Brochure/Catalogue

How do overvoltages occur?
How do overvoltages occur?
Surges are primarily caused by:
• Transient switching operations
• Lightning due to atmospheric discharges
• Electrostatic discharges
• Faulty switching operations
Lightning
Bolts of lightning comprise extremely high currents. They
can cause a large voltage drop and a large rise in potential,
even in well-earthed buildings or systems, despite low
earthing resistances.
This can then result in a galvanic, inductive or capacitive
coupling of surge voltages within the circuits of electrical or
electronic facilities. Any insulation will also be penetrated.
So, in reality, there are no electrical isolation methods
which provide reliable protection against surge voltages.
Analogue converters, relays or opto modules are important
for separating potentials, but they are denitely not surge
protection components.
A natural lightning strike consists of a main discharge and
a time-shifted post discharge. The strength of this second
discharge is usually far below the energy level of the main
discharge. Both discharges, however, have enough power to
cause signicant damage.
Our table below shows results from a natural lightning strike
and a lightning current generator which simulates a lightning
pulse.
The various forms of coupling must be considered in order to
understand the effects of lightning.
Conductive coupling
i
1
Z
g
i
2
i
g
Surges are transferred directly into circuits via common
earthing impedances. The magnitude of the overvoltage
depends on the amperage of the lightning and the earthing
conditions. The frequency and the wave behaviour are
mainly determined by the inductance and the speed of
the current rise. Even distant lightning strikes can lead to
overvoltages in the form of travelling waves, which affect
different parts of electrical systems by way of conductive
coupling
Inductive coupling
i
S
i
ind
H
A high-amperage lightning strike generates a strong
magnetic eld. Starting from here, overvoltages reach nearby
circuits by means of an induction effect (e.g. directly earthed
conductor, power supply lines, data lines, etc.). According to
the transformer principle, the coupling of induced voltages
is considerable owing to the high-frequency current di/dt –
even when primary and secondary windings consist of only
a single winding each, i.e. the inductance is low.
Capacitive coupling
C
P
C
P
C
P
A capacitive coupling of overvoltages is also possible. The
high voltage of the lightning generates an electric eld with
a high eld strength. The transport of electrons can cause a
capacitive decay to circuits with lower potentials and raise
the potential concerned to an overvoltage level.
-
-
0
Main discharge
I
imp
I/kA
Period between the discharges
and the several follow-up
discharges
0 10 20 30 40 50 60 70 80 90 t/µ 110
The discharge curve of natural lightning (red) and a simulated lightning strike
from lightning current generator (green)
W
The basics of lightning and surge protection
W.52028840000