode. π 3.14 earth electrodes
If different earth electrodes (vertical and horizontal) are com- r ≥ l 1 r = 5.89 m are required!
bined, the equivalent total length should be taken into account.
The minimum earth electrode length can be disregarded if an
earth resistance of less than 10 Ω is achieved. Figure 5.5.1.3 Type B earth electrode – Determination of the mean
radius – Sample calculation
In general, earth rods are vertically driven deeply into natu-
ral soil which typically starts below foundations. Earth elec-
trode lengths of 9 m have proven to be advantageous. Earth
rods have the advantage that they reach soil layers in greater the connecting cable of the single earth electrodes can also be
depths whose resistivity is generally lower than in the areas implemented in the structure.
close to the surface.
It is recommended that the first metre of a vertical earth elec- Type B earth electrodes
trode is considered ineffective under frost conditions. Type B earth electrodes are ring earth electrodes encircling the
Type A earth electrodes do not meet the requirements with object to be protected or foundation earth electrodes. In Ger-
regard to equipotential bonding between the down conduc- many, the requirements for earth-termination systems of new
tors and potential control. Single earth electrodes of type A buildings are described in DIN 18014.
must be interconnected to ensure that the current is evenly If it is not possible to encircle the structure by means of a
split. This is important for calculating the separation distance s. closed ring, the ring must be complemented by means of con-
Type A earth electrodes can be interconnected below or on the ductors inside the structure. Pipework or other permanently
surface of the earth. When retrofitting existing installations, conductive metal components can also be used for this pur-

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