CB780/CB784 RELAY MODULES
32-00207—01 18
3. The Lockout Interlock or Running Interlock inputs
(interlock circuit including Airflow Switch) must
close by ten seconds into PREPURGE; otherwise, a
safety shutdown occurs.
4. After the firing rate motor reaches the PREPURGE
rate position and PREPURGE timing is completed,
the firing rate motor will drive to the low fire position.
5. When the firing rate motor reaches low fire position,
the Low Fire Switch, terminal 18, input must be
energized before entering the Ignition Trial state.
IGNITION TRIALS
Pilot Flame
Establishing
Period (PFEP):
1. With the firing
rate motor at the
low fire position:
a. The pilot valve and the ignition transformer, ter-
minals 8, 10 and 21, are energized. The
CB780/CB784 has a fifteen second interrupted
pilot valve, terminal 21 and a ten second inter-
rupted pilot valve/ignition, terminal 8.
b. During PFEP, the Low Fire Switch must remain
closed. If it opens, a safety shutdown occurs.
c. The Preignition Interlock input is ignored
throughout the Ignition Trial state.
2. Flame must be proven by the end of the four or ten
second PFEP to allow the sequence to continue. If
flame is not proven by the end of PFEP, a safety
shutdown occurs.
3. With flame
proven, the
ignition,
terminal 10,
is
de-energized for early spark termination.
4. Main Flame Establishing Period (MFEP):
a. The CB780/CB784 has a selectable ten second
or fifteen second MFEP. After the Ignition Trials,
and with the presence of flame, the main fuel
valve, terminal 9, is powered. If a flameout
occurs, the CB780/CB784 will lockout within 3
seconds.
RUN
A ten second
stabilization
period occurs
at the beginning
of the RUN period.
1. The firing rate motor releases to modulation.
2. The CB780/CB784 is now in RUN and will remain in
RUN until the controller input, terminal 6, opens,
indicating that the demand is satisfied or a limit has
opened.
POSTPURGE
The CB780/CB784
provides a fifteen
second POST-PURGE
following the completion
of the RUN period. The blower motor output is powered to
drive all products of combustion and any unburned fuel
from the combustion chamber. It also supplies
combustion air to burn fuel being purged from the fuel
line downstream of the fuel shutoff valve.
1. The main fuel valve and intermittent pilot valve, ter-
minals 9 and 21, are de-energized and the firing rate
motor is commanded to the low fire position to begin
the POSTPURGE period.
2. The Preignition Interlock closes within the first five
seconds of POSTPURGE.
3. After the fifteen second POSTPURGE period is com-
pleted, the CB780/CB784 reenters STANDBY.
MAIN IGN 00:15
Flame Signal 2.7V
M20284A
RUN
Expanded Annun.
M20285A
POST PURGE 00:12
Device RM7800L
M20286A
PILOT IGN 00:10
Fault History
M20283A
To Next Page
Loading...
Need help?
General