Siemens SIMATIC ET 200SP System Manual page 54

Wiring
5.2 Additional rules and regulations for the operation of the ET 200SP with fail-safe modules
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.
Fast reacting actuators may briefly drop out or be activated during the 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 VDC (e.g., 230 VDC), the outputs of a fail-
safe output module and the parts carrying a higher voltage must be electrically isolated
(according to IEC 60664-1).
This is generally the case for relays and contactors. Particular attention must be paid to this
with 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.
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WARNING
Distributed I/O system
System Manual, 12/2015, A5E03576849-AF