Appendix
A.2 Protection against electromagnetic interference
S7-300, CPU 31xC and CPU 31x: Installation
A-12 Operating Instructions, Edition 08/2004, A5E00105492-05
A.2.6 Cable shielding
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 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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