KROHNE TT 51 SERIES EN - Manual (Page 5)

TERMS AND DEFINITIONS
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TT 51 SERIES
www.krohne.com 09/2010 - 4000869801 - AD TT 51 SIL R01 en
Terms and definitions
Used abbreviations
DC
D
Diagnostic Coverage of dangerous failures.
Diagnostic coverage is the ratio of the detected failure rate to the total failure rate.
FIT Failure In Time (1x10 -9 failures per hour)
FMEA Failure Modes Effects Analysis is a structured qualitative analysis of a system,
subsystem, process, design or function to identify potential failure modes, their
causes and their effects on (system) operation.
FMEDA Failure Modes Effects and Diagnostic Analysis adds a qualitative failure data for all
components being analyzed and ability of the system to detect internal failures via
automatic on-line diagnostics parts to FMEA.
HFT Hardware Fault Tolerance
Low demand mode Mode, where the frequency of demand for operation made on a safety-related
system is not greater than one per year and not greater than twice the proof-test
frequency.
High demand
mode
Mode, where the frequency of demands for operation made on a safety-related
system is greater than one per year and greater than twice the proof-check
frequency.
MTBF Mean Time Between Failure is average time between failure occurrences.
MTTR Mean Time To Restoration is average time needed to restore normal operation after
a failure has occurred.
PFD
AVG
Probability of Failure on Demand is the average probability of a system to fail to
perform its design function on demand.
PFH Probability of Failure per Hour is the probability of a system to have a dangerous
failure occur per hour.
SFF Safe Failure Fraction summarizes the fraction of failure, which lead to a safe state
and the fraction of failures which will be detected by diagnostic measures and lead
to a defined safety action.
SIF Safety Instrumented Function
SIL Safety Integrity Level
Type A component "Non-complex" subsystem (all failure modes are well defined);
for details see 7.4.3.1.2 of IEC 61508-2.
Type B component "Complex" subsystem (at least one failure mode are not well defined);
for details see 7.4.3.1.3 of IEC 61508-2.
T[Proof] Proof Test Interval
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