Siemens SIMATIC S7-400 Hardware And Installation Manual page 74

Wiring the S7-400
Power Supply Units
When using power supply units, ensure that the secondary winding is not
connected to the protective ground conductor.
Filtering the 24 VDC Supply
When you power the S7-400 from a battery with the ungrounded configuration, you
must provide interference suppression for the 24 VDC supply. Use a Siemens
power cable filter, such as the B84102-K40.
Insulation Monitoring
If a double fault could cause a hazardous state in the installation, you must provide
insulation monitoring.
Example of Ungrounded Operation
If you have configured an S7-400 with a local connection and you only wish to
ground the overall installation at the CR, you can operate the ERs in an
ungrounded configuration.
Note
If you connect an ER via a local connection with 5 V transfer, ungrounded
operation is mandatory for the ER.
Connecting a Programming Device with Ungrounded Configuration
To connect a programming device to an S7-400 in ungrounded configuration,
please observe the following note:
Note
To connect a programming device to an S7-400 in ungrounded configuration, you
must connect the programming device via an RS 485 repeater.
4-12
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S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01