Siemens SITRANS P500, Operating Instructions Manual, Page: 233 / 238

Glossary
SITRANS P500 with HART
Operating Instructions, 12/2014, A5E02344528-09
233
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)
Total error
Total Error is sum of Total Performance and the long-term stability.
Total Error(s)
→
Total error
Total Performance
Total Performance is the square root of the sum of the squares of the three deviations
resulting from the influence of the static pressure, the temperature and the characteristic
deviation.