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The loss of economic value forms an exception. For this type of A = L W + 2 (3 H) (L +W)+ (3 H) 2
loss, protection measures should be based on economic con- D
siderations. If the data for this analysis are not available, the Nearby lightning strikes
-3
representative value of tolerable risk R T = 10 specified in the For nearby lightning strikes with magnetic effects we have:
IEC standard may be used.
In the EN standard, there is no tolerable risk R T . Therefore, it is N M = N G A M 10 -6
advisable to perform a cost benefit analysis.
A M is obtained from drawing a line around the structure at a
distance of 500 m (Figure 3.2.3.3). Lightning strikes to the
3.2.3 Frequency of dangerous events area A M magnetically induce surges in installation loops in the
The following frequencies of dangerous events can be relevant structure.
for a structure: Lightning strikes to lines
N D Caused by direct lightning strikes to a structure; For direct lightning strikes to an incoming line we have:
N M Caused by nearby lightning strikes with magnetic ef-
fects; N = N A C C C 10 -6
L G L I E T
N L Caused by direct lightning strikes to incoming lines;
N I Caused by lightning strikes near incoming lines. where N L is the annual number of surges on the line section
with a maximum value of at least 1 kV.
A detailed calculation can be found in Annex A of IEC 62305-2 C I is the installation factor of the line (Table 3.2.3.2) which
(EN 62305-2). takes into account whether an overhead line or a buried cable
The average annual number N of dangerous events resulting is used. If a medium-voltage line is installed in the area A L ra-
from lightning strikes influencing a structure to be protected ther than a low-voltage line, the required transformer reduces
depends on the thunderstorm activity of the region where the the surges at the entry point into the structure. In such cases,
structure is located and on the structure’s physical character- this is taken into account by the line type factor C T (Table
istics. To calculate the number N, the ground flash density N G 3.2.3.3). C E is the environmental factor (Table 3.2.3.4) which
should be multiplied by an equivalent collection area of the defines the “building density” near the line and thus the prob-
structure, taking into account correction factors for the struc- ability of a lightning strike.
ture’s physical characteristics. The ground flash density N G is For the collection area for direct lightning strikes to the line
the number of lightning strikes per km per year (e.g. Figure (Figure 3.2.3.3) we have:
2
3.2.3.1).
This value is available from ground flash location networks in A = 40 L
many areas of the world. If a map of N G is not available, in L L
temperate regions it may be estimated by: where L L is the length of the line section. If the length of the
line section is unknown, a worst case value of L L = 1000 m
N 0.1 T should be assumed.
G D
As a rule, lightning strikes within the area A L lead to a high-
where T D is the thunderstorm days per year (which can be ob- level discharge which can cause fire, explosion or a mechanical
tained from isokeraunic maps). or chemical reaction in the relevant structure. Therefore, the
frequency N L does not only include surges resulting in faults on
Direct lightning strikes or damage to the electrical and electronic systems, but also in
For direct lightning strikes to the structure we have: mechanical and thermal effects which arise in case of lightning
interference.
N = N A C 10 -6
D G D D
For lightning strikes near an incoming line with a maximum
A D is the equivalent collection area of the isolated structure in value of at least 1 kV, which cause surges on this line, we have:
m (Figure 3.2.3.2). C D is a location factor which considers
2
the influence of the surroundings (buildings, terrain, trees, etc.) N = N G A C C E C T 10 6
I
I
I
(Table 3.2.3.1).
The collection area for an isolated rectangular structure with a where the same boundary conditions and correction factors
length L, width W and height H on a plane surface is calculated (Tables 3.2.3.2 to 3.2.3.4) apply as in case of direct light-
as follows: ning strikes.

34 LIGHTNING PROTECTION GUIDE www.dehn-international.com