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Acronyms and abbreviations
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3. Acronyms and abbreviations
Acronym / Abbreviation
Designation
Description
Element
Term defined by IEC 61508 as “part of a
subsystem comprising a single component or
any group of components that performs one or
more element safety functions”.
PFD
Probability of Failure on
Demand
This is the likelihood of dangerous safety
function failures occurring on demand.
PFH
Probability of dangerous
Failure per Hour
The term “Probability” is misleading, as
IEC 61508 defines a rate.
SFF
Safe Failure Fraction
Safe Failure Fraction summarizes the fraction
of failures which lead to a safe state and the
fraction of failures which will be detected by
diagnostic measures and lead to a defined
safety action.
SIF
Safety Integrity Function
Function that provides fault detection (to
ensure the necessary safety integrity for the
safety functions).
SIL
Safety Integrity Level
The international standard IEC 61508
specifies four discrete safety integrity levels
(SIL 1 to SIL 4). Each level corresponds to a
specific probability range regarding the failure
of a safety function.