Siemens ULTRAMAT 6F Sicherheitshandbuch - Seite 21
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Glossary
Dangerous failure
Failure with the potential to bring the safety-instrumented system into a
dangerous or non-functional status
Low Demand Mode
The frequency of demands for operation made on a safety related system is
no greater than one per year and no greater than twice the proof-test
frequency;
Safety function
Defined function executed by a safety-instrumented system with the
objective of achieving or maintaining a safe system status taking into account
a defined dangerous occurrence.
Example:
Monitoring of user defined threshold values.
Safety Integrity Level
SIL
Safety-instrumented system
A safety-instrumented system excludes the safety functions that are required
to achieve or maintain a safe status in a system. It consists of a sensor, logic
unit/ control system and final controlling element.
Example:
A safety-instrumented system is made up of a pressure transmitter, a limit
signal sensor and a control valve.
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 the probability that the required safety
function will work.
The achievable SIL is determined by the following safety characteristics:
•
•
•
Average probability of dangerous failure of a safety function in case of
demand (PFD
)
AVG
Hardware fault tolerance (HFT)
Safe failure fraction (SFF)
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SIL Safety Manual ULTRAMAT 6 Gas analyzers
A5E00729183-02