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2-High Pressure Switch S4
The high pressure switch is an automatic reset SPST N.C.
switch which opens on a pressure rise. S4 is located in
the compressor discharge line and wired in series with the
compressor contactor coil.
When discharge pressure rises to 640 ± 10 psig (4412 ±
69 kPa) (indicating a problem in the system) the switch
opens and the respective compressor is de-energized
(the economizer can continue to operate).
When discharge pressure drops to 475 ± 20 (3275 ± 138
kPa) psig, the switch closes and the compressor is en-
ergized. The CMC1 board monitors the pressure switch
when the compressor demand Y1 is active, allowing five
strike lockout. The compressor is shut down indefinitely
in this condition. A pressure switch may open and close
again four times during a current demand cycle without
causing a lockout condition by resetting the count at the
end of the demand cycle (CMC1 Y1 input OFF). The five-
strike lockout can only be reset by one of the following
actions:
-Power cycle the controller
-Apply the TEST mode
3-Low Ambient Switches S11 (field-installed option)
The low ambient switch is an auto-reset SPST N.O. pres-
sure switch which allows for mechanical cooling operation
at low outdoor temperatures. The switch is located in the
liquid line in the compressor section.
On P and Y volt units, S11 is wired in series with the com-
mon (black) lead to K10 outdoor fan motor.
On G and J volt units, S11 is wired in series with outdoor
fan relay K10 coil and when opened breaks 24 volts to the
coil, de-energizing outdoor fan B4.
When liquid pressure rises to 450 ± 10 psig (3102 ± 69
kPa), the switch closes and the condenser fan is ener-
gized. When discharge pressure in drops to 240 ± 10 psig
(1655 ± 69 kPa), the switch opens and the condenser fan
is de-energized. This intermittent fan operation results in
higher evaporating temperature allowing the system to op-
erate without icing the evaporator coil and losing capacity.
4-Compressor Low Discharge Temperature Limit S3
(field-supplied option)
S3 is a thermostat which opens on temperature drop. It is
wired in line with the 24VAC compressor contactor.
5-Compressor High Temperature Limit S5
The compressor thermal protector is located on top of the
compressor. S5 is wired in series with S4 high pressure
limit. The protector opens at 248°F + 9°F (120°C + 5°C)
and closes at 169°F + 18°F (76°C + 10°C).
C-Blower Compartment
All units are equipped with belt drive blowers. See unit
nameplate for blower type.
1-Blower Wheels
ZGB-036, -048 and -060 belt drive units use 10" x 10" (254
mm x 254 mm) blower wheels. ZGB -074 units use 15 x 9
(381 x 229 mm) blower wheels.
2-Indoor Blower Motor B3
Belt drive units use single or three phase motors (same
as supply voltage). CFM adjustments are made by adjust-
ing the motor pulley (sheave). Motors are equipped with
sealed ball bearings. All motor specifications are listed in
the Specifications (see table of contents) section in the
front of this manual. Units may be equipped with motors
manufactured by various manufacturers, therefore elec-
trical FLA and LRA specifications will vary. See unit rating
plate for information specific to your unit.
hree phase scroll compressors must be phased
sequentially for correct compressor and blower
rotation. Follow "COOLING START-UP" section
of
installation
compressor and blower operation.
A-Blower Operation
Initiate blower demand at thermostat according to instruc-
tions provided with thermostat. Unit will cycle on thermo-
stat demand. The following steps apply to applications
using a typical electro-mechanical thermostat.
1 - Blower operation is manually set at the thermostat
subbase fan switch. With fan switch in ON position,
blowers will operate continuously.
2 - With fan switch in AUTO position, the blowers will
cycle with demand. Blowers and entire unit will be
off when system switch is in OFF position.
B-Determining Unit CFM
B-Determining Unit CFM
IMPORTANT - ZGB074S4T blower (G thermostat) CFM
MUST BE ADJUSTED IN HIGH SPEED. Disconnect fac-
tory-installed J350 low speed connector from P350. Con-
nectors are located near the bottom of the control box.
Connect J351 high speed connector to P350. Once blow-
er CFM is set, J350 can be reconnected to operate the
blower on low during ventilation only demands. See table
4.
TWO-SPEED BLOWER OPERATION
Thermostat
G(P350/J350)*
G(P350/J351)
W1
W2
Y1
Y2
*Factory-installed jack/plug connection.
Page 29
IMPORTANT
instructions
to
ensure
TABLE 4
ZGB074S4T UNITS
Blower Speed
Low
High
High
High
Low
High
proper
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