BCU 580 · Edition 11.15
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19 .12 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 coverage
could exist for sensors and/or automation system and/or
control elements. Unit: %.
from EN ISO 13849-1:2008
19 .13 Operating mode
IEC 61508 describes two modes of operation for safety func-
tions. These are low demand mode and high demand or
continuous mode.
In low demand mode, the frequency of demands for operation
made on a safety-related system is not greater than one per
year and is not greater than twice the proof-test frequency.
In high demand mode or continuous mode, the frequency of
demands for operation made on a safety-related system is
greater than one per year or greater than twice the proof-test
frequency.
See also IEC 61508-4
19 .14 Safe failure fraction SFF
Fraction of safe failures related to all failures, which are as-
sumed to appear
from EN 13611/A2:2011
19 .15 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
19 .16 Mean time to dangerous failure MTTF
d
Expectation of the mean time to dangerous failure
from EN ISO 13849-1:2008
Glossary