Siemens CALOMAT 6 - Glossary

Siemens AG
SIL Safety Manual CALOMAT 6 Gas Analyzers 18
A5E00729184-02, 07/2010
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)