Terminology; Abbreviations And Concepts - AUMA AC 01.2-22X Manual

Terminology

1. Terminology
Information sources
1.1.

Abbreviations and concepts

SIL
Safety function
Safety instrumented
function (SIF)
Safety instrumented
system (SIS)
Safety-related system
4
IEC 61508-4, Functional safety of electrical/electronic/programmable electronic
safety-related systems
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.
Table 1: Abbreviations of safety figures
Abbrevi- Full expression
ation
λ Lambda Safe
S
λ Lambda Dangerous
D
λ Lambda Dangerous Undetected
DU
λ Lambda Dangerous Detected
DD
DC Diagnostic Coverage
MTBF Mean Time Between Failures
SFF Safe Failure Fraction
PFD Average Probability of dangerous Fail-
avg
ure on Demand
HFT Hardware Fault Tolerance
T Proof test interval
proof
Safety Integrity Level
The international standard IEC 61508 defines 4 levels (SIL 1 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. An 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.
A safety-related system can comprise stand-alone systems dedicated to perform a
particular safety function or can be integrated into a plant.
SA .1 with AC 01.2-22X/-22Y/ACExC 01.2-22X/-22Y
Part 4: Definitions and abbreviations
Description
Number of safe failures
Number of dangerous failures
Number of undetected dangerous fail-
ures
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 com-
ponent or subsystem. The diagnostic
coverage does not include any failures
detected during proof tests.
Mean time between the occurence of
two subsequent failures
Fraction of safe failures as well as of
detectable dangerous failures
Average probability of dangerous fail-
ures on demand of a safety function.
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.
Interval for proof test
Multi-turn actuators