Siemens SITRANS P DS III Operating Instructions Manual page 230

List of abbreviations
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
AVG
Failure on Demand
SFF Safe Failure Fraction
SIL Safety Integrity Level
SIS Safety Instrumented System
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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"
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.
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-
instrumented system into a dangerous or non-permissible
functional status.
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.
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 sensors, logic unit/control system and
final controlling elements.
Operating Instructions, 06/2013, A5E00047092-09
SITRANS P DS III with HART