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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
12-4 A5E00105492-03
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Depending on the emitting media (line or isolated) and the distance between the
interference source and the device, four different coupling mechanisms can
influence the PLC.
Table 12-5 Coupling mechanisms
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Electrical
coupling
Galvanic or mechanical
coupling always occurs when
two circuits use one common
cable.
• Clocked devices (influence on the
network due to converters and
third-party power supply modules)
• Starting motors
• Potential differences on component
enclosures with common power
supply
• Static discharge
Capacitive
coupling
Capacitive or electrical
coupling occurs between
conductors connected to
different potentials.
The coupling effect is
proportional to voltage
change over time.
• Interference coupling due to
parallel routing of signal cables
• Static discharge of the operator
• Contactors
Inductive
coupling
Inductive or magnetic
coupling occurs between two
current circuit loops. Current
flow in magnetic fields
induces interference voltages.
The coupling effect is
proportional to current change
over time.
• Transformers, motors, arc welding
devices
• Power supply cables routed in
parallelism
• Switched cable current
• High-frequency signal cable
• Coils without suppression circuit
Radio frequency
coupling
Radio frequency coupling
occurs when an
electromagnetic wave
reaches a conductor system.
This wave coupling induces
currents and voltages.
• Neighboring transmitters (e.g. radio
phones)
• Sparking (sparkplugs, collectors of
electrical motors, welding devices)
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