WIKA T32.xS - Manual (Page 5)

GB
5 WIKA safety manual temperature transmitter T32.xS
11583631.02 11/2010 GB/D/F/E
1.4 Abbreviations
Abbreviation Description
HFT Hardware Fault Tolerance, capability of a functional unit to continue
the execution of the demanded function when faults or anomalies
exist.
MTBF Mean interval between two failures
MTTR Mean interval between the occurrence of the failure in a device or
system and its repair
PFD Likelihood of dangerous safety function failures occurring on
demand
PFD
avg
Average likelihood of dangerous safety function failures occurring
on demand
SIL Safety Integrity Level, the international standard IEC 61508
defines four discrete safety integrity levels (SIL1 to SIL4). Each
level corresponds to a specific probability range with respect to
the failure of a safety function. The higher the integrity level of the
safety-related system, the lower the likelihood of the demanded
safety functions not occurring.
SFF Safe Failure Fraction, the proportion of failures without the potential
to put the safety-related system into a dangerous or impermissible
functional state.
T
Proof
In accordance with IEC 61508-4, chapter 3.5.8, TProof is defined
as the periodic testing to expose errors in a safety-related system.
XooY Classification and description of the safety-related system with
respect to redundancy and the selection procedure used. "Y"
indicates how often the safety function is carried out (redundancy).
"X" determines how many channels must work properly.
λ
sd
und λ
su
λ
sd
Safe detected + λ
su
Safe undetected
Safe failure (IEC 61508-4, chapter 3.6.8):
A safe failure is present when the measuring system switches to the
defined safe state or the fault signalling mode without the process
demanding it.
λ
dd
+λ
du
λ
dd
Dangerous detected + λ
du
Dangerous undetected
Unsafe failure (IEC 61508-4, chapter 3.6.7):
Generally a dangerous failure occurs if the measuring system
switches into a dangerous or functionally inoperable condition.
λ
du
λ
du
Dangerous undetected
A dangerous undetected failure occurs if the measuring system does
not switch into a safe condition or into the error mode on a demand
from the process.
For further relevant abbreviations, see IEC 61508-4.
1. General information