Shielding Of Cables And Lines - Siemens S5-135U/155U System Manual

3.3.3

Shielding of Cables and Lines

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:
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).
System Manual
C79000-G8576-C199-06
an equipotential bonding conductor cannot be laid;
analog signals (of a few mV or A) are to be transmitted;
foil (static) shields are used.
Installation Guidelines
3-15