Siemens SITRANS TR200 Produktinformation Seite 13

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
required a maximum of once per year on average.
Table 1 Safety Integrity Level
SIL
4
3
2
1
The "average probability of dangerous failures of the entire safety-instrumented system" (PFD
the three sub-systems in the following figure.
Figure 2 Example of PFD distribution
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)
According to IEC 61511-1, Section 11.4.4, the hardware fault tolerance (HFT) can be reduced by one (values in brackets) for
sensors and final controlling elements with complex components if the following conditions are applicable for the device:
● The device is proven-in-use.
● The user can configure only the process-related parameters, e.g. control range, signal direction in case of a fault, limiting
values, etc.
● The configuration level of the firmware is blocked against unauthorized operation.
● The function requires SIL of less than 4.
The device fulfills these conditions.
Functional safety for SITRANS TR200/TR300
A5E03263689-02, 08/2010
) The table deals with "Low demand mode", i.e. the safety function is
AVG
Interval
10 ≤ PFD < 10
-5 -4
AVG
10 ≤ PFD < 10
-4 -3
AVG
10 ≤ PFD < 10
-3 -2
AVG
10 ≤ PFD < 10
-2 -1
AVG
HFT
0
Not permitted
SIL 1
SIL 2
SIL 3
1 (0)
1)
SIL 1
SIL 2
SIL 3
SIL 4
) is normally split between
AVG
2 (1)
1)
SIL 2
SIL 3
SIL 4
SIL 4
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