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Glossary
23 .10 Lifting
After positioning the actuator IC 20, the BCU checks by
means of brief lifting whether its feedback input (termi-
nals 40 and 41) has been activated by the correct out-
put signal from the actuator. The signal of the relevant
control output (ignition, OPEN, CLOSE) is switched off
briefly for this purpose. While the signal is switched off,
the BCU may not detect a signal at the feedback input.
23 .11 Air actuator
The air actuator can be used
– for cooling,
– for purging,
– to control the burner capacity in ON/OFF mode and
in High/Low mode when using a pneumatic air/gas
ratio control system.
23 .12 Safe failure fraction SFF
Fraction of safe failures related to all failures, which are
assumed to appear
from EN 13611/A2:2011
23 .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
BCU 460, BCU 465 · Edition 11.19
NOTE: Diagnostic coverage can exist for the whole or
parts of a safety-related system. For example, diagnos-
tic coverage could exist for sensors and/or automation
system and/or control elements. Unit: %.
from EN ISO 13849-1:2008
23 .14 Mode of operation
IEC 61508 describes two modes of operation for safety
functions. These are low demand mode and high de-
mand or continuous mode.
In low demand mode, the frequency of demands for op-
eration 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 continu-
ous 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
143
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