Siemens simatic c7-621 Hardware Installation Manual page 32

Installing and Preparing the C7
All the doors and metal panels (side, rear and top panels) of the wiring closet
should be connected at least three times to the closet frame (keep the
connections short and make sure that contact is over a large area with bare,
unpainted metal).
Note
If you use equipment that generates high electrostatic voltages (for example
textile machinery), the grounding cables of the machine parts producing high
voltage interference signals should be connected to a separate ground
(surface grounding with building construction, reinforcement) from the
grounding point of the wiring closet.
The manual /70/ contains installation guidelines and deals with measures for
protecting from interference.
Overvoltage Measures to prevent overvoltage and lightning protection are described in the
Protection guidelines in /70/ .
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C7-621 / C7-621 AS-i Control Systems
C79000-G7076-C621-01