Functional Safety; B.2 Functional Safety - Siemens SITRANS P DS III Operating Instructions Manual

B.2

Functional safety

Abbreviation Full term in English
CFC Continuous Function Chart
FIT Failure in Time
HFT Hardware Fault Tolerance
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
SITRANS P DS III/P410 with PROFIBUS PA
Operating Instructions, 02/2016, A5E00053276-08
Meaning
Software package for graphical, technology-oriented configura-
tion of automation tasks
Frequency of failure
Number of faults withing 10
Hardware fault tolerance:
Capability of a function unit to continue executing a required
function in the presence of faults or deviations.
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-
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 func-
tional 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 func-
tions 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.
List of abbreviations/acronyms

B.2 Functional safety

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