Siemens SIMATIC ET 200AL, System Manual

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Designing interference-free controllers
Function Manual, 06/2014, A5E03461486-AB
15
Cable shielding
4
Shielding is a measure to attenuate (dampen) magnetic, electrical or electromagnetic
interference fields.
Interference currents on cable shields are diverted from the shield connection to ground. To
avoid interference as a result of these currents, it is imperative to provide a low-impedance
connection to the protective conductor.
Rules
Ideally, you should use cables equipped with a protective braided shield. The shield's
contacting must be more than 80% at the contact point. Avoid tensile and compression strain
on cables with a foil shield. This type of strain can damage the foil shield very slightly at the
fastening point. The result is a reduced shielding effect.
Shield connection with grounding at both ends
Always connect the cable shields to the ground at both ends. Low and high frequency
interference can be reduced only by connecting both ends of the shield to the ground.
Note
Laying an additional equipotential bonding cable
Whenever potential differences develop between the grounding points, an equalizing current
flows on a shielding that is connected at both ends. In this case, lay an additional
equipotential bonding cable. You can find more information on this topic in the chapter
Equipotential bonding (Page 23).