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be determined at the base of the air-termination rod. Current potential level is equal to the height of the building. No sepa-
paths with different conductor lengths are formed due to the ration distances must be maintained.
finely meshed conductor routing on the roof surface. Moreover,
the lightning current is split as follows according to the nodes: In general, the requirements of the IEC 62305-3 (EN 62305-3)
¨ 1. Base of the air-termination rod (two conductors) standard must be observed. The DEHN Distance Tool of the
DEHNsupport software allows to easily calculate the separa-
k c1 = 0.5 with a conductor length l 1 of 8.0 m
tion distance based on nodal analysis as described in 3.3.2.1.
¨ 2. Node 1 (two conductors)
k c2 = 0.25 with a conductor length l 2 of 4.0 m
¨ 3. Node 2 (two conductors) 5.7 Step and touch voltage
k c3 = 0.125 with a conductor length l 3 of 10.0 m
IEC 62305-3 (EN 62305-3) points out that, in special cases,
¨ 4. Node 3 (three conductors) touch or step voltage outside a building in the vicinity of the
k c4 = 0.063 with a conductor length l 4 of 10.0 m down conductors can present a life hazard even though the
¨ 5. Node 4 (three conductors) lightning protection system was designed according to the lat-
k c5 = 0.042 with a conductor length l 5 of 8.0 m est standards.
Special cases are, for example, the entrance areas or covered
areas of highly frequented structures such as theatres, cin-
The separation distance is calculated as follows:
emas, shopping centres or nursery schools where bare down
k (k l +k l +...+k l ) conductors and earth electrodes are present in the immediate
s = i c1 1 c2 2 cn n vicinity.
k Measures against impermissibly high step and touch voltages
m
may also be required for structures which are particularly ex-
0.06(0.5 8m + 0.25 4m + 0.125 10m + 0.063 10m + 0.042 8m) posed (prone to lightning strikes) and freely accessible to the
s =
0.5 general public.
s = 0.87 in case of solid material These measures (e.g. potential control) are primarily taken for
churches, observation towers, shelters, floodlight pylons in
sports grounds and bridges.
A separation distance of 0.87 m (solid material) must be main-
tained at the base of the air-conditioning system. The number of people can vary from place to place (e.g. in the
entrance area of shopping centres or in the staircase of obser-
Determination of the zero potential level vation towers). Therefore, measures to reduce step and touch
To calculate the separation distance, it is important to deter- voltage are only required in areas which are particularly at risk.
mine the zero potential level. The zero potential level of build-
ings is located at the same height as the foundation or ring Possible measures are potential control, standing surface insu-
earth electrode. Thus, the definition of the zero potential level lation or additional measures which will be described below.
is decisive for the separation distance s. The individual measures can also be combined with each other.

Buildings with a wall and ceiling reinforcement, which is inter- Definition of touch voltage
connected in such a way that it is capable of carrying lightning Touch voltage is a voltage acting on a person between its
currents, can be used as a down-conductor system. Thus, no standing surface on earth and when touching the down con-
separation distances must be maintained due to the constant ductor. The current path leads from the hand via the body to
potential. However, the roof surfaces are typically covered with the feet (Figure 5.7.1).
insulations and roof membranes on which meshed air-termina-
tion systems are installed. These meshed air-termination sys- For structures with a steel frame or reinforced concrete con-
tems are connected to the reinforcement in the vicinity of the struction, there is no risk of impermissibly high touch voltages
roof parapet. In case of a lightning strike, separation distances provided that the reinforcement is safely interconnected or the
must be maintained from the meshes and the conductors. down conductors are laid in concrete.
Therefore, it is recommended to install insulated conductors Moreover, the touch voltage can be disregarded in case of
which allow to maintain the separation distances. metal façades if they are integrated in the equipotential bond-
In case of buildings with an interconnected steel frame con- ing system and / or used as natural components of the down
struction and a metal roof, it can be assumed that the zero conductor.

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