Shielding Cables; A.5 Shielding Cables - Siemens S7-400 Installation Manual

A.5

Shielding Cables

Purpose of the Shielding
A cable is shielded to attenuate the effects of magnetic, electrical, and
electromagnetic interference on this cable.
Principle of Operation
Interference currents on cable shields are discharged to ground via the shield bus
which is electrically connected to the housing. To prevent these interference
currents themselves from becoming an interference source, a low-impedance
connection to the protective ground conductor is particularly important.
Suitable Cables
If possible, only use cables with a braided shield. The coverage density of the
shield should be at least 80%. Avoid cables with a foil shield because the foil can
be easily damaged by tensile and compressive stress at the securing points; this
can reduce the shielding effect.
Grounding the Cable Shields
Generally, you should always connect both ends of the shield to the chassis ground
(that is, at the beginning and end of the cable). Grounding the shields at both ends
is essential to achieve a good degree of suppression of interference in the higher
frequency region.
In exceptional cases, you can connect only one end of the shield to the chassis
ground (for example, at the beginning or end of the cable). However, you only
achieve attenuation of lower frequencies. Single-ended connection of the shield
may be advantageous when
• An equipotential bonding conductor cannot be laid
• Analog signals of a few mA or mA are transmitted
• Foil shields (static shields) are used.
For data cables in serial communication, only use metallic or metallized
connectors. Secure the shield of the data cable to the connector housing. Do not
connect the shield to Pin 1 of the connector.
For stationary operation, you should strip the shielded cable without damaging the
shield and connect it to the shield/protective ground bar.
Note
In the event of potential differences between grounding points, a circulating current
may flow via the shield connected at both ends. In this case, install an additional
equipotential bonding conductor (see Section A.6).
S7-400 and M7-400 Programmable Controllers Hardware and Installation
A5E00069481-04
Assembling and Installing Systems
A-13