1-8 750-166
CB780/CB784
SAFETY PROVISIONS
MANDATORY PURGE
If lockout occurs after the initiation of ignition trials,
(or at anytime during a sequence when the fuel valves may
have been energized), a mandatory POSTPURGE period is
imposed.
OFF CYCLE (STANDBY OR PREPURGE) FLAME
SIGNAL CHECK
The flame detection subsystem (flame detector and am-
plifier) is monitored during STANDBY. If a flame simulat-
ing condition or an actual flame exists, a system hold occurs
and start-up is prevented. If the flame signal exists at any
time after the first 40 seconds of STANDBY, a safety shut-
down will occur and be annunciated. A shutter-check ampli-
fier and self-checking detector are energized for the first
40 seconds during STANDBY and the last two seconds
before exiting STANDBY. If a flame exists, a safety shut-
down occurs. An Auto-Check Amplifier is energized con-
tinually through STANDBY and PREPURGE to detect any
possibility of a runaway detector or a flame. If either situa-
tion happens, a safety shutdown occurs. A standard amplifier
is energized continually through STANDBY and
PREPURGE. If either situation happens, a safety shutdown
occurs.
PREIGNITION OUTPUT CIRCUIT CHECK
At the end of PREPURGE, the Dynamic Safety Relay
operation is checked. Also, all safety critical loads, terminals
8, 9, 10 and 21 are checked to verify the terminals are not
powered. If the Dynamic Safety Relay operation is faulty, or
if any of the safety critical loads are powered, safety shut-
down occurs and is annunciated.
TAMPER RESISTANT TIMING AND LOGIC
Safety and logic timings are inaccessible and cannot be
altered or defeated.
VERIFIED SPARK TERMINATION
The ignition terminal is monitored to verify early spark
termination (five seconds ignition and pilot and five seconds
pilot only).
FIRST-OUT ANNUNCIATION AND
SELF-DIAGNOSTICS
Sequence Status Lights (LEDs) provide positive visual
indication of the program sequence: POWER, PILOT,
FLAME, MAIN and ALARM. The green POWER LED
blinks every four seconds to signify the CB780/CB784
hardware is running correctly.
Multi-function Keyboard Display Module (standard with
CB780, optional with CB784) shows elapsed time during
PREPURGE, PILOT IGN, MAIN IGN, and POSTPURGE.
As an additional troubleshooting aid, it provides sequence
timing, diagnostic information, historical information and
expanded annunciator information when a safety shutdown
or hold or normal operation occurs.
First-out Annunciation reports the cause of a safety shut-
down or identifies the cause of a failure to start or continue
the burner control sequence with an English text and num-
bered code via the Keyboard Display Module. It monitors all
field input circuits, including the Flame Signal Amplifier
and Firing Rate Position Switches. The system distinguishes
118 modes of failure and detects and annunciates difficult-
to-find intermittent failures.
Self-Diagnostics adds to the First-out Annunciation by
allowing the CB780/CB784 to distinguish between field
(external device) and internal (system related) problems.
Faults associated within the flame detection subsystem,
CB780/CB784 or plug-in Purge Card, are isolated and re-
ported by the Keyboard Display Module. See the Diagnos-
tics and Troubleshooting section of the CB780/CB784 Op-
eration and Maintenance Manual, Manual Number 750-166.
INTERLOCK REQUIREMENTS
The following interlock inputs are provided:
Low Fire Interlock
This interlock verifies the firing rate motor is in the low
fire position before and during ignition trials.
High Fire Interlock
This interlock verifies the firing rate motor is in the high
fire position prior to and during PREPURGE.
Lockout Interlock
This interlock (ILK) input signifies a Lockout Interlock.
If the Lockout Interlock is open for more than ten seconds
into PREPURGE, the CB780/CB784 will lockout. After
entering PREPURGE, if the Lockout Interlock opens during
the first ten seconds, the purge timer will be reset. This
provides a continuous PURGE to occur without interruption
before the Pilot Flame Establishing Period. If a Lockout
Interlock opens anytime after ten seconds into PURGE,
during the Ignition Trials or Run, it causes a lockout.
A typical Lockout Interlock string contains an airflow
switch (see Fig. 7). The Interlock Check is a site configur-
able option (see Table 7). If this feature is enabled, the
CB780/CB784 will lockout after 120 seconds whenever
control terminal 6 is energized, and the Lockout Interlock
string (including airflow switch) is closed during STANDBY.
Preignition Interlock
The Preignition Interlock input is typically connected to
proof-of-closure switches for fuel valve(s). The Preignition
Interlock must be energized throughout PREPURGE. If the
Preignition Interlock opens during STANDBY, it causes a
hold (30 seconds). The CB780/CB784 will lockout if the
interlock does not close within 30 seconds during STANDBY.
If the Preignition Interlock opens during PREPURGE, it
will lockout. If the Preignition Interlock is open after five sec-
onds into POSTPURGE, the CB780/CB784 will lockout.
The Preignition Interlock is ignored during the ignition trials
state and during RUN.
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