Siemens SITRANS P DS III Operating Instructions Manual page 131

Description
The following table shows the dependency of the SIL on the "average probability of
dangerous failures of a safety function of the entire safety-instrumented system" (PFD
The table deals with "Low demand mode", i.e. the safety function is required a maximum of
once per year on average.
Table 8- 1
The "average probability of dangerous failures of the entire safety-instrumented system"
(PFD
Image 8-2
The following table shows the achievable Safety Integrity Level (SIL) for the entire safety-
instrumented system for type B devices depending on the safe failure fraction (SFF) and the
hardware fault tolerance (HFT).
● Type B devices include analog transmitters and shut-off valves with complex
components, e.g. microprocessors (also see IEC 61508, Section 2).
● For detailed information on values and hardware/firmware versions for your device, refer
to the manufacturer declaration for the device (Declaration of Conformity, Functional
Safety according to IEC 61508 and IEC 61511): Certificates
(http://www.siemens.com/processinstrumentation/certificates).
SFF
< 60 %
60 to 90 %
90 to 99 %
> 99 %
Operational reliability in accordance with IEC 61511-1, Section 11.4.4
1)
SITRANS P DS III/P410 with PROFIBUS PA
Operating Instructions, 02/2016, A5E00053276-08
Safety Integrity Level
SIL Interval
4 10 ≤ PFD
-5
3 10 ≤ PFD
-4
2 10 ≤ PFD
-3
1 10 ≤ PFD
-2
) is normally split between the following three components:
AVG
PFD distribution
HFT for type B devices
0
Not permitted
SIL 1
SIL 2
SIL 3
< 10
-4
AVG
< 10
-3
AVG
< 10
-2
AVG
< 10
-1
AVG
1 (0)
SIL 1
SIL 2
SIL 3
SIL 4
8.2 Safety Integrity Level (SIL)
1)
Functional safety
).
AVG
2 (1)
1)
SIL 2
SIL 3
SIL 4
SIL 4
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