Siemens SIMATIC S7-400 Hardware And Installation Manual page 245

Rule 4
Employ special EMC measures for particular applications (see Section 4.11).
Fit suppressors to all inductors which are not controlled by S7-400 modules.
Use incandescent bulbs or suppressed fluorescent lamps in the immediate
vicinity of your controller for illuminating cabinets or housings.
Rule 5
Create a standard reference potential; ground all electrical apparatus if possible
(see Sections 4.10 and 4.12).
Install equipotential bonding conductors of sufficient rating when potential
differences exist or are expected between sections of your system.
Use specific grounding measures. Grounding the control system is a protective
and functional measure.
Connect the system sections and cabinets containing central racks and
expansion racks to the grounding/protective ground system in a star
configuration. This prevents the formation of ground loops.
S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01
www.gkbpx.com
Assembling and Installing Systems
A-5