Shielding Of Cables And Lines - Siemens S5-155U System Manual

3.3.3
Shielding of Cables and
Lines
System Manual
C79000-B8576-C199-03
Wiring of Programmable Controllers for EMC
Shielding is a method of attenuating magnetic, electrical or electro-
magnetic interference fields.
Interference currents on cable shields are given a path to ground via
the shield bar which is electrically connected to the housing. A
low-impedance connection to the protective conductor is particularly
important so that these interference currents themselves do not
become an interference source.
Where possible, only use lines with a braided shield. The coverage
density of the shield should be more than 80 %. Avoid lines with a foil
shield because the foil can be very easily damaged by tensile strain
and compression during fitting; this results in reduced effectiveness of
the shield.
As a rule, line shields should always be connected at both ends. This
is the only way to achieve a good degree of interference suppression
in the higher frequency region.
Only in exceptional cases should the shield be connected at one end
only, as this only achieves attenuation of the low frequencies.
Single-ended shield connection may be more advantageous when:
• • an equipotential bonding conductor cannot be laid;
• • analog signals (of a few mV or µA) are to be transmitted;
• • foil (static) shields are used.
With data lines for serial communication, always use metal or
metallized connectors. Secure the shield of the data line to the
connector case. Do not connect the shield to Pin 1 of the connector.
For stationary operation, it is advisable to fully strip the insulation
from the shielded cable and connect it to the shield/protective
conductor bar.
Note
In the event of potential differences between ground points, a
circulating current may flow through the shield connected at both
ends. In this case, install an additional equipotential bonding
conductor (see Section 3.3.2).
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