Siemens SIMATIC S7 Functional Safety Manual

Fail-Safe CPU and signal module (SM) installation
4.2 Fail-Safe system electrical design rules
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA
91
WARNING
All power supply and fail-safe signal module circuits must be connected together to a
common voltage reference or must be isolated SELV circuits.
The power supply M terminals on the fail-safe CPU and the fail-safe SMs must be
connected together or isolated as SELV. Failure to do so can result in unexpected machine
or process operation, which may cause death or serious injury to personnel, and/or damage
to equipment.
Connecting all M terminals together or isolating with approved SELV isolation prevents
unwanted current flows in the event of a single fault in the CPU to SM isolation boundary.
4.2.2
Power budget
4.2.2.1
Connecting power to the S7-1200 system
Fail-safe CPUs require an external SELV/PELV 24 VDC power supply for the CPU's 24 VDC
power input (the 24 VDC L+ and M terminals, with an
arrow pointing
into
the CPU module).
The external 24 VDC supply powers the CPU's internal 5 VDC supply that provides power
for the CPU, signal modules (SM), signal board (SB), and communication modules (CM). 24
VDC power is available at the CPU's 24 VDC power output (the 24VDC L+ and M terminals,
with an
arrow pointing
out of
the CPU module).
Note
Sensor supply connections for safety inputs
If you want to use a sensor supply source from the PLC for safety inputs, you should use the
sensor supply provided on the fail
-safe DI signal modules.
The purpose of a power budget calculation is to ensure that the power requirements of all
CPU powered elements (fail-safe CPU, fail-safe SM, standard SM, and CM) do not exceed
the available CPU power:
● You may not be able to connect the maximum number of signal modules and
communication modules, if you exceed the CPU's 5 VDC power output capability.
● You may need to use external 24 VDC power for some system elements, if you exceed
the CPU's 24 VDC power output capability.
Note
Power limits for 5 VDC and 24 VDC supplied from a fail-safe CPU
.
Exceeding the power budget of the CPU may result in not being able to connect the allowed
maximu
m number of add-on modules.

Siemens SIMATIC S7 Specifications

General

Brand	Siemens
Model	SIMATIC S7
Category	Controller
Language	English