Functional Safety KCD2-STC-(Ex)1(.SP)(-Y1), HiC2025(Y1)
Planning
2018-08
11
3.2 Assumptions
The following assumptions have been made during the FMEDA:
• Failure rate based on the Siemens standard SN29500.
• Failure rates are constant, wear is not considered.
• External power supply failure rates are not included.
• The devices are not protected against power supply failures. It is within the responsibility
of the user to ensure that low supply voltages are detected and adequate reaction on
this fault is implemented.
• The safety-related device is considered to be of type A device with a hardware fault
tolerance of 0.
• 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 HART function is only used for configuration, calibration and diagnostics, not during
operation.
• The indication of a dangerous failure (via fault bus) is detected within 1 hour by the
programmable logic controller (PLC).
SIL 2 Application
• The device shall claim less than 10 % of the total failure budget for a SIL 2 safety loop.
• 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
.
• For a SIL 2 application operating in high demand mode the total PFH value of the SIF
should be smaller than 10
-6
per hour, hence the maximum allowable PFH value would then
be 10
-7
per hour.
• Since the safety loop has a hardware fault tolerance of 0 and it is a type A device,
the SFF must be > 60 % according to table 2 of IEC/EN 61508-2 for a SIL 2 (sub) system.
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