Siemens SITRANS P Betriebsanleitung Seite 141

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The "average probability of dangerous failures of the entire safety-instrumented system"
(PFD
AVG
) is normally split between the three sub-systems in the following figure.
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3)'$9*IUDFWLRQ
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HOHPHQW
  
Figure 8-3 PFD distribution
The following table shows the achievable Safety Integrity Level (SIL) for the entire safety-
instrumented system for type A/B subsystems depending on the safe failure fraction (SFF) and
the hardware fault tolerance (HFT).
Type A sub-systems include analog transmitters and shut-off valves without complex
components, e.g. microprocessors (see also IEC 61508, Section 2).
Type B subsystems include analog transmitters and shut-off valves with complex
components, e.g. microprocessors (also see IEC 61508, Section 2).
SFF HFT for type A sub-system HFT for type B sub-system
0 1 2 0 1 (0)
1)
2 (1)
1)
< 60 % SIL 1 SIL 2 SIL 3 Not permitted SIL 1 SIL 2
60 to 90 % SIL 2 SIL 3 SIL 4 SIL 1 SIL 2 SIL 3
90 to 99 % SIL 3 SIL 4 SIL 4 SIL 2 SIL 3 SIL 4
> 99 % SIL 3 SIL 4 SIL 4 SIL 3 SIL 4 SIL 4
1)
Operational reliability in accordance with IEC 61511-1, Section 11.4.4
Operational reliability
According
to IEC 61511-1, Section 11.4.4, the hardware fault tolerance (HFT) can be reduced
by one (values in brackets) for transmitters and final controlling elements with complex
components if the following conditions apply to the device:
The device is proven in operation.
The user can configure only the process-related parameters, e.g. control range, signal
direction in case of a fault, limiting values, etc.
The configuration level of the firmware is blocked against unauthorized operation.
The function requires SIL of less than 4.
Functional safety
8.1 General safety instructions
SITRANS P, DS III series with HART
Operating Instructions, 09/2012, A5E00047092-08 139
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