General safety instructions
2.2 Safety Integrity Level (SIL)
Functional safety for SITRANS P, series DS III PROFIsafe
2-2
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Figure 2-1
Example of a safety-instrumented system with a bus
Functioning of the system as shown in the example
The transmitter generates a process-specific measured value. The programmable logic
control (PLC) monitors this value in case it exceeds a preset limit value. If a fault occurs, the
PLC generates a shutdown signal for the connected positioner. The positioner switches the
corresponding valve to the defined safety position. The communication with the positioner
via PROFIBUS PA or FOUNDATION Fieldbus remains unrestricted, i.e. position and status
messages are still possible.
2.2
Safety Integrity Level (SIL)
Definition: 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. The higher the SIL of the safety-instrumented system, the higher probability that the
required safety function will work.
