Cable Shielding - Siemens SIMATIC S5 Manual
Distributed i/o system
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5.1.5
Cable shielding
Definition
Shielding is a means of weakening (attenuating) magnetic, electric and
electromagnetic interference fields.
Interference currents on cable shields are diverted to ground via the shield
busbar, which forms a conductive connection with the housing. It is
particularly important to ensure a low-ohmic connection to the protective
conductor, as otherwise the interference currents themselves may become a
source of interference.
Measures for cable
Note the following points:
shielding
Note
If a potential difference occurs between the grounding points, an
equalization current can flow through a shield connected at both ends.
In this case, install an additional potential equalization line.
ET 200 Distributed I/O System
EWA 4NEB 780 6000-02a
Routing cables; connecting and installing bus connectors
If possible, use only cables with braided shields. The shield density
should be more than 80 %.
Avoid using cables with foil shielding, because tensile and compressive
loads at the attachments can easily damage the foil. The result is a
reduction in shield efficiency.
Always connect the cable shields at both ends. It is only when the
shielding is connected at both ends that interference suppression is
effective at the high end of the frequency range.
Exceptions:It may be better to connect the shield at one end only, if
– for some reason it is not possible to install potential equalization lines,
– analog signals (in the low mV or A range) are transmitted, or
– foil shielding (static shields) is used.
Note, however, that if the shield is connected at one end only it can
suppress only low-frequency interference.
Connect the shield of the data line to the plug body.
If the system is installed for stationary operation, it is advisable to remove
the insulation from the shielded cable without interruption and connect
the cable to the shielding/protective conductor busbar.
5
5-9
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