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Glossary
19 .10 Lifting
After positioning the actuator IC 20, the BCU checks by
means of brief lifting whether its feedback input (termi-
nal 52) has been activated by the correct output 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.
19 .11 Air valve
The air valve 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.
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, diagnos-
tic coverage could exist for sensors and/or automation
system and/or control elements. Unit: %.
from EN ISO 13849-1:2008
BCU 580 · Edition 11.15l
19 .13 Operating mode
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
129
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