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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
12-12 A5E00105492-03
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A cable is shielded to attenuate the effects of magnetic, electrical and
electromagnetic interference on the cable.
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Interference currents on cable shielding is diverted to ground conductive
interconnection between the shielding and the cabinet. To avoid interference as a
result of these currents, it is imperative to provide a low-impedance connection to
the protective conductor.
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Whenever possible, use cables equipped with a shielding braid. Shielding density
should be at least 80%. Avoid cables with film shielding, because the film can be
easily damaged by tensile or pressure stress, thus reducing its shielding effect.
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Note the following points on handling the shielding:
• Always use metal clamps to mount shielding braid. The clamps must contact a
large area of the shielding and provide appropriate contact force.
• Directly behind the cabinet's cable entry, terminate the shielding on a shielding
bus. Then, route the cable to the module; however, do not connect the shielding
once again to ground in this place.
• In installations outside of cabinets (e.g. for wall-mounting) you can also
terminate the shielding on a cable duct.
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