Siemens SIMATIC S7-400 Reference Manual page 22

General specifications
1.2 Electromagnetic compatibility
Additional measures
If you want to connect an S7-400 system to the public network, you must ensure compliance
with class B pursuant to EN 55032.
Appropriate additional measures are to be taken if you need to increase the immunity of the
system in the light of high external interference.
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S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08