Siemens SIMATIC S7-400 Hardware And Installation Manual page 78

Wiring the S7-400
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 4.10).
Observe the following points with regard to the shield:
Only use cable clamps made of metal to secure braided shields. The clamps
must surround the shield over a large area and provide good contact.
Connect the shield to a shield bus immediately after entry of the cable into the
cabinet. Route the shield to the module but do not connect it there again to the
chassis ground or the shield bus.
Shown in the following figure are some methods of securing shielded cables with
cable clamps.
For installation other than in cabinets (for example, wall mounting), you can
provide contact between the cable shields and the cable duct.
4-16
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S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01