Siemens SIMATIC S7 System Manual page 23

Connecting an external 24 VDC power supply in parallel with the 24 VDC sensor supply
can result in a conflict between the two supplies as each seeks to establish its own
preferred output voltage level.
The result of this conflict can be shortened lifetime or immediate failure of one or both
power supplies, with consequent unpredictable operation of the PLC system. Unpredictable
operation could result in death, severe personal injury and/or property damage.
The DC sensor supply and any external power supply should provide power to different
points.
Some of the 24 VDC power input ports in the S7-1200 system are interconnected, with a
common logic circuit connecting multiple M terminals. For example, the following circuits are
interconnected when designated as "not isolated" in the data sheets: the 24 VDC power
supply of the CPU, the power input for the relay coil of an SM, or the power supply for a non-
isolated analog input. All non-isolated M terminals must connect to the same external
reference potential.
Connecting non-isolated M terminals to different reference potentials will cause unintended
current flows that may cause damage or unpredictable operation in the PLC and any
connected equipment.
Failure to comply with these guidelines could cause damage or unpredictable operation
which could result in death or serve personal injury and/or property damage.
Always ensure that all non-isolated M terminals in an S7-1200 system are connected to the
same reference potential.
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
WARNING
WARNING
Installation
23