Siemens SIMATIC PCS 7 Operating Manual page 56

Configuring S7F/FH hardware
6.4 Setting system parameters for F-signal modules
Light test activated
The module conducts complete bit pattern tests within the configured maximum test time. If
the output is active in the "good condition", a dark test is always conducted. If the output is
not active in the "good condition", activate the light test with this parameter. If the signal
changes daily or more often, SIL3/Kat.4/PLe can be achieved even without a light test. If this
is not reached with a "0" signal, the light test must be activated which then fulfills this
condition.
Maximum light test time
With the parameter "maximum test time(s)", you determine the time within which the light
and dark tests are to be conducted (in all combinations) for the whole module. After this time
elapses the tests are repeated.
Light periods arise during the complete bit pattern test. In this process, a test signal is
connected to the output from the fail-safe output module while the output is inactive ("0"
output signal). This then activates the output briefly (= "light period"). A sufficiently inactive
actuator does not respond to this and remains deactivated.
Each output channel has its own configurable maximum light test time.
The maximum light test time should be set sufficiently long enough if the affected channel
activates large capacitive loads. If the maximum light test time is set too short for a controlled
capacitive load, the output channel is passivated, because loading does not occur within the
parameterized time.
In the event of faulty readback signals, the light test signal is available for the parameterized
maximum light test time at the output before the error "Short circuit to M" leads to the
passivation of the output channel.
Set maximum light test time:
1. If an output channel is continuously being passivated with an intact F-SM, this can be in
contact with a capacitive component of the controlled load that is too large.
In such an instance, configure the maximum light test time to the maximum value of 5 ms.
If passivation of the channel continues to occur, either an external fault is present or the
connected capacity lies outside of the permitted range.
2. Since the fault response time is extended by the configured maximum light test time, the
light test time should be set as short as possible when testing (but long enough so the
output channel is not passivated).
3. If you have set a maximum light test time which leads to the output channel becoming
passivated sporadically, set the next highest value of the maximum light period.
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Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB