Safety-instrumented system in single-channel operation (SIL 2)
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Automation system
Transmitter
Final controlling element
Figure 9-2 Safety-instrumented system in single-channel operation
The combination of transmitter, automation system and final controlling element forms a safety-
instrumented system that performs a safety function.
The transmitter generates a process-related measured value that is transferred to the
automation system. The automation system monitors this measured value. If the measured
value violates the high or low limit, the automation system generates a shutdown signal for the
connected final controlling element, which switches the corresponding process valve to the
specified safety position.
9.3 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 a range of probability for failure of a safety function.
Description
The following table shows the dependency of the SIL on the "average probability of dangerous
failure of a safety function of the entire safety-instrumented system" (PFD
AVG
). "Low demand
mode" is examined. The safety function is required a maximum of once per year on average.
Table 9-1 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 failure of the entire safety-related system" (PFD
AVG
) is
normally split between the following three components:
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Functional safety
9.3 Safety Integrity Level (SIL)
SIPART PS2 with 4 to 20 mA/HART
Operating Instructions, 11/2019, A5E00074631-AE 193
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