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8.3
Safety Integrity Level (SIL)
The international standard IEC 61508 defines four discrete Safety Integrity Levels (SIL) from
SIL 1 to SIL 4. Every level corresponds to a 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 8- 1
The "average probability of dangerous failures of the entire safety-instrumented system"
(PFD
Figure 8-2
The following table shows the achievable Safety Integrity Level (SIL) for the entire safety-
instrumented system for type A devices depending on the safe failure fraction (SFF) and the
hardware fault tolerance (HFT).
● Type A devices include analog transmitters and shut-off valves without complex
components, e.g. microprocessors (see also IEC 61508, Section 2).
● The specific values for your device are listed in the manufacturer's declaration
(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 %
SIPART PS2 with and without HART
Operating Instructions, 09/2014, A5E00074631-12
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 A devices
0
SIL 1
SIL 2
SIL 3
SIL 3
< 10
-4
AVG
< 10
-3
AVG
< 10
-2
AVG
< 10
-1
AVG
1
SIL 2
SIL 3
SIL 4
SIL 4
Functional safety
8.3 Safety Integrity Level (SIL)
2
SIL 3
SIL 4
SIL 4
SIL 4
).
AVG
141
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