Siemens SIMATIC S7-400 Hardware And Installation Manual page 243

Coupling Mechanisms
Interference can reach the programmable controller via four different coupling
mechanisms, depending on the transmission medium (conducted or
non-conducted) and distance between interference source and the equipment.
Coupling Mechanism
Direct Coupling Direct or metallic coupling always
occurs when two circuits have a
common conductor.
Capacitive Coupling Capacitive or electrical coupling
occurs between conductors which are
at different potentials.
The degree of coupling is proportional
to the voltage variation as a function of
time.
Inductive Coupling Inductive or magnetic coupling occurs
between two conductor loops through
which current is flowing. Interference
voltages are induced by the magnetic
fluxes associated with the currents.
The degree of coupling is proportional
to the current variation as a function of
time.
Radiated Interference There is a radiation path when a
conductor is subjected to an
electromagnetic wave. Impinging of
the wave results in induced currents
and voltages.
S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01
www.gkbpx.com
Cause
Assembling and Installing Systems
Typical Interference Sources
Switched devices (supply affected
by inverters and external power
supply units)
Motors being started
Different potentials of component
cases with a common power
supply
Static discharges
Interference pickup via parallel
signal cables
Static discharge of the operator
Contactors
Transformers, motors, electric
welders
Parallel AC supply cables
Cables with switched currents
Signal cables with a high
frequency
Unconnected coils
Local transmitters (for example,
two-way radios)
Spark gaps (spark plugs,
collectors of electric motors,
welders)
A-3