Siemens SIPART Series - Safety Integrity Level (SIL)

Functional safety
8.3 Safety Integrity Level (SIL)
SIPART PS2 with and without HART
Operating Instructions, 10/2013, A5E00074631-11
143
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
AVG
).
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 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 following three components:
Figure 8-2 PFD distribution
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
HFT for type A devices
0
1
2
< 60 %
SIL 1
SIL 2
SIL 3
60 to 90 %
SIL 2
SIL 3
SIL 4
90 to 99 %
SIL 3
SIL 4
SIL 4
> 99 %
SIL 3
SIL 4
SIL 4