Page 22 - 35_DS702_E_2014_Lightning_Protection_Guide

molten metal
Q
lightning current
current 10.00 mm
U A,C 10.00 mm
Q short = ∫idt
Aluminium Copper
time d = 0.5 mm; 200 A, 350 ms d = 0.5 mm; 200 A, 180 ms
long stroke current
current Q long = ∫idt

time
tip of the lightning protection system 10.00 mm 10.00 mm
Stainless steel Steel
Figure 2.4.1 Energy conversion at the point of strike due to the d = 0.5 mm; 200 A, 90 ms d = 0.5 mm; 200 A, 100 ms
charge of the lightning current

10.00 mm
Galvanised steel
d = 0.5 mm; 200 A, 100 ms
Figure 2.4.3 Plates perforated by the effects of long stroke arcs

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
Galvanised steel Copper
100 kA (10/350 µs) 100 kA (10/350 µs) which is capable of melting or vaporising large volumes of
material. Figures 2.4.2 and 2.4.3 show a comparison be-
Figure 2.4.2 Effect of a short stroke arc on a metal surface
tween the effects of the short stroke charge Q short and the
long stroke charge Q long .
product of the charge Q and the anode / cathode drop voltage
U A,C , which is in the micrometre range (Figure 2.4.1). 2.5 Specific energy
The average value of U A,C is some 10 V and depends on influ-
ences such as the current intensity and wave form: The specific energy W/R of a short stroke is the energy the short
stroke converts into a resistance of 1 Ω. This energy conversion
W =Q U is the integral of the square of the short stroke over time for
A,C
the duration of the short stroke:
Q Charge of the lightning current
2
U A,C Anode / cathode drop voltage W = i dt
R
Consequently, the charge of the lightning current causes the
components of the lightning protection system directly struck Therefore, this specific energy is frequently referred to as cur-
by lightning to melt and also stresses isolating and protective rent square impulse. It is relevant for the temperature rise in
spark gaps as well as spark-gap-based surge protective de- conductors carrying lightning impulse currents as well as for
vices. Recent tests have shown that, because the arc persists the force exerted between conductors carrying lightning im-
for a longer time, it is mainly the long stroke charge Q long pulse currents (Figure 2.5.1).

www.dehn-international.com LIGHTNING PROTECTION GUIDE 21