2019-10
10
Functional Safety KFD2-UT2-(Ex)*(-1)
Planning
3.2 Assumptions
The following assumptions have been made during the FMEDA:
• Failure rate based on the Siemens standard SN 29500.
• Failures during configuration are not considered.
• The collective error message output is not considered in the FMEDA and
in the calculations.
• Propagation of failures is not relevant.
• External power supply failure rates are not included.
• The collective error message output is not safety relevant.
• The indication of a dangerous failure (via fault bus) is detected within 1 hour
by the programmable logic controller (PLC).
• 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.
• The application program in the programmable logic controller (PLC) is configured to detect
underrange and overrange failures. These failures have been classified as dangerous
detected failures.
SIL 2 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 PFDavg/PFH.
• For a SIL 2 application operating in low demand mode the total PFD
avg
value of
the SIF (Safety Instrumented Function) should be smaller than 10
-2
, hence the maximum
allowable PFD
avg
value would then be 10
-3
.
• According to IEC/EN 61511-1 section 11.4.5, a HFT = 0 can be considered sufficient for
SIL 2 applications in low demand mode of operation. The evaluation is based on failure
rates as described in section 11.9.
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