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rolling sphere head of the
downward downward leader
point afar from leader
the head of the starting
downward leader upward leader
starting closest point to
upward leader the head of the
downward leader

final striking
distance h B
A rolling sphere cannot only touch the
steeple, but also the nave of the church at
multiple points, as this model experiment
shows. All these points are potential
points of strike.
Figure 5.1.1.1 Starting upward leader defining the point of strike Figure 5.1.1.2 Model of a rolling sphere;
source: Prof. Dr. A. Kern, Aachen

downward leader. The head of the downward leader can only between the peak value I of the lightning current and the final
approach the earth within a certain distance. This distance is striking distance h B (= radius of the rolling sphere):
defined by the continuously increasing electrical field strength
of the ground as the head of the downward leader approach- r = 10 I 0.65
es. The smallest distance between the head of the downward r in m
leader and the starting point of the upward leader is called I in kA
the final striking distance h B (corresponds to the radius of the
rolling sphere). The protection of buildings against lightning is described in the
Immediately after the electrical insulation strength is exceeded IEC 62305-1 (EN 62305-1) standard. Among other things, this
at one point, the upward leader, which leads to the final strike standard defines the classification into the individual lightning
and manages to cross the final striking distance, is formed. protection levels / classes of LPS and stipulates the resulting
Observations of the protective effect of earth wires and pylons lightning protection measures.
were used as the basis for the so-called electro-geometric It differentiates between four classes of LPS. Class of LPS I
model. provides the most protection and a class of LPS IV, by com-
parison, the least. The interception effectiveness E i of the air-
This is based on the hypothesis that the head of the downward termination systems is concomitant with the class of LPS, i.e.
leader approaches the objects on the ground, unaffected by which percentage of the prospective lightning strikes is safely
anything, until it reaches the final striking distance. controlled by the air-termination systems. From this, the final
The point of strike is then determined by the object closest to striking distance and hence the radius of the rolling sphere
the head of the downward leader. The upward leader starting is obtained. The relationships between lightning protection
from this point “forces its way through” (Figure 5.1.1.2). level / class of LPS, interception effectiveness of the air-termi-
nation systems, final striking distance / radius of the rolling
Classes of LPS and radius of the rolling sphere sphere and current peak value are shown in Table 5.1.1.1.
As a first approximation, a proportionality exists between the
peak value of the lightning current and the electrical charge Taking as a basis the hypothesis of the electro-geometric mod-
stored in the downward leader. Furthermore, the electrical field el that the head of the downward leader approaches the ob-
strength of the ground as the downward leader approaches is jects on the earth in an arbitrary way, unaffected by anything,
also linearly dependent on the charge stored in the downward until it reaches the final striking distance, a general method
leader, to a first approximation. Thus there is a proportionality can be derived which allows the volume to be protected of

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