Shielding Of Cables - Siemens SIMATIC S7-300 Hardware And Installation Manual

Appendix
11.2.5

Shielding of cables

Purpose of the shielding
A cable is shielded to attenuate the effects of magnetic, electrical and
electromagnetic interference on the cable.
Operating principle
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.
Suitable cables
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.
Handling of the shielding
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
busbar. 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.
The figure below shows some options of mounting shielded cables, using cable
clamps.
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S7-300 Programmable Controller Hardware and Installation
A5E00105492-01