Siemens S7-400 Shielding Cables, Purpose of the Shielding, Principle of Operation

Assembling and Installing Systems

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 degA-19A-19A-19A-19ree 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 μA 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).

Automation System S7-400	Hardware and Installation	A-13
A5E00850741-01