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• 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:
Safety Integrity Level
The international standard IEC 61508 defines 4 levels (SIL1 through SIL 4).
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.
A SIS consists of sensor(s), logic system and actuator(s).
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.
λ
S
Lambda Safe
Number of safe failures
λ
D
Lambda Dangerous
Number of dangerous failures
λ
DU
Lambda Dangerous Undetected
Number of undetected dangerous failures
λ
DD
Lambda Dangerous Detected
Number of detected dangerous failures
DC
Diagnostic Coverage
Diagnostic Coverage - ratio between the failure
rate of dangerous failures detected by diagnostic
tests and total rate of dangerous failures of the
component or subsystem. The diagnostic
coverage does not include any failures detected
during proof tests.
MTBF
Mean Time Between Failures
Mean time between the occurrence of two
subsequent failures
SFF
Safe Failure Fraction
Fraction of safe failures as well as of detectable
dangerous failures
PFD
avg
Average Probability of dangerous
Failure on Demand
Average probability of dangerous failures on
demand of a safety function.
HFT
Hardware Failure Tolerance
Ability of a functional unit to execute a required
function while faults or deviations are present.
HFT = n means that the function can still be safely
executed for up to n faults occurring at the same
time.
T
proof
Proof test interval Interval for proof test
Multi-turn actuators
SA series with Control 3.XX /NI/SIL
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