Safety Integrity Level (Sil) - Siemens CALOMAT 6 Instructions Manual

Siemens AG
2.2

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 in a safety function. The higher the SIL of the safety-instrumented system,
the higher the probability that the required safety function will work.
The achievable SIL is determined by the following safety characteristics:



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 safety function is required a maximum of once per year on average.
SIL
4
3
2
1
Table 2-1 Safety Integrity Level
The "average probability of dangerous failures of the entire safety
instrumented system" (PFD
in the following figure.
Sensor
e.g. Gas
Analyzer
Figure 2-2 PFD distribution
The following table shows the achievable Safety Integrity Level (SIL) for the
entire safety-instrumented system for type B systems depending on the
proportion of safe failures (SFF) and the hardware fault tolerance (HFT). Type
B systems include sensors and positioners actuators with complex
components, e.g. microprocessors (see also IEC 61508, Section 2).
SFF
<60%
60 to 90%
90 to 99%
>99%
SIL Safety Manual CALOMAT 6 Gas Analyzers
A5E00729184-02, 07/2010
Average probability of dangerous failure of a safety function in case of
demand (PFD )
AVG
Hardware fault tolerance (HFT)
Safe failure fraction (SFF)
AVG
PFD
AVG
≥ 10 ... < 10
-5 -4
≥ 10 ... < 10
-4 -3
≥ 10 ... < 10
-3 -2
≥ 10 ... < 10
-2 -1
AVG
Control
system or
logic unit
e.g. PLC
0
Not allowed
SIL1
SIL2
SIL3
). The table deals with "Low demand mode", i.e.
) is normally spilt between the three subsystems
HFT
1 2
SIL1 SIL2
SIL2 SIL3
SIL3 SIL4
SIL4 SIL4
8
Final control-
ling element
e.g. valve