Reverse Voltage Protection; Dc Outputs - Siemens SIMATIC S7 Functional Safety Manual

Technical specifications
A.1 General technical specifications
A.1.12

Reverse voltage protection

Reverse voltage protection circuitry is provided on each terminal pair of +24 VDC power or
user input power for CPUs, signal modules (SM), and signal boards (SB). It is still possible to
damage the system by wiring different terminal pairs in opposite polarities.
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 severe 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.
A.1.13

DC outputs

Short-circuit protection circuitry is not provided for DC outputs on CPUs, signal modules
(SM) and signal boards (SB).
140
WARNING
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA