Siemens S7-300 Installation And Operating Manual page 49

Additional requirements of actuators
The fail-safe output modules perform a cyclic test of the outputs. The module briefly disables
the activate outputs and enables the inactive outputs. The test pulses have the following
duration:
● Dark period < 1 ms
● Light period < 1 ms
You can configure a light and dark period for the SM 326; F-DO 10 x DC 24V/2A PP. You
can find additional information in the SM 326; F-DO 10 x DC 24V/2A PP online help.
High-speed actuators may briefly drop out or be activated during this test. If your process
does not tolerate this action you must use actuators with a sufficient lag (> 1 ms).
The outputs of a fail-safe output module must be electrically isolated to EN 50178 from
components which carry higher voltages if the actuators are operated at voltages higher
than 24 V DC, for example, at 230 V DC, or are being used to switch higher voltages.
Relays and contactors usually comply with this rule. This aspect is of particular importance
when using semiconductor switchgear.
Avoiding dark periods when operating in safety mode
If you are using actuators that respond too fast (i.e. < 1 ms) to "dark period" test signals,
you can nonetheless use the internal test coordination by wiring two opposite outputs in
parallel using a series diode. Parallel wiring suppresses the dark periods (see chapter
"Parallel Connection of Two Outputs for Dark Period Suppression (Page 151)").
Technical data of sensors and actuators
Refer to the module chapters which contain the technical data for selecting sensors and
actuators.
Fail-safe signal modules
Installation and Operating Manual, 01/2010, A5E00085586-10
WARNING
WARNING
5.4 Sensor and Actuator Requirements for F-SMs in Safety Mode
Wiring
49