Planning; Assumptions - Pepperl+Fuchs HiD2022 Series Manual

Functional Safety HiD2022*, KFD2-STC5-(Ex)*, KFD2-STV5-(Ex)*

Planning

3 Planning
3.1

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.
• External power supply failure rates are not included.
• 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.
• If you are using the HART function, observe that the HART signal superimposes
an analog signal on the signal line. This is negligible for slow acquisition system
(typically < 25 acquisition/s). If in doubt, use HART filters at the analog input
of the receiving device.
• The application program in the programmable logic controller (PLC) is configured
to detect underrange and overrange failures.
• Both channels of the 2-channel device can be used in the same safety function.
Observe that both devices use common components and that a simultaneous failure
is possible with a common cause factor of 15 %.
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 PFD
• 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.
SIL 3 Application
SIL 3 can be reached if the two outputs of the device are connected to the same control
interface and evaluated if the deviation remains below 2 %.
• 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
• 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.
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