Coupling mechanisms andtypical interference sources ata glanceShown in the following table are the four different couplingmechanisms, their causes and possible interference sources.Coupling Mechanism Cause Typical Interference Sources•• Direct Coupling Direct or metallic couplingalways occurs when two circuitshave a common conductor.• Switched devices (supplyaffected by inverters andexternal power supply units)• Motors being started• Different potentials ofcomponent cases with acommon power supply• Static discharges•• Capacitive Coupling Capacitive or electrical couplingoccurs between conductors whichare at different potentials.The degree of coupling isproportional to the voltagevariation as a function of time.• Interference pickup viaparallel signal cables• Static discharge of theoperator• Contactors•• Inductive Coupling Inductive or magnetic couplingoccurs between two conductorloops through which current isflowing. Interference voltages areinduced by the magnetic fluxesassociated with the currents. Thedegree of coupling is proportionalto the current variation as afunction of time.• Transformers, motors, electricwelders• Parallel AC supply cables• Cables whose currents areswitched• Signal cables with a highfrequency• Unconnected coils•• Radiated Interference There is a radiation path when aconductor is subjected to anelectromagnetic wave. Impingingof the wave results in inducedcurrents and voltages.• Local transmitters(e.g. two-way radios)• Spark gaps (spark plugs,collectors of electric motors,welders)SIMATIC S5InterferenceCapacitiveCoupling PathSIMATIC S5InterferenceDirect CouplingPathSIMATIC S5InterferenceRadiationPathSIMATIC S5InterferenceInductiveCoupling PathSignalPrinciples of Installation of Systems for EMCSystem Manual3 - 6 C79000-B8576-C199-03
