Terminology; Abbreviations And Concepts - AUMA SA3 Manual

1

Terminology

Information sources
1.1

Abbreviations and concepts

SIL
Safety function
Safety instrumented
function (SIF)
Safety instrumented
system (SIS)
Safety-related system
• 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.
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:
Abbreviation Full expression
λ Lambda Safe
S
λ Lambda Dangerous
D
λ DU
Lambda Dangerous Undetected
λ DD
Lambda Dangerous Detected
Diagnostic Coverage
DC
MTBF Mean Time Between Failures
SFF Safe Failure Fraction
PFD avg Average Probability of dangerous
Failure on Demand
Hardware Failure Tolerance
HFT
T proof
Proof test interval
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.
Function with specified safety integrity level (SIL) to achieve functional safety.
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.
Description
Number of safe failures
Number of dangerous failures
Number of undetected dangerous failures
Number of detected dangerous failures
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.
Mean time between the occurrence of two
subsequent failures
Fraction of safe failures as well as of detectable
dangerous failures
Average probability of dangerous failures on
demand of a safety function.
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.
Interval for proof test
5
Multi-turn actuators
SA series with Control 3.XX /NI/SIL