Assumptions - Pepperl+Fuchs SIL KCD2-STC-1.HC Safety Manual

Safety Manual SIL KCD2-STC-(Ex)1.HC(.SP), HiC2025HC
Planning
2.2

Assumptions

The following assumptions have been made during the FMEDA analysis:
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 circuit 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
SIL2 (sub)system.
The stress levels are average for an industrial environment and can be

compared to the Ground Fixed Classification of MIL-HNBK-217F.
Alternatively, the assumed environment is similar to:
• IEC 60654-1 Class C (sheltered location) with temperature limits within
During normal operation any change of the operating function (DIP switch

modification) must be prevented.
It was assumed that the appearance of a safe error (e. g. output in safe state)

would be repaired within 8 hours (e. g. remove sensor burnout).
During the absence of the device for repairing, measures have to be taken to

ensure the safety function (for example: substitution by an equivalent device).
The HART protocol is only used for setup, calibration, and diagnostic

purposes, not during normal operation.
The application program in the logic solver must be configured to detect

underrange and overrange failures.
the manufacturer's rating and an average temperature over a long period
of time of 40 ºC. Humidity levels are assumed within manufacturer's
rating. For a higher average temperature of 60 ºC, the failure rates should
be multiplied with an experience based factor of 2.5. A similar multiplier
should be used if frequent temperature fluctuation must be assumed.
value would then be 10
avg
-6
per hour, hence the
-7
per hour.
value
avg
-2
, hence
-3
.
7