PFU 780 · Edition 02.12l
51
Project planning information
6 .3 Minimum combustion time
Even if the start-up signal (
ϑ
) is applied only briefly, the
time set under parameter 20 elapses before the burner
control unit shuts down the burner or signals a fault. To
stabilise the burner operation, a minimum combustion
time can be set independently of the central control
system. If the start-up signal (
ϑ
) drops once the sec-
ond safety time t
SA2
has started to elapse, the burner
remains in operation for at least time t
B
. The minimum
combustion time t
B
starts to elapse following controller
enable. If the start-up signal drops before the second
safety time t
SA2
, e.g. during pre-purge, the control unit
reverts directly to standby and the burner is not ignited.
The signal inputs for the pilot/main burner start-up sig-
nal cannot be used for a safety shut-down because the
unit controls the valves until the minimum combustion
time has elapsed.
04
06
07
t
t
B
26a
16c
18e
2c–4c
18a
14a
88
10e
26e
30a
2e–4e
6a–6e
V1
1
V2
2
ϑ
1
ϑ
2
2
1
In the case of pilot/main burner monitoring, the mini-
mum combustion time only has an effect on the be-
haviour of the main burner. The minimum combustion
time for the pilot burner is limited to the safety time on
start-up (t
SA1
).
Background: The pilot burner is only used in single-
stage operation.
6 .4 Safety interlocks (Limits)
The limiters in the safety interlock (linking of all the rel-
evant safety control and switching equipment for the
use of the application, e.g. safety temperature limiter,
minimum and maximum gas pressure, tightness con-
trol) must isolate terminal 26e from the voltage supply.
If the safety interlock is interrupted, this is indicated by
a blinking
51
on the display.
If the safety interlocks fail, an immediate program abort
with switch-off of all outputs occurs (even during the
safety time). If the safety interlocks are operational
again or the unit is switched back on, the program run is
restarted in standby.