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Functional Safety KFD0-SD2-Ex2.1*45
Planning
3.2
Assumptions
The following assumptions have been made during the FMEDA:
•
Failure rate based on the Siemens standard SN 29500.
•
Failure rates are constant, wear is not considered.
•
The safety-related device is considered to be of type A device with a hardware
fault tolerance of 0.
•
Only one input and one output are part of the safety function (only for 2-channel version).
•
The device will be used under average industrial ambient conditions comparable
to the classification stationary mounted according to MIL-HDBK-217F.
Alternatively, operating stress conditions typical of an industrial field environment similar
to IEC/EN 60654-1 Class C with an average temperature over a long period of time
of 40 ºC may be assumed. For a higher average temperature of 60 ºC, the failure rates
must be multiplied by a factor of 2.5 based on experience. A similar factor must be used
if frequent temperature fluctuations are expected.
SIL 3 Application
•
To build a SIL safety loop for the defined SIL, it is assumed as an example that this device
uses 10 % of the available budget for PFD
•
For a SIL 3 application operating in low demand mode the total PFD
of the SIF (Safety Instrumented Function) should be smaller than 10
hence the maximum allowable PFD
•
For a SIL 3 application operating in high demand mode the total PFH value
of the SIF should be smaller than 10
would then be 10
•
Since the safety loop has a hardware fault tolerance of 0 and it is a type A device,
the SFF must be > 90 % according to table 2 of IEC/EN 61508-2 for a SIL 3 (sub) system.
3.3
Safety Function and Safe State
Safe State
The safe state of the output is the de-energized state. The output current is less than 50 A.
Safety Function
When the input signal is active, the output is activated. The safe state is introduced
when the input voltage is below 5 V DC.
Reaction Time
The reaction time for all safety functions is < 1 s.
Note
See corresponding datasheets for further information.
10
avg
value would then be 10
avg
-7
per hour, hence the maximum allowable PFH value
-8
per hour.
/PFH.
avg
-3
,
-4
.
value
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