DG · Edition 08.13
27
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9
Glossary
9 .1 Diagnostic coverage DC
Measure of the effectiveness of diagnostics, which may be
determined as the ratio between the failure rate of detected
dangerous failures and the failure rate of total dangerous
failures
NOTE: Diagnostic coverage can exist for the whole or parts
of a safety-related system. For example, diagnostic cover-
age could exist for sensors and/or logic system and/or final
elements. Unit: %.
from EN ISO 13849-1:2008
9 .2 Mode of operation
High demand mode or continuous mode
Operating mode, where the frequency of demands for opera-
tion made on a safety-related system is greater than one per
year or greater than twice the proof-test frequency
from EN 61508-4:2008
9 .3 Category
Classification of the safety-related parts of a control system
in respect of their resistance to faults and their subsequent
behaviour in the fault condition, and which is achieved
by the structural arrangement of the parts, fault detection
and/or by their reliability
from EN ISO 13849-1:2008
9 .4 Common cause failure CCF
Failures of different items, resulting from a single event, where
these failures are not consequences of each other
from EN ISO 13849-1:2008
9 .5 Fraction of undetected common cause failures
Fraction of undetected failures of redundant components due
to a single event, whereby these failures are not based on
mutual causes
NOTE:
is expressed as a fraction in the equations and as a
percentage elsewhere.
from EN 61508-6:2010
9 .6 B
10d
value
Mean number of cycles until 10% of the components fail dan-
gerously
from EN ISO 13849-1:2008
9 .7 T
10d
value
Mean time until 10% of the components fail dangerously
from EN ISO 13849-1:2008
9 .8 Hardware fault tolerance HFT
A hardware fault tolerance of N means that N + 1 is the mini-
mum number of faults that could cause a loss of the safety
function
from IEC 61508-2:2010
