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9−Blower Contactor K3 (CAV units)
Blower contactor K3, used in all units, is a three-pole-double-
break contactor with a 24VAC coil used to energize the indoor
blower motor B3 in response to blower demand. K3 is ener-
gized by main control panel (A55).
10−Relay K203 (MSAVt units)
Blower relay K203, used in all MSAV units, is a 24volt single
pole relay used to energize the indoor blower motor B3 in re-
sponse to blower demand. K203 is energized by main control
panel (A55).
11−Outdoor Fan Relay K10, K68 (all units)
K149, K150, K152, K153 (SCA240/SCB288
units)
Outdoor fan relays K10, K68, K149, K150, K152 and K153
are DPDT relays with a 24VAC coil. In all units K10 and K68
energizes condenser fans B4 and B5. In addition to K10 and
K68 the SCA240/SCB288 is equipped with K149, K150,
K152 and K153 which energizes condenser fans B21, B22,
B23 and B24.
12−Power Exhaust Relay K65 (PED units)
Power exhaust relay K65 is a N.O. DPDT relay with a
24VAC coil. K65 is used in all units equipped with the op-
tional power exhaust dampers. K65 is energized by the
economizer control panel (A56), after the economizer
dampers reach 50% open (adjustable in ECTO). When
K65 closes, the exhaust fans B10 and B11 are en-
ergized.
ELECTRIC HEAT CONTROL HAT SECTION
(45 − 90 kW electric heat only)
13−Electric Heat Relay K9
All SCA/SCB series units with 45 − 90 kW electric heat use an
electric heat relay K9. K9 is a N.O. SPST pilot relay intended to
electrically isolate the unit’s 24V circuit from the electric heat
24V circuit. K9 is energized by the main control board A55.
K9−1 closes, enabling T2 to energize the electric heat con-
trol panel A60 and contactors K17 and K18.
14−Electric Heat Transformer T2
All SCA/SCB series units with 45 − 90 kW electric heat
use a single line voltage to 24VAC transformer mounted
in the electric heat control hat section in the control box.
The transformer supplies power to all electric heat con-
trols (contactors and coils). The transformer is rated at
70VA and is protected by a 3.5 amp circuit breaker CB13.
The 208/230 (Y) voltage transformers use two primary
voltage taps as shown in figure 5. Transformer T2 is iden-
tical to T1.
INTEGRATED MODULAR CONTROL BOARDS
Figures 6 and 7
The Integrated Modular Control (IMC) is a series of control
boards which integrates most control functions required for the
SCA/SCB units. The control boards are located in the upper
left hand corner of the control box. The control includes
complete unit diagnostics with permanent code storage,
field programmable control parameters and control op-
tions, on-site testing, and serial communications. For fur-
ther information refer to Integrated Modular Control Guide sent
with each unit.
15−Main Control Module A55 (all units)
The main control module A55 is the heart of the system. It
controls one compressor, one bank of electric heat, one
outdoor fan, and one blower. A55 includes the thermostat
inputs, serial communications ports, diagnostic code display,
control pushbutton, system configuration dip switches, and
four expansion ports. A diagnostic code list is located on the
back side of the left access panel.
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