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Figure 2.1.5 Discharge mechanism of a negative upward flash Figure 2.1.6 Discharge mechanism of a positive upward flash (earth-
(earth-to-cloud flash) to-cloud flash)

combinations of partial lightning strikes are shown in Figure conductive parts, a voltage drop across the part carrying the
2.1.7 for downward flashes, and in Figure 2.1.8 for upward current occurs due to the amplitude of the current and the
flashes. impedance of the conductive part carrying the current. In
The lightning currents consisting of both short strokes and the simplest case, this relationship can be described using
long strokes are impressed currents, i.e. the objects struck Ohm´s Law.
have no effect on the lightning currents. Four para meters
which are important for lightning protection can be obtained U = I R
from the lightning current curves shown in Figures 2.1.7 I Peak value of the lightning current
and 2.1.8: R Earth resistance
¨ The peak value of the lightning current I
¨ The charge of the lightning current Q flash consisting of the If a current is formed at a single point on a homogeneously
charge of the short stroke Q short and the charge of the long conducting surface, a potential gradient area arises. This ef-
stroke Q long fect also occurs when lightning strikes homogeneous ground
¨ The specific energy W/R of the lightning current (Figure 2.2.1). If living beings (persons or animals) are
¨ The steepness di/dt of the lightning current rise. inside this potential gradient area, step voltage is formed
which can cause electric shock (Figure 2.2.2). The higher
The following chapters show which of the individual para- the conductivity of the ground, the flatter is the potential
meters are responsible for which effects and how they influ- gradient area. The risk of dangerous step voltages is thus
ence the dimensioning of lightning protection systems. also reduced.
If lightning strikes a building which is already equipped with a
lightning protection system, the lightning current flowing via
2.2 Peak value of the lightning current the earth-termination system of the building causes a voltage
drop across the earth resistance R E of the earth-termination
Lightning currents are impressed currents, in other words a system of the building (Figure 2.2.3). As long as all exposed
lightning discharge can be considered to be an almost ideal conductive parts in the building are raised to the same high
current source. If an impressed electric current flows through potential, persons inside the building are not in danger. There-

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