PFU 780 · Edition 02.12
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12 .13 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
12 .14 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 61508-4:2001
12 .15 Safe failure fraction SFF
Fraction of safe failures related to all failures, which are as-
sumed to appear
from EN 13611/A2:2011
12 .16 Probability of dangerous failure PFH
D
Value describing the likelihood of dangerous failure per hour
of a component for high demand mode or continuous mode.
Unit: 1/h
from EN 13611/A2:2011
12 .17 Mean time to dangerous failure MTTF
d
Expectation of the mean time to dangerous failure
from EN ISO 13849-1:2008
Glossary