Siemens SITRANS P DS III Operating Instructions Manual page 148

Functional safety
8.2 Safety Integrity Level (SIL)
The "average probability of dangerous failures of the entire safety-instrumented system"
(PFD
Figure 8-3
The following table shows the achievable Safety Integrity Level (SIL) for the entire safety-
instrumented system for type B devices depending on the safe failure fraction (SFF) and the
hardware fault tolerance (HFT).
● Type B devices include analog transmitters and shut-off valves with complex
components, e.g. microprocessors (also see IEC 61508, Section 2).
● For detailed information on values and hardware/firmware versions for your device, refer
to the manufacturer declaration for the device (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 %
Operational reliability in accordance with IEC 61511-1, Section 11.4.4
1)
Operational reliability
According to IEC 61511-1, Section 11.4.4, the hardware fault tolerance (HFT) can be
reduced by one (values in brackets) for transmitters and final controlling elements with
complex components if the following conditions apply to the device:
● The device is proven in operation.
● 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.
148
SIL Interval
2 10 ≤ PFD
-3
1 10 ≤ PFD
-2
) is normally split between the following three components:
AVG
PFD distribution
HFT for type B devices
0
Not permitted
SIL 1
SIL 2
SIL 3
< 10
-2
AVG
< 10
-1
AVG
1 (0)
SIL 1
SIL 2
SIL 3
SIL 4
1)
SITRANS P DS III with HART
Operating Instructions, 06/2013, A5E00047092-09
2 (1)
1)
SIL 2
SIL 3
SIL 4
SIL 4