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Function
Principle of self-
supervision
Prerequisites for
burner startup
Siemens Building Technologies
HVAC Products
In contrast to conventional amplifiers, the signal delivered by the flame detector is han-
dled dynamically and not statically. The flame detector signal is converted to a se-
quence of control pulses and then fed to the flame relay circuit. The latter is designed
such that the flame relay can only be energized by a flame signal of the described form.
If the pulses change due to a faulty detector or faulty detector cables, the relay will be
deenergized and the burner control triggers the required safety actions. In the case of
UV supervision, it must also be ensured that self-ignition of the UV tube (e.g. due to ag-
ing) does not simulate a flame signal. For that reason, incident radiation at the UV cell
is periodically interrupted by a shutter. In addition to the self-checking facility, the flame
signal circuit is subjected to a functional test during the prepurge time. If it does not op-
erate correctly, the startup sequence will be aborted or lockout initiated. Furthermore, if
mains voltage drops to a level where safe operation of the burner control is no longer
ensured, the burner will automatically shut down. When mains voltage returns to the
normal level, the burner control repeats the startup sequence. If the detector signals
are only slightly above the minimum levels, such mains voltage fluctuations can also
give rise to burner lockout however.
The burner control is reset and in the start position (terminals 11 and 12 must re-
-
ceive power)
-
The air damper is closed. End switch «z» for the fully closed position must feed
power from terminal 11 to terminal 8
-
All control contacts between terminals 12 and 5 (limit thermostat, control thermo-
stat, etc.) must be closed
A
Start
When «R» closes, the burner control's sequence switch starts running. At the same
time, the fan motor connected to terminal 6 (only prepurging) receives power and,
on completion of «t7», the fan motor or flue gas fan at terminal 7 (pre- and post-
purging) also receives power. On completion of «t16», the control command to
open the air damper is given via terminal 9. During the running time of the motor,
the sequence switch does not operate, as terminal 8, via which the motor of the
sequence switch first receives power, is not live during that period of time. The se-
quence switch starts again and programs only after the air damper is fully open and
end switch «a» has changed over to feed power to terminal 8.
t1 Prepurge time with air damper fully open (nominal amount of combustion air)
Shortly after the start of the prepurge time, air pressure switch «LP» must change
over, thus interrupting the current path between terminals 4 and 13. Otherwise, the
burner control would go to lockout (start of air pressure check). At the same time,
terminal 14 must be live since this current path is used to power the ignition trans-
former and the fuel valves.
t3´ With the LOK16..., an ignition transformer connected to terminal 15 is therefore
switched on at this point in time (long preignition). If there is no «LP», the ignition
transformer receives power already with the start command. On completion of the
prepurge time, the burner control via terminal 10 runs the air damper into the low-
fire position, which is determined by the changeover point of auxiliary switch «m».
During the positioning time, the sequence switch stops again until terminal 8 re-
ceives power from «m».
t5 Interval
On completion of «t5», terminal 20 receives power. At the same time, control out-
puts 9 to 11 and input 8 are galvanically separated from the unit's control section,
so that the latter is protected against reverse voltages from the load control circuit.
The startup sequence of the burner control ends with the release of load controller
«LR» at terminal 20. The sequence switch switches itself automatically off, depend-
ing on the time variant used, either immediately or after some so-called «idle
steps», that is, without changing the contact positions.
9/20
CC1N7785en
15.10.2003
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