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
β
D
Of those failures that are detected by the diagnostic tests, the fraction that have
a common cause – generally it is assumed β = 2 x β
D
C
PT
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
λ
DD
Detected dangerous failure rate (this is the sum of all the detected dangerous
failure rates for a component)
λ
DF
Dangerous systematic failure rate
λ
DU
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 so
that, 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