1. Terminology
Information sources
●
IEC 61508-4, Functional safety of electrical/electronic/programmable electronic
safety-related systems – Part 4: Definitions and abbreviations
●
IEC 61511-1, Functional safety - Safety instrumented systems for the process
industry sector – Part 1: Framework, definitions, system, hardware and software
requirements
1.1. Abbreviations and concepts
To evaluate safety functions, the lambda values or the PFD value (Probability of
Dangerous Failure on Demand) and the SFF value (Safe Failure Fraction) are the
main requirements. Further figures are required to assess the individual components.
These figures are explained in the table below.
Table 1: Abbreviations of safety figures
DescriptionFull expressionAbbrevi-
ation
Number of safe failuresLambda Safeλ
S
Number of dangerous failuresLambda Dangerousλ
D
Number of undetected dangerous fail-
ures
Lambda Dangerous Undetectedλ
DU
Number of detected dangerous failuresLambda Dangerous Detectedλ
DD
Diagnostic Coverage - ratio between
the failure rate of dangerous failures
detected by diagnostic tests and total
rate of dangerous failures of the com-
ponent or subsystem.The diagnostic
coverage does not include any failures
detected during proof tests.
Diagnostic CoverageDC
Mean time between the occurence of
two subsequent failures
Mean Time Between FailuresMTBF
Fraction of safe failures as well as of
detectable dangerous failures
Safe Failure FractionSFF
Average probability of dangerous fail-
ures on demand of a safety function.
Average Probability of dangerous Fail-
ure on Demand
PFD
avg
Ability of a functional unit to execute a
required function while faults or devi-
ations are present. HFT = n means that
the function can still be safely executed
for up to n faults occurring at the same
time.
Hardware Fault ToleranceHFT
Interval for proof testProof test intervalT
proof
SIL
Safety Integrity Level
The international standard IEC 61508 defines 4 levels (SIL 1 through SIL 4).
Safety function
Function to be implemented by a safety-related system for risk reduction with the
objective to achieve or maintain a safe state for the plant/equipment with respect to
a specific dangerous event.
Safety instrumented
function (SIF)
Function with specified safety integrity level (SIL) to achieve functional safety.
Safety instrumented
system (SIS)
Safety instrumented system for executing a single or several safety instrumented
functions. An SIS consists of sensor(s), logic system and actuator(s).
Safety-related system
A safety-related system includes all factors (hardware, software, human factors)
necessary to implement one or several safety functions. Consequently failures of
safety function would result in a significant increase in safety risks for people and/or
the environment.
A safety-related system can comprise stand-alone systems dedicated to perform a
particular safety function or can be integrated into a plant.
5
Part-turn actuators
with AC 01.2-SIL/ACExC 01.2-SIL Terminology
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