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" inde‐
pendent 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-instrumen‐
ted system decides that a specified pressure limit has been ex‐
ceeded 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 Dangerous Failure on
Demand
Probability of dangerous failures of a safety function on demand
PFD
AVG
Average Probability of Dangerous Fail‐
ure 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-in‐
strumented system into a dangerous or non-permissible func‐
tional status.
SIL Safety Integrity Level The international standard IEC 61508 defines four discrete Safe‐
ty 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 low‐
er the probability that it will not execute the required safety func‐
tions.
SIS Safety Instrumented System A safety-instrumented system (SIS) executes the safety func‐
tions that are required to achieve or maintain a safe status in a
system. It consists of sensors, logic unit/control system and final
controlling elements.
Abbreviations
G.3 Abbreviations for functional safety
SIPART PS2 with PROFIBUS PA
Operating Instructions, 05/2019, A5E00127926-AC 365
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