750-166 1-7
CB780/CB784
SAFETY PROVISIONS
Safety Provisions
DYNAMIC SELF-CHECK SAFETY CIRCUIT
The microcomputer tests itself and related hardware
while at the same time the safety relay system tests the micro-
computer operation. If a microcomputer or safety relay
failure occurs and does not allow proper execution of the
self-check routine, safety shutdown will occur and all safety
critical loads will be de-energized.
EXPANDED SAFE-START CHECK
The conventional safe-start check, which prevents burner
start-up if flame is indicated at start-up, is expanded to
include a flame signal check during STANDBY, a preigni-
tion interlock check, an interlock check, and a safety critical
load check.
HIGH FIRE PURGE AND LOW FIRE START
SWITCH TESTS
High Fire Purge Switch Test examines the Purge Posi-
tion Interlock Switch at the moment the firing rate motor is
commanded to the high fire position. If the switch is by-
passed, welded or otherwise closed prematurely, the system
will automatically add 30 seconds to allow additional drive
time for the firing rate motor to reach or near the open
position before starting the purge timing; otherwise, purge
timing starts when the High Fire Switch is closed. This
switch will also cause a hold (four minutes, fifteen seconds)
condition when the switch is open before purge or opens
during purge. The CB780/CB784 will lockout and annunci-
ate an alarm if the switch fails to close within the hold time
period.
Low Fire Start Switch Test examines the Low Fire Start
Switch at the moment PREPURGE is completed. If the
switch is bypassed, welded or otherwise prematurely closed,
the system automatically adds 30 seconds to allow the firing
rate motor additional time to reach or near the low fire start
position before ignition trials; otherwise, ignition trials start
after the Low Fire Switch closes. The test also is used to prove
that the firing rate motor is at low fire position throughout
the ignition trial period. This switch will also cause a hold
(four minutes, fifteen seconds) condition if the switch opens
after purging is complete. The CB780/CB784 will lockout
and annunciate an alarm if the switch fails to close within the
hold time period.
INTERNAL HARDWARE STATUS MONITORING
The CB780/CB784 checks the purge card for correct
parity to prevent purge timing shifts and circuitry failures. It
also analyzes the integrity of the configuration jumpers and
internal hardware. The POWER LED blinks every four
seconds, signifying an internal hardware check.
CLOSED LOOP LOGIC TEST
The test verifies the integrity of all safety critical loads,
terminals 8, 9, 10 and 21. If the loads are not energized
properly; i.e., the main valve terminal is powered during
PREPURGE, the CB780/CB784 will lockout on safety shut-
down. The CB780/CB784 must react to input changes but
avoid the occurrence of nuisance shutdown events. Signal
conditioning is applied to line voltage inputs to verify proper
operation in the presence of normal electrical line noise such
as transient high voltage spikes or short periods of line
dropout. Signal conditioning is tolerant of synchronous noise
(line noise events that occur at the same time during each line
cycle).
DYNAMIC AUTO-CHECK
Dynamic Auto-Check circuitry tests the flame signal
amplifier during burner operation and shuts down the CB780/
CB784 if the flame amplifier fails.
DYNAMIC FLAME AMPLIFIER AND SHUTTER
CHECK
Self-checking circuitry tests all electronic components in
the flame detection system and amplifier 10 to 12 times per
minute and shuts down the CB780/CB784 if the detection
system fails.
DYNAMIC INPUT CHECK
All system input circuits are examined to verify that the
CB780/CB784 is capable of recognizing the true status of
external controls, limits and interlocks. If any input fails this
test, a safety shutdown occurs and the fault is annunciated.
DYNAMIC SAFETY RELAY TEST
Checks the ability of the dynamic safety relay contacts to
open and close. It also verifies that the safety critical loads,
terminals 8, 9, 10 and 21, can be de-energized, as required, by
the Dynamic Self-Check logic.
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