Siemens SIMATIC S7-1500 System Manual page 78

Additional requirements for actuators
The fail-safe output modules test the outputs at regular intervals. The F-module briefly
switches off the activated outputs and, if necessary, switches on the deactivated outputs.
You can assign the maximum duration of the test pulses (dark and light period) with
parameters.
High-speed actuators may briefly drop out or be activated during this test. If your process
does not tolerate this, set the pulse duration of the light or dark test correspondingly or use
actuators that have sufficient lag.
If the actuators switch voltages greater than 24 V DC (e.g. 230 V AC), the outputs of a fail-
safe output module and the parts carrying a higher voltage must be electrically isolated
(acc. to standard IEC 60664-1:2010).
This is generally the case for relays and contactors; particular attention must be paid to this
requirement for semiconductor switching devices.
Technical specifications of sensors and actuators
Refer to the manuals of the fail-safe modules for technical specifications to assist you in
selecting sensors and actuators.
SIMATIC Drive Controller
System Manual, 11/2019, A5E46600094-AA
WARNING
6.2 Additional rules and regulations for operation
Connecting
77