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8.1.2
Safety Integrity Level (SIL)
Definition: 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. The higher the SIL of the safety-instrumented system, the higher probability that the
required safety function will work.
The achievable SIL is determined by the following safety characteristics:
● Average probability of dangerous failure of a safety function in case of demand (PFD
● Hardware fault tolerance (HFT)
● Safe failure fractions (SFF)
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
SIL
4
3
2
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 sub-systems depending on the proportion of safe failures
(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).
SIPART PS2 with PROFIBUS communication
Operating Instructions, 09/2007, A5E00127926-07
Safety Integrity Level
PFD
AVG
≥ 10
-5
≥ 10
-4
≥ 10
-3
≥ 10
-2
) is normally split between the three sub-systems in the following figure.
AVG
PFD distribution
...< 10
-4
...< 10
-3
...< 10
-2
...< 10
-1
Functional safety
8.1 General safety notes
AVG
)
AVG
109
)
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