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Radiation coupling
E / H
Electromagnetic wave elds (E/H eld), that also ensue
during lightning (distant eld condition, E/H eld vectors
perpendicular to each other), affect conductor structures
in such a way that coupled overvoltages must be expected
even without direct lightning strikes. Permanent wave elds
from strong transmitters are also able to cause coupled
interference voltages in lines and circuits.
Switching operations – transients
More often, it is switching operations that cause interference
rather than lightning. High-amperage shutdowns in the
mains in particular can generate considerable overvoltages
(e. g. welding equipment). Switching operations generate
overvoltages because, due to their construction, switching
contacts that switch the current on or off do not operate
in synchronisation with the current zero of an alternating
current. This means that in the majority of cases there is
a very rapid change of current, from a high value to zero
(di/dt). Owing to the impedances in the circuit concerned,
this leads to transient overvoltages with high-frequency
oscillations and high voltage peaks. These can reach
electrical components by conductive, inductive or capacitive
means and endanger or damage them. The situation is
similar in the case of short- circuits in the mains because
these also represent a rapid switching operation.
Electrostatic discharges – ESD
Electrostatic discharges (ESD) caused by frictional charges
are well known. You can experience them when getting out
of a car or walking across a carpet. These discharges can
be over 10,000 volts in strength. We speak of ESD when
these discharge to a lower potential. If such a charge strikes,
for example, electronic components, then these can be
completely ruined. Special care is taken, for example, with
ESD issues when manufacturing electronic circuit boards.
Faulty switching operations
Again and again, we experience faulty switching operations
in the 50/60 Hz mains. These can be caused by a failed
power supply unit controller or incorrect wiring in a panel.
The relatively high voltages that can occur as a result also
represent dangerous overvoltages. Protection against these
is vital.
Description of interference voltages
Surge voltages that occur between live conductors, or
between a live conductor and the neutral conductor, are
called transverse voltage or symmetric interference [UQ].
i
S
i
S
U
Q
Surge voltages that occur between a live conductor and the
PE conductor are called longitudinal voltage or asymmetric
interference [UL].
i
S
i
S
U
L
U
L
The forms of interference voltage
Coupled transient surge voltages are basically either
symmetric (differential-mode interference) or asymmetric
(common-mode) interferences, which are measured as
longitudinal or transverse voltages.
Normal-mode interference (symmetrical interference)
A voltage between supply and return conductor, differential
mode voltage/current. Occurs mainly at low interference
frequencies in the existing lines. The interference current
causes an interference voltage U
Q
directly at the interference
sink (between the input terminals). With galvanic or
inductive coupling, both the effective sources and the
interference sources are connected serially. The load
and interference sources are connected in series as an
inductive (magentic eld) or conductive coupling (common
impedance). In symmetrical circuits (non-earthed or virtual
potential earthed), the normal-mode interference occurs as
symmetrical voltages.
In unsymmetrical circuits (earthed one side), the normal-
mode interference occurs as unsymmetrical voltages.
How do overvoltages occur?
W
The basics of lightning and surge protection
W.6 2028840000