Siemens SIPART PS2 Operating Instructions Manual page 365

Abbreviation Full term in English
MooN "M out of N" voting
MTBF Mean Time Between Failures
MTTR Mean Time To Restoration
PFD Probability of Dangerous Failure on
Demand
PFD Average Probability of Dangerous Fail‐
AVG
ure on Demand
SFF Safe Failure Fraction
SIL Safety Integrity Level
SIS Safety Instrumented System
SIPART PS2 with PROFIBUS PA
Operating Instructions, 05/2019, A5E00127926-AC
G.3 Abbreviations for functional safety
Meaning
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.
Average period between two failures
Average period between the occurrence of a fault in a device or
system and restoration of functionality
Probability of dangerous failures of a safety function on demand
Average probability of dangerous failures of a safety function on
demand
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.
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.
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
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