Siemens ULTRAMAT 6F Руководство по технике безопасности - Страница 20
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Siemens AG
B
List of Abbreviations/Acronyms
B.1
Abbreviations
Abbreviation Full term in English
FIT
Failure in Time
HFT
Hardware Fault Tolerance
MTBF
Mean Time Between Failures
MTTR
Mean Time To Repair
PFD
Probability of Failure on Demand
PFD
Average Probability of Failure on Demand
AVG
PLC
Programmable Logic Controller
SIL
Safety Integrity Level
SFF
Safe Failure Function
TI
Test Interval
XooY
"X out of Y" voting
SIL Safety Manual ULTRAMAT 6 Gas analyzers
A5E00729183-02
Meaning
Frequency of failure of the protective function
Hardware fault tolerance: Capability of a func-
tion unit to continue executing a required func-
tion in the presence of faults or deviations.
Average period between two failures
Average period between the occurrence of a
fault on a device or system and the repair
Probability of dangerous failures of a safety
function on demand
Average probability of dangerous failures of a
safety function on demand
The international standard IEC 61508 defines
four discrete Safety Integrity Levels (SIL 1 to
SIL 4). Each level corresponds to a range of
probability for failure of a safety function. The
higher the Safety Integrity Level of the safety-
instrumented system, the lower the probability
that it will not execute the required safety func-
tions.
Proportion of safe failures: Proportion of failures
without the potential to bring the safety instru-
mented system into a dangerous on no permis-
sible functional status.
Testing interval of the protective function
Classification and description of the safety-
instrumented system in terms of redundancy
and the selection procedures used.
"Y" -Specifies how often the safety function is
executed (redundancy).
"X" -Determines how many channels have to
work correctly.
Example:
Pressure measurement: 1oo2 architecture. A
safety instrumented system decides that a
specified pressure limit has been exceeded if
one out of two pressure sensors reaches this
limit. In a 1oo1 architecture, there is only one
pressure sensor.
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