PFU 780 · Edition 02.12
50
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 second 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 signal can-
not 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 minimum
combustion time only has an effect on the behaviour 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 relevant
safety control and switching equipment for the use of the
application, e.g. safety temperature limiter, minimum and
maximum gas pressure, tightness control) must isolate ter-
minal 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.
6 .5 Emergency stop
6 .5 .1 In the event of fire or electric shock
If there is a risk of fire, electric shock or similar, inputs L1, N
and 26e (safety interlocks) of the PFU should be disconnected
from the electrical power supply – this should be reflected in
the wiring on site.
6 .5 .2 Via the safety interlocks (limits)
The safety interlock turns off the power to input 26e, such as
in the event of air deficiency or similar.