Siemens SIMATIC Manual page 243

Wiring non-isolated transducer to a non-isolated AI
Always operate the CPU / IM 153 in grounded mode if you connect non-isolated transducers
to non-isolated modules.
Figure 4-4
Note
You may not connect non-isolated 2-wire transducers/resistive transducers to non-isolated
analog inputs!
S7-300 Module data
Manual, 06/2017, A5E00105505-AJ
Wiring non-isolated transducers to a non-isolated AI
Principles of analog value processing
4.2 Wiring and connecting transducers to analog inputs
243