CB780/CB784 RELAY MODULES
7
32-00207—01
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 Ampli-Check™
Dynamic AMPLI-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.
Dynamic Self-Check Safety Circuit
The microcomputer tests itself and related hardware while
at the same time the safety relay system tests the
microcomputer 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
preignition 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 Position
Interlock Switch at the moment the firing rate motor is
commanded to the high fire position. If the switch is
bypassed, 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 annunciate 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.
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
amplifier) is monitored during STANDBY. If a flame
simulating 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 shutdown will occur and be annunciated. A shutter-
check amplifier 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
shutdown occurs. An
AMPLI-CHECK™ Amplifier is energized continually
through STANDBY and PREPURGE to detect any
possibility of a runaway detector or a flame. If either
situation 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 shutdown occurs and is annunciated.
Tamper-Resistant Timing and Logic
Safety and logic timings are inaccessible and cannot be
altered or defeated.
