Functional Safety 2016

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Functional Safety 2016
November, 2016 - London
Page 17 of 17
Glossary of Terms
Term Description
β Beta Factor - the fraction of undetected failures that have a common cause
βD
Of those failures that are detected by the diagnostic tests, the fraction that have
a common cause – generally it is assumed β = 2 x βD
CPT Proof Test Coverage
Competency
The combination of training, skills, experience and knowledge that a person has
and their ability to apply them to perform a task safely. Other factors, such as
attitude and physical ability, can also affect someone’s competence.
DC Diagnostic Coverage - Ratio of the Detected Dangerous Failure rate to the Total
Failure Rate provided by the Diagnostics of the system or Subsystem.
FMEDA Failure Modes, Effects and Diagnostics Analysis
λ D Dangerous failure rate for a component
λDD
Detected dangerous failure rate (this is the sum of all the detected dangerous
failure rates for a component)
λDF Dangerous systematic failure rate
λDU
Undetected dangerous failure rate (this is the sum of all the undetected
dangerous failure rates for a component)
MTBF Mean Time Between Failures
MT Mission Time
MTTF Mean Time To Failure
MTTR Mean Time to Restoration
MRT Mean Repair Time
PFD AVG Average probability of failure on demand
Proof Test Test performed to reveal undetected faults in a safety-instrumented system sothat, if necessary, the system can be restored to its design functionality.
Proof Test
Coverage
Is the ratio of the total dangerous failures detected by the proof test against the
dangerous failures not detected by the proof test
RRF Risk Reduction Factor
SIF Safety Instrumented Function
SIS Safety Instrumented System
SIL Safety Integrity Level
TI Proof Test Interval