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Properties of the structure R A R B R V 3.3.2 DEHN Distance Tool; calculation of
or internal systems the separation distance according to
IEC 62305-3 (EN 62305-3)
Protection measures The DEHN Distance Tool is another module of the DEHNsupport
Physical restrictions, insulation, Toolbox software. In addition to the conventional calculation
warning notice, potential control • formulas for determining the separation distance and thus
on the ground the partitioning coefficient k c , more exact calculations can be
Lightning protection system (LPS) • • • b performed according to the standard. The calculation of the
separation distance is based on nodal analysis.
Surge protective devices for lightning
equipotential bonding • • •
b Due to equipotential bonding
3.3.2.1 Nodal analysis
Table 3.3.1.1 DEHN Risk Tool, measures (excerpt) Kirchhoff’s first law defines that at any node the sum of the
currents flowing into that node is equal to the sum of currents
flowing out of that node (nodal rule) (Figure 3.3.2.1.1).
¨ Costs of the content c c This rule can also be used for buildings with external lightning
¨ Total costs of the structure (c a + c b + c c + c s ) c t protection system. In case of a simple building with one air-
termination rod (Figure 3.3.2.1.2), the lightning current is
It must be observed that the costs also include replacement
costs, downtime costs and follow-up costs. These costs must distributed at the base in the event of a lightning strike to the
be evenly allocated to the types of costs. air-termination rod. This lightning current distribution depends
on the number of down conductors, also referred to as current
The EN 62305-2 standard allows to define these values ac- paths. Figure 3.3.2.1.2 shows a node with four conductors
cording to tables if they are unknown. The following procedure (current paths).
must be observed: In case of a meshed external lightning protection system with
down conductors, the lightning current is distributed in each
¨ Assessment of the total value c t of the structure (Table junction and at the connection point of the air-termination sys-
3.2.9.1) tem. To this end, a clamp connection according to IEC 62305-3
¨ Proportional assessment of the values of c a , c b , c c and c s (EN 62305-3) is required.
based on c t (Table 3.2.9.2) The closer the mesh or the higher the number of nodes, the
In addition to c a , c b , c c and c s , the costs of protection measures better is the lightning current distribution. The same applies to
c p must also be defined. To this end, the the entire conductor routing (Figure 3.3.2.1.3).
¨ Interest rate i Nodal analysis is used to calculate the exact lightning cur-
¨ Maintenance rate m and rent distribution and the resulting separation distances. This
method is used for network analysis in electrical engineering
¨ Amortisation rate a and is a transmission line method. If nodal analysis is used for
must be defined. a building with external lightning protection system, each line
(current path) is shown in the form of a resistor. Thus, the vari-
The result of the consideration of loss of economic value is: ety of meshes and down conductors in a lightning protection
¨ Costs of the total loss C L without protection measures system forms the basis for nodal analysis. The lines of a light-
ning protection system, for example of a mesh, are typically
¨ Remaining loss costs C RL despite of protection measures
divided into many individual line sections by means of nodes
¨ Annual costs C PM of the protection measures
(junctions). Each line section represents an electric resistance R
¨ Savings (Figure 3.3.2.1.4). When using nodal analysis, the reciprocal
The calculated values are displayed in € / year. value of the resistance, also referred to as conductance G, is
If the assessment leads to positive savings S M , the protection used for calculation:
measures make economic sense. If it leads to negative savings
S M , the protection measures do not make economic sense. Af- R = 1 G = 1
ter performing a risk analysis, a detailed report or a summary G R
can be printed (rtf file).

52 LIGHTNING PROTECTION GUIDE www.dehn-international.com