Assumptions - Pepperl+Fuchs KCD2-SR-1.LB Safety Manual

Safety Manual SIL KCD2-SR-(Ex)*(.LB)(.SP), HiC282*
Planning
2.2

Assumptions

The following assumptions have been made during the FMEDA analysis:
Only one input and one output are part of the considered safety function

(only 2-channel version).
The device shall claim less than 10 % of the total failure budget for a SIL2

safety loop.
For a SIL2 application operating in Low Demand Mode the total PFD

of the SIF (Safety Instrumented Function) should be smaller than 10
the maximum allowable PFD
For a SIL2 application operating in High Demand Mode of operation the total

PFH value of the SIF should be smaller than 10
maximum allowable PFH value would then be 10
Failure rate based on the Siemens SN29500 data base.

Failure rates are constant, wear out mechanisms are not included.

External power supply failure rates are not included.

The safety-related device is considered to be of type A components with a

Hardware Fault Tolerance of 0.
Since the loop has a Hardware Fault Tolerance of 0 and it is a type A

component, the SFF must be > 60 % according to table 2 of IEC 61508-2 for a
SIL2 (sub)system.
It is assumed that the device will be used under average industrial ambient

conditions, which are comparable with the classification "stationary mounted"
in MIL-HDBK-217F. Alternatively, the following ambient conditions are
assumed:
• IEC 60654-1 Class C (sheltered location) with temperature limits in the
It is assumed that any safe failures that occur (e.g., output in safe condition)

will be corrected within eight hours (e.g., correction of a sensor fault).
While the device is being repaired, measures must be taken to maintain the

safety function (e.g., by using a replacement device).
The indication of a dangerous fault (via fault bus) is detected within 1 hour by

the programmable logic controller (PLC).
range of the manufacturer's specifications and an average temperature
of 40 ºC over a long period. A moisture level within the manufacturer's
specifications is assumed. For a higher average temperature of 60 ºC,
the failure rates must be multiplied by a factor of 2.5 based on empirical
values. A similar multiplier must be used if frequent temperature
fluctuations are expected.
value would then be 10
avg
-6
per hour, hence the
-7
per hour.
value
avg
-2
, hence
-3
.
7