Siemens SIMATIC S7-1500 System Manual page 262

Safe electrical isolation (SELV in accordance with IEC 61131-2 or IEC 61010-2-201)
System power/load current supplies with 24 V DC output voltage feature safe electrical
separation and voltage limiting (extra-low voltage). System power/load current supplies with
a 24 V DC output voltage are not connected to the protective conductor.
In accordance with IEC 61131-2 / IEC 61010-2-201, this protection is referred to as SELV
(Safety Extra Low Voltage).
The wiring of SELV circuits must be safely separated from the wiring of other circuits that are
not SELV, or the insulation of all conductors must be dimensioned for the higher voltage.
Protective extra-low voltage (PELV in accordance with IEC 61131-2 or IEC 61010-2-201)
Load current supplies with grounded 24 V DC output voltage require a safe connection to the
protective conductor and voltage limiting (extra low voltage).
In accordance with IEC 61131-2 / IEC 61010-2-201, this protection is referred to as PELV
(Protective Extra Low Voltage).
Either the wiring of PELV circuits must be safely isolated from the wiring of other circuits that
are not PELV, or the insulation of all wires must be dimensioned for the higher voltage.
Reference potential of the controller
The reference potential of the S7-1500R/H redundant system is connected to the rail via a
high-resistance RC combination in the R/H-CPU. This connection conducts high-frequency
interference currents and prevents electrostatic charges. Despite the grounded rail, the
reference potential of the S7-1500R/H redundant system has to be considered as ungrounded
due to the high-resistance connection.
If you want to configure the S7-1500R/H redundant system with a grounded reference
potential, establish an electrical connection between the M connection of the CPU and the
protective conductor.
You can find a simplified representation of the potentials in the section Electrical
configuration (Page 264).
S7-1500R/H redundant system
System Manual, 01/2024, A5E41814787-AF
8.2 Operation on grounded infeed
Wiring
261