Safety function
Defined function executed by a safety-instrumented system with the objective of attaining or
maintaining a safe system state by taking a defined hazardous incident into account.
Example:
Limit pressure monitoring
Safety Integrity Level
→
SIL
Safety-instrumented system
A safety-instrumented system (SIS) executes the safety functions that are required to achieve
or maintain a safe state 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.
Sensor
Converter that converts mechanical or other non-electric variables into electrical signals.
SIL
The international standard IEC 61508 defines four discrete safety integrity levels (SIL) from SIL
1 to SIL 4. Each level corresponds to a 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 SIL which can be achieved is determined by the following safety-instrumented
characteristics:
● Average probability of failure on demand (PFD
AVG
)
● Hardware fault tolerance (HFT)
● Safe failure fraction (SFF)
Spread
Ratio of the set measuring span to the maximum set measuring span.
Total error
Total Error is sum of Total Performance and the long-term stability.
Glossary
SITRANS P320/P420 with 4 to 20 mA/HART
Operating Instructions, 11/2019, A5E44852162-AC 275
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