Honeywell 7800 Series Manual page 24

RM7838B
OPERATION
Sequence of Operation
The RM7838B has the following operating sequences,
see Figs. 21 and 22 and Table 3.
INITIATE
The RM7838B enters the INITIATE sequence when the
Relay Module is powered. The RM7838B can also enter the
INITIATE sequence if the Relay Module verifies input
voltage fluctuations of +10/-15% or frequency fluctuations
of +/-10% during any part of the operating sequence. The
INITIATE sequence lasts for ten seconds unless the voltage
or frequency tolerances are not met. When the tolerances are
not met, a hold condition will be initiated and will be dis-
played on the VFD for at least five seconds. When the toler-
ances are met, the INITIATE sequence will restart. If the
condition is not corrected and the hold condition exists for
four minutes, the RM7838 will lockout. Causes for hold
conditions in the INITIATE sequence:
a. AC line dropout is detected.
b. AC line frequency error caused by using a 60 Hz device
on a 50 Hz line, or vice versa.
c. AC line noise that can prevent a sufficient reading of
the line voltage inputs.
d. Brownouts caused by a low line voltage.
The INITIATE sequence also delays the recirculating/
exhaust blower starter from being energized and de-ener-
gized from an intermittent AC line input or control input.
The Alarm is powered during INITIATE.
STANDBY
The RM7838B will remain in STANDBY until the
Burner Control Switch is closed.
Two seconds before leaving STANDBY, the shutter cir-
cuit is energized to verify no flame is present.
The Alarm is powered during Standby.
NORMAL START-UP
PURGE
The RM7838B provides PURGE timing selectable from
two seconds to 30 minutes.
1. Command to start PURGE is obtained by closing the
Burner Control Switch, which starts the recirculating/ex-
haust blower. Power is applied to the RM7838B through the
recirculating/exhaust blower airflow switch, stop station
and the limit string, to terminal no. 6. The Run/Test Switch
and all microcomputer monitored circuits must also be in the
correct operating state.
2. The RM7838B commands the firing rate motor to high
purge rate position by closing a circuit between terminals
no. 12 and 13. Purge timing will not begin until the High Fire
Switch closes, providing input to the RM7838B terminal
no. 19. Four minutes, 15 seconds are provided for the High
Fire Switch to be closed. A jumpered High Fire Switch will
add 30 seconds to the PURGE timing.
65-0120—1
3. When the PURGE timing is complete, the RM7838B
commands the firing rate motor to low fire position by
closing a circuit between terminals 13 and 14. Four minutes,
15 seconds are provided for the motor to reach the low fire
position.
4. When the low fire proving switch is proven (input on
terminal no. 18), the RM7838B will command the Combus-
tion Blower through terminal no. 5. A jumpered Low Fire
Switch will add 30 seconds to the PURGE timing.
5. Five seconds are provided for the Combustion Blower
Airflow Switch at terminal no. 7 to close and stabilize. Safety
shutdown will occur if the Combustion Blower Airflow
Switch does not close or if it should open during ignition
trials or Run.
6. The RM7838B will wait indefinitely for the Start Input
to terminal no. 21.
7. The Preignition Interlock must remain closed through-
out PURGE timing or a safety shutdown will occur.
8. The Alarm is powered during PURGE timing.
IGNITION TRIALS
Ignition trials begin when the Start input is received at
terminal 21 of the RM7838B. The RM7838B will lock out
with a jumpered Start Switch.
a. Pilot Flame Establishing Period (PFEP)
1. When the PFEP begins:
a. The pilot valve and ignition transformer (termi-
nals 8 and 10) are energized.
b. Five seconds into PFEP, Ignition terminal 10 is
de-energized.
c. Flame must be proven by the end of the ten
second PFEP (four seconds if Configuration
Jumper JR1 is clipped) to allow the sequence to
continue. If a flame is not proven by the end of
PFEP, a safety shutdown will occur.
2. During the PFEP, the Low Fire Switch must remain
closed. If it opens, a safety shutdown occurs.
3. The Preignition Interlock input (terminal 20) is
ignored throughout PFEP.
4. The alarm output will stop when flame has been
proven.
5. The RM7838B will :
a. Hold indefinitely, waiting for Manual Open
Valve command (terminal 17).
b. Hold indefinitely with a Pilot Valve Hold com-
mand present on terminal 16.
c. Automatically sequence to MFEP if the con-
figuration jumper 3 has been clipped and the
Pilot Valve Hold Command is not present.
Manual Open Valve input, terminal 17, must be
closed at the end of the MFEP.
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Operation