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RM7897A,C 7800 SERIES RELAY MODULES
NOTE: Interrupting power will cause electrical resets, but
does not reset a safety shutdown (lockout) condi-
tion.
OPERATION
Sequence of Operation
The RM7897A,C has the operating sequences listed
below; see Fig. 2 and 3. The RM7897A,C LED provide
positive visual indication of the program sequence:
POWER, PILOT, FLAME, MAIN and ALARM.
Initiate
The RM7897A,C Relay Module enters the INITIATE
sequence when the relay module is initially powered. The
RM7897A,C can also enter the INITIATE sequence if the
relay module verifies 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 not met, a hold condition is initiated and
displayed on the optional KDM for at least five seconds;
when met, the INITIATE sequence restarts. If the condition
is not corrected and the hold condition exists for four
minutes, the RM7897A,C locks out. Causes for hold
conditions in the INITIATE sequence:
1. AC line dropout detection.
2. AC line noise that can prevent a sufficient reading of
the line voltage inputs.
3. Low line voltage brownouts.
The INITIATE sequence also delays the burner motor
starter from being energized and de-energized from an
intermittent AC line input or control input.
Standby
The RM7897A,C is ready to start an operating sequence
when the operating control input determines a call for
heat is present. The burner switch, limits, operating limit
control and all microcomputer-monitored circuits must be
in the correct state for the relay module to continue into
the PREPURGE sequence.
Normal Start-Up Prepurge
The RM7897A,C Relay Module provides PREPURGE
timing, selectable with ST7800 Purge Timer cards, from
two seconds to thirty minutes with power applied and the
operating control indicating a call for heat.
1. The Airflow Interlock, burner switch, Run/Test switch
and all microcomputer-monitored circuits must also
be in the correct operating state.
2. The motor output, terminal 4, is powered to start the
PREPURGE sequence.
3. The Airflow Interlock input closes ten seconds into
PREPURGE or within the specified purge card tim-
ing; otherwise, a recycle to the beginning of PRE-
PURGE or lockout occurs, depending on how the
Airflow Switch selectable jumper (JR3) is configured.
32-00158—03
Ignition Trials
1. Pilot Flame Establishing Period (PFEP):
a. When the PFEP begins:
(1) The pilot valve and ignition transformer,
terminals 8 and 10 (also terminal 21,
RM7897A), are energized. The RM7897A has
an intermittent pilot valve, terminal 8, and
interrupted pilot valve on terminal 21. The
RM7897C has an interrupted pilot valve,
terminal 8.
(2) 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 occurs.
(3) the Pre-Ignition Interlock is ignored through-
out the Trial for Ignition State.
2. Main Flame Establishing Period (MFEP):
a. After PFEP, and with the presence of flame,
the main fuel valve, terminal 9, is powered. If a
flameout occurs, the relay module locks out
within 0.8 or 3 seconds, depending on the Flame
Failure Response Time (FFRT) of the amplifier.
b. The RM7897A (terminal 21) and RM7897C
(terminal 8) have a ten second MFEP
(RM7897C1018 is four seconds). After the
Ignition Trials, the interrupted pilot valve,
terminal 8 or 21, is de-energized. If a flameout
occurs, the relay module recycles (locks out if
jumper JR2 clipped) within 0.8 or 3 seconds,
depending on the Flame Failure Response Time
(FFRT) of the amplifier.
Run
1. The RM7897C has a delayed main valve that is
energized once the RUN period is entered.
2. The relay module is now in RUN and remains in RUN
until the controller input, terminal 6, opens, indicat-
ing that the demand is satisfied or a limit has
opened.
Post Purge
After the demand is satisfied or a limit opens, de-
energizing terminal 6, the Ignition/Pilot valve, main valve
and delayed main valve, terminals 8, 9 and 21, are de-
energized. The blower motor, terminal 4, remains powered
during the specified programmed time. The Pre-Ignition
Interlock closes within the first five seconds of Post Purge.
Run/Test Switch
The Run/Test Switch is located on the top side of the relay
module, see Fig. 12. The Run/Test Switch allows the
burner sequence to be altered as follows:
1. In the measured PREPURGE sequence, the
Run/Test Switch, placed in the TEST position,
causes the PREPURGE timing to stop.
2. In the Pilot Flame Establishing Period, the Run/Test
Switch, placed in the TEST position, stops the timer
during the first eight seconds of a ten-second PFEP
selection or during the first three seconds of a
four-second PFEP selection. It also allows for pilot
turn-down test and other burner adjustments. This
activates a fifteen-second flameout timer that
permits pilot flame adjustment without nuisance
14
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