Siemens S7-400 installation manual Coupling Mechanisms, Capacitive Coupling, Inductive Coupling

Assembling and Installing Systems

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	Cause		Typical Interference Sources
Direct Coupling	Direct or metallic coupling always	•	Switched devices (supply affected
	occurs when two circuits have a		by inverters and external power
	common conductor.		supply units)
		•	Motors being started
		• Different potentials of component
			cases with a common power
			supply
		•	Static discharges
Capacitive Coupling	Capacitive or electrical coupling	•	Interference pickup via parallel
	occurs between conductors which are		signal cables
	at different potentials.	•	Static discharge of the operator
	The degree of coupling is proportional	•	Contactors
	to the voltage variation as a function
	of time.
Inductive Coupling	Inductive or magnetic coupling occurs	•	Transformers, motors, electric
	between two conductor loops through		welders
	which current is flowing. Interference	•	Parallel AC supply cables
	voltages are induced by the magnetic	•	Cables with switched currents
	fluxes associated with the currents.	•	Signal cables with a high
	The degree of coupling is proportional
	to the current variation as a function of		frequency
	time.	•	Unconnected coils
Radiated Interference	There is a radiation path when a	•	Local transmitters (for example,
	conductor is subjected to an		two-way radios)
	electromagnetic wave. Impinging of	•	Spark gaps (spark plugs,
	the wave results in induced currents		collectors of electric motors,
	and voltages.
			welders)

A-6	Automation System S7-400	Hardware and Installation
		A5E00850741-01