Siemens SIMATIC S7 System Manual page 788

Calculating a power budget
B.1 Power budget
If you require an external 24 V DC power supply, ensure that the power supply is not
connected in parallel with the sensor supply of the CPU. For improved electrical noise
protection, it is recommended that the commons (M) of the different power supplies be
connected.
Connecting power supplies safely
Connecting an external 24 V DC power supply in parallel with the 24 V DC sensor supply of
the CPU 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 or serious injury to personnel, and/or damage to equipment.
The DC sensor supply of the CPU and any external power supply should provide power to
different points. A single connection of the commons is allowed.
Some of the 24 V DC power input ports in the S7-200 SMART 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 V
DC power supply of the CPU, the power input for the relay coil of an EM, or the power supply
for a non-isolated analog input. All non-isolated M terminals must connect to the same
external reference potential.
Avoiding unwanted current flow
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 severe personal injury and/or property damage.
Always ensure that all non-isolated M terminals in an S7-200 SMART system are
connected to the same reference potential.
Refer to the technical specifications for your particular CPU (Page 685) to determine the
24 V DC sensor supply power budget, the 5 V DC logic budget supplied by your CPU and
the 5 V DC power requirements of the expansion modules and signal boards.
788
WARNING
WARNING
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART