Shielding Of Cables - GE RSTi-EP User Manual

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 aluminium) 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.
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
Caution
• 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
make contact with 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.
When shielding field-bus cables, the installation guidelines for the
respective field buses must be observed. (Refer the websites of the field bus
Attention
organizations.)
If it is only possible to have a one-sided shield connection for reasons specific
to the circuit or equipment, the second side of the cable shield can be routed to
the local reference potential via a capacitor (with short connections). To
prevent disruptive discharges when interference pulses occur, a varistor or a
resistor can also be wired in parallel to the capacitor.
Caution
As an alternative, a doubled version (galvanically isolated) can be used,
whereby the inner shield is connected on one side and the outside shield is
connected on both sides.
GFK-2958E RSTi-EP User Manual
Chapter 7 Earthing and Shielding
338