Glossary
Functional safety for SITRANS P, series DS III PROFIsafe
Glossary-2
Product information, 07/2006, A5E00849297-01
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:
Limit pressure monitoring
Safety Integrity Level
Safety-instrumented system
A safety-instrumented system executes 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.
Sensor
Converter that converts mechanical or other non-electrical 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 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.
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 fractions (SFF)
