Brochure/Catalogue

How do overvoltages occur?
Transverse voltage U
Q
(normal-mode voltage)
This is a transient coupled interference between two active
conductors. For asymmetric circuits with ground potential,
the transverse voltage is equal to the longitudinal voltage
[U
Q
= U
L
].
A remedy or limitation may be achieved by twisting the
corresponding wires together and shielding or multiple
shielding with cable sheath. This reduces the induction of
transverse voltages.
Common-mode interference
(unsymmetrical interference)
Voltage between conductor and reference potential (earth),
common-mode voltage/current. Mainly caused by a
capacitive coupling (electrical eld).
Therefore, signicant common-mode interference currents
only ow at higher interference frequencies. The interference
voltage at the potentially susceptible device is caused by
different voltage drops at the supply and return conductors
(in each case between input terminal and reference earth).
The source of interference can be between a signal wire and
reference conductor increase in reference potential between
separate earths.
In symmetrical circuits, common-mode interference occurs
as asymmetrical voltages between the d.c. offset of the
circuit and the reference earth. The forward and return
conductors have the same interference voltages compared
to the reference ground. In unsymmetrical circuits, common-
mode interference occurs as unsymmetrical voltages
between the individual conductors and the reference earth.
Longitudinal voltage U
L
(common-mode voltage)
A coupled transient interference voltage between an active
conductor and the earth potential. As a rule, the longitudinal
voltage is higher than the transverse voltage (transverse
voltage is lower owing to cable shielding and twisting).
Longitudinal voltages caused by lightning currents on cable
shielding can assume quite high values, especially in the
case of long lines entering a building from the outside.
Consequences
The impedances and stray capacitances are equal in ideal
circuits. This means that the currents in the supply and
return conductors generated by coupled overvoltages are
also equal and so do not generate any interference voltage.
However, in practice the impedances and stray capacitances
in the supply and return conductors are different. This results
in unequal currents which cause different voltages to earth
in the supply and return conductors.
This means that the unequal impedances lead to the
common-mode voltage becoming, for the most part, a
normal-mode voltage because of the dissimilarity in the
voltages to earth of the supply and return conductors.
U
push-pull
U
common mode
U
sym.
U
unbalanced 1
U
unbalanced 2
U
asym.
Z / 2
Z / 2
Electrical system
MSR
0
1
2
3
4
5
SURGE PROTECTION
SURGE PROTECTION
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The basics of lightning and surge protection
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