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7.9 Abbreviations
Abbreviation Meaning
DC Diagnostic
Coverage
Diagnostic coverage of safe
failures (DC
S
) or dangerous
failures (DC
D
)
FIT Failure in Time Number of failures that occur
in 10
9
hours
HFT Hardware Fault
Tolerance
Hardware fault tolerance: abil-
ity of a function unit to continue
with the execution of a de-
manded function despite exist-
ing faults or deviations
MTBF Mean Time
Between
Failures
Indicates the average time pe-
riod between one failure and
the next. This includes the
length of time that the compo-
nent functions for until the fail-
ure and the downtime.
MTTR Mean Time To
Repair
Indicates the average time
needed in order to repair a
faulty component.
PFD
avg
Average Proba-
bility of danger-
ous Failure on
Demand
Average probability of failure
on demand of a safety function
PFH Probability of a
Dangerous Fail-
ure per Hour
Probability of failure per hour
for the safety function
SFF Safe Failure
Fraction
Proportion of safe failures:
Proportion of failures without
the potential to set the safety-
related system to a dangerous
or impermissible function state
SIL Safety Integrity
Level
IEC/EN 61508 defines four
safety integrity levels (SIL 1 to
4). The higher the Safety Integ-
rity Level of the safety-related
system, the lower the probabil-
ity range for the failure of a
safety function.
λ The failure rate λ indicates the number of errors
per time unit.
DD Dangerous Detected
DU Dangerous Undetected
SD Safe Detected
SU Safe Undetected
