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tection against direct lightning strikes needed to be improved,
this could be done (Figure 5.1.1.5).

Example 2: Aachen Cathedral
The cathedral stands in the middle of the old quarter of Aachen
surrounded by several high buildings. Adjacent to the cathe-
dral is a scale model (1:100) whose purpose is to make it easier
for visitors to understand the geometry of the building.

The buildings surrounding Aachen Cathedral provide a degree
of natural protection against lightning strikes. To demonstrate
the natural protection and the effectiveness of lightning pro-
tection measures, a model of the most important elements of
the surrounding buildings was made on the same scale (1:100)
Figure 5.1.1.4 New administration building: Model with rolling (Figure 5.1.1.6).
sphere according to class of LPS I;
source: WBG Wiesinger
Figure 5.1.1.6 also shows rolling spheres for classes of LPS II
and III (i.e. with radii of 30 cm and 45 cm) on the model.
The aim here was to demonstrate the increasing requirements
on the air-termination systems as the radius of the rolling
sphere decreases, i.e. which areas of Aachen Cathedral had
additionally to be considered at risk from lightning strikes if
a class of LPS II providing a higher degree of protection was
used.
The rolling sphere with the smaller radius (according to a class
of LPS providing a higher lightning protection level) naturally
also touches the model at all points already touched by the
rolling sphere with the larger radius. It is thus only necessary
to determine the additional contact points.
As demonstrated, the sag of the rolling sphere is decisive when
dimensioning the air-termination system for a structure or a
Figure 5.1.1.5 New DAS administration building: Areas threatened roof-mounted structure.
by lightning strikes for class of LPS I, top view
(excerpt); source: WBG Wiesinger The following formula can be used to calculate the penetration
depth p of the rolling sphere when the rolling sphere rolls “on
rails”, for example. This can be achieved by using two spanned
wires, for example.
 d  2
2
p = r − r −  
2
 
r Radius of the rolling sphere
d Distance between two air-termination rods or two
parallel air-termination conductors

Figure 5.1.1.7 illustrates this approach.
Air-termination rods are frequently used to protect the surface
Figure 5.1.1.6 Aachen Cathedral: Model with surroundings and roll- of a roof or roof-mounted structures against a direct lightning
ing spheres of classes of LPS II and III; strike. The square arrangement of the air-termination rods, over
source: Prof. Dr. A. Kern, Aachen which no cable is generally spanned, means that the sphere

66 LIGHTNING PROTECTION GUIDE www.dehn-international.com