Siemens SIMATIC ET 200S Manual page 70

Configuration Options
Warning
!
If you connect the AUX1 bus to common potential independently of the P1/P2
buses (different voltages), there is no safe electrical isolation between the AUX1
bus and the P1/P2 buses.
Connecting different potentials to the AUX1 bus
Note
If you apply different potentials to the AUX1 bus in an ET 200S station, you must
isolate the potential groups by means of a power module with the terminal module
TM-P15S23-A0.
3-6
ET 200S Distributed I/O System
EWA-4NEB 780602402-12