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31CG-PRC007-EN
ELECTRICAL CONTROL SYSTEM
The controls used on CGA 10 and 15-ton
units are classified either as “safety”
controls or “operational” controls. Brief
descriptions of the specific safety and
operating controls used in the CGA
control scheme are provided in the
following paragraphs.
Refer to the following control
descriptions for control settings.
UNIT SAFETY CONTROLS
Low Pressure Cutout (LPC01, LPC02)
Mounted below the unit control box are
two low pressure cutouts that open and
stop compressor operation if the
operating pressure drops below 38.5
+ 1
psig. The cutout automatically resets
when the pressure reaches 44.5
+ 2 psig.
The LPCO is a SPDT device. If it opens at
low ambient start-up, it will energize ODF
relay, stopping the outdoor fan(s) while
the compressor remains energized
through the LAST (Low Ambient Start
Timer).
High Pressure Cutout (HPC01, HPC02)
These units have high pressure cutouts
that open and stop compressor
operation if the discharge pressure
reaches 400
+ 10 psig. The cutout
automatically resets when pressure
drops to 250
+ 15 psig.
Reset Relays (RRS, RR2)
If the unit is shut down by the low
pressure cutout (or high pressure
cutout), the reset relay locks out the
compressor contactor (CCS, CC2). This
prevents the system from recycling until
the condition that caused the low (or
high) pressure cutout to trip is
determined and corrected.
CAUTION: To prevent unit damage, do
not reset the control circuit until the
cause of the safety lockout is identified
and corrected.
To reset RR1 and RR2, open and reclose
the unit disconnect switch.
Low Temperature Cutout (LTC)
The LTC is designed to disable the unit if
the leaving water temperature falls too
low. The LTC’s remote sensing bulb is
Controls (10, 15 Ton)
mounted at the outlet end of the
evaporator, where it monitors leaving
water temperature.
During normal unit operation, if the low
temperature cutout (LTC) senses a
temperature falling to 36°F
+ 3.0°F, the
LTC will open to interrupt compressor
operation. (Manual reset is required.)
Motor Overloads
These units have internal compressor
and condenser fan motor overloads.
These overloads protect the motors from
overcurrent and overheating conditions
and automatically reset as soon as they
cool sufficiently.
UNIT OPERATIONAL CONTROLS
Water Temperature Thermostat (WTT)
System operation for 10 and 15-ton CGA
units is governed by a two-stage water
temperature thermostat (WTT). The
remote sensing bulb of this device is
factory-installed in a bulb well located on
the evaporator water inlet; here, it
monitors the temperature of the water
returning to the evaporator.
Low Ambient Start Timer (LAST)
When one of the two timers energizes,
the low pressure control is bypassed for
four minutes, this allows time for suction
pressure to build sufficiently for the low
pressure cutout contacts to close.
Hot Gas Bypass Timer, Solenoid (HGBT,
HGBS)
The hot gas bypass option is factory-
installed only, and is used in a chilled
water system to keep the first stage
compressor on-line during short no-load
or light-load conditions. When Water
Temperature Thermostat (WTT) first
stage opens, 24-volt power is supplied to
the Compressor Contactor (CC1) through
Hot Gas Bypass Timer (HGBT) pins one
and four. Power is also applied from
WTT-B to HGBT coil (fixed 30-minute
time delay pickup) and to the Hot Gas
Bypass Solenoid (HGBS) through HGBT
pins eight and five. If first stage cooling
remains satisfied for 30 minutes, HGBT
will energize and shut down the
compressor. If there is a call for cooling
during HGBP mode, the unit will return
to cooling mode.
The adjustable hot gas bypass valve is
factory set at 70 psig.
Note: Hot gas bypass is available only
on the lead compressor circuit.
Anti-Short Cycle Timers (ASCT1, ASCT2)
An anti-short cycle timer is provided in
each compressor control circuit to
protect the compressors from starting
too frequently. This can occur as a result
of just over 0%, or just over 50% of the
unit capacity, or because of sudden
power outages of short duration.
Whenever the contacts of the water
temperature thermostat (WTT) open —
or when there is a momentary power
outage — the anti-short cycle timer will
lock out compressor operation for three
minutes.
Delay Between Compressors (DBC)
The delay between compressors
prevents both compressors from starting
at the same time by delaying
compressor number two for 30 seconds.
Definition of Abbreviations:
ASCT — Anti-Short Cycle Timer
CC — Compressor Contactor
CWFIR — Chilled Water Flow
Interlock Relay
CWPS — Chilled Water Pump
Starter
DBC — Time Delay Between
Compressors
HGBT — Hot Gas Bypass Timer
HPCO — High Pressure Cutout
LAST — Low Ambient Start
Timer
LPCO — Low Pressure Cutout
LTC — Low Temperature
Cutout
ODF — Outdoor Fan
RR — Reset Relay
SPDT — Single Pole, Double
Throw
WTT — Water Temperature
Thermostat