Shielding Of Cables - Emerson PACSystems RSTi-EP User Manual

User Manual
GFK-2958J
7.4

Shielding of Cables

To prevent the coupling of interference voltages and the decoupling of interference fields
in cables, only shielded cables made from well-conducting material (copper or aluminum)
with braided shielding and a coverage of at least 80 % should be used in the design of a cable
shield.
Only when a cable shield is connected to the local reference potential on both sides is it
possible to achieve optimal shielding against electric and magnetic fields. Exceptions are
possible, for example, with high-impedance, symmetrical or analogue signal cables. If a
shield is attached on only one side, this merely achieves an isolation against electric fields.
CAUTION
Requirements for effective shielding design:
• The shield connection to the shield bus should be low impedance
• The shield must be connected directly at its entrance into the system
• Keep cable ends as short as possible
• Do not use cable shields for equipotential bonding
When connecting a data cable using a sub-D connector, the connection must be made
through the connector's shield collar and never through pin 1.
The data cable's shield must be attached to the shield bus with the insulation stripped away.
The shield is to be connected and attached with clamping brackets or similar metal fixing
devices. The shield bus must be connected to the reference potential surface through a low
impedance [e.g. fastening point with a separation of 10 to 20 cm (3.94" x 7.87")]. The
brackets must surround and contact a large part of the shield.
Isolation of the cable shield should be avoided. Instead, it should be routed into the system
(for example, the switch cabinet) up to the interface connection.
Earthing and shielding
When shielding field-bus cables, the installation
guidelines for the respective field buses must be
observed. (Refer the websites of the field bus
organizations.)
Attention
Chapter 7
Oct 2019
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