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Function principle of multi-channel operation
Automation system
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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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