Wiring
5.1 Safe Functional Extra-Low Voltage for Fail-Safe Signal Modules
Fail-safe signal modules
44
Installation and Operating Manual, 01/2010, A5E00085586-10
5.1
Safe Functional Extra-Low Voltage for Fail-Safe Signal Modules
PELV
WARNING
Fail-safe signal modules must be operated with safe functional extra low voltage (SELV,
PELV). That is, the fail-safe modules may only be exposed to a maximum fault voltage of
V
m
. Rule for all fail-safe signal modules:
V
m
< 60.0 V
Additional information about PELV is available in the data sheets of the power supply
modules to be installed, for example.
All system components which supply electrical power in any form must meet this condition.
Each additional power circuit (24 V DC) installed in the system must be operated with safe
functional extra low voltage (SELV, PELV). Refer to the data sheets or contact the
manufacturer for information.
You should also note that the sensors and actuators connected to the I/O modules may be
connected to an auxiliary power supply. Make sure that such auxiliary power supplies
conform to PELV requirements. The process signal of a 24 V digital module may not develop
a fault voltage in excess of V
m
.
WARNING
All power sources such as the internal and external 24 V DC load voltage supplies, and the
5 VDC bus voltage must be galvanically interconnected so that any development of
cumulative voltage at the individual voltage sources which may lead to a fault voltage
higher than V
m
is safely ruled, irrespective of any potential differences.
Make sure that the conductor cross-sections used for the galvanic interconnections are
compliant with S7-300 installation guidelines (see the
S7-300 Automation System:
CPU 31xC and CPU 31x: installation Operating Instructions).
The fail-safe signal modules and all their standard components can be operated in standard
and safety mode on a single or on several shared power supply modules.
