Wiring the S7-400
4-15
S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01
4.9 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.
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