
Function principle of multi-channel operation
Automation system
0HDVXUHG
Y
DOXH
6KXWGRZQVLJQDO
Transmitter
Final controlling elements
Figure 8-2 Safety-instrumented system (SIL3)
The transmitter generates process-related measured values that are transferred to the
automation
system. The automation system monitors these measured values. In the event of
a fault, the automation system generates shutdown signals for connected final controlling
elements that set the associated valve to the defined safety position.
Faults are:
● Violations of the preset high or low limits
● Simultaneously incoming measured values which are different from one another
8.1.2 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
Functional safety
8.1 General safety instructions
SITRANS P, DS III series with HART
138 Operating Instructions, 09/2012, A5E00047092-08
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