Shielding Cables - Siemens SIMATIC S5-100U System Manual

Installation Guidelines

3.4.4 Shielding Cables

Shielding is a measure to weaken (attenuate) magnetic, electric or electromagnetic interference
fields.
Interference currents on cable shields are discharged to ground over the shield bar which has a
conductive connection to the housing. So that these interference currents do not become a source
of noise in themselves, a low-resistance connection to the protective conductor is of special
importance.
Use only cables with shield braiding if possible. The effectiveness of the shield should be more than
80%. Avoid cables with foil shielding since the foil can easily be damaged by tension and pressure;
this leads to a reduction in the shielding effect.
As a rule, you should always shield cables at both ends. Only shielding at both ends provides good
suppression in the high frequency range.
As an exception only, you can connect the shielding at one end. However, this attenuates only the
lower frequencies. Shielding at one end can be of advantage in the following cases:
• If you cannot run an equipotential bonding conductor
• If you are transmitting analog signals (e.g. a few microvolts or microamps)
• If you are using foil shields (static shields).
Always use metallic or metalized connectors for data lines for serial connections. Secure the shield
of the data line at the connector housing. Do not connect the shield to the PIN1 of the connector
strip!
In the case of stationary operation, you are recommended to insulate the shielded cable without
interrupt and to connect it to the shield/protective ground bar.
Note
If there are potential differences between the earthing points, a compensating current
can flow over the shielding that is connected at both ends. For this reason, connect an
additional equipotential bonding conductor.
3-32
S5-100U
EWA 4NEB 812 6120-02b