
Abbreviation Full term in English Meaning
MooN "M out of N" voting Classification and description of the safety-instrumented system
in terms of redundancy and the selection procedures used.
A safety-instrumented system or part that consists of "N"
independent channels. The channels are connected to each
other
in such a way that "M" channels are in each case sufficient
for the device to perform the safety instrumented function.
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.
MTBF Mean Time Between Failures Average period between two failures
MTTR Mean Time To Restoration Average period between the occurrence of a fault in a device or
system and restoration of functionality
PFD Probability of Failure on Demand Probability of dangerous failures of a safety function on demand
PFD
AVG
Average Probability of Failure on
Demand
Average probability of dangerous failures of a safety function on
demand
SFF Safe Failure Fraction Proportion of safe failures:
Proportion of failures without the potential to bring the safety-
instrumented system into a dangerous or non-permissible
functional status.
SIL Safety Integrity Level 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
functions.
SIS Safety Instrumented System A safety-instrumented system (SIS) 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.
TI Test Interval Testing interval of the protective function
List of abbreviations
B.1 Functional safety
SITRANS P, DS III series with HART
222 Operating Instructions, 09/2012, A5E00047092-08
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