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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
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
SIL
4
3
2
1
The "average probability of dangerous failures of the entire safety-instrumented system"
(PFD
Figure 7-2
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).
SFF
< 60 %
60 to 90 %
90 to 99 %
> 99 %
As per IEC 61511-1, Section 11.4.4
1)
SITRANS TH200/TH300
Operating Instructions, 06/2010, A5E00393069-03
Safety Integrity Level
Interval
10
≤ PFD
-5
10
≤ PFD
-4
10
≤ PFD
-3
10
≤ PFD
-2
) is normally split between the three sub-systems in the following figure.
AVG
Example of PFD distribution
HFT
0
Not permitted
SIL 1
SIL 2
SIL 3
< 10
-4
AVG
< 10
-3
AVG
< 10
-2
AVG
< 10
-1
AVG
1 (0)
1)
SIL 1
SIL 2
SIL 3
SIL 4
Functional safety
7.1 General safety information
2 (1)
1)
SIL 2
SIL 3
SIL 4
SIL 4
)
AVG
37
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