Siemens sitrans tr300, Manual

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Functional safety
7.1 General safety information
SITRANS TR200/TR300
Operating Instructions, 06/2010, A5E01071992-03
35
7.1.2 Safety Integrity Level (SIL)
The international standard IEC 61508 defines four discrete Safety Integrity Levels (SIL) from
SIL 1 to SIL 4. Each level corresponds to the probability range for the failure of a safety
function.
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
AVG
)
The table deals with "Low demand mode", i.e. the safety function is required a maximum of
once per year on average.
Table 7- 1 Safety Integrity Level
SIL Interval
4 10
-5
≤ PFD
AVG
< 10
-4
3 10
-4
≤ PFD
AVG
< 10
-3
2 10
-3
≤ PFD
AVG
< 10
-2
1 10
-2
≤ PFD
AVG
< 10
-1
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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Figure 7-2 Example of PFD distribution
The following table shows the achievable Safety Integrity Level (SIL) for the entire safety-
instrumented system for type B subsystems depending on the safe failure fraction (SFF) and
the hardware fault tolerance (HFT). Type B subsystems include analog transmitters and
shut-off valves without complex components, e.g. microprocessors (also see IEC 61508,
Section 2).
HFT SFF
0 1 (0)
1)
2 (1)
1)
< 60 % Not permitted SIL 1 SIL 2
60 to 90 % SIL 1 SIL 2 SIL 3
90 to 99 % SIL 2 SIL 3 SIL 4
> 99 % SIL 3 SIL 4 SIL 4
1)
As per IEC 61511-1, Section 11.4.4