036-21338-001-C-0703
Unitary Products Group 7
SEQUENCE OF OPERATION
UNIT OPERATION - 7-1/2
When the external control calls for cooling at terminal Y1
:
The system controller (SC) is energized. The system control-
ler starts the tandem compressors and enables the con-
denser fans by energizing contactor 1M (2M on the 208/230
volt models).
Condenser fan motor #1 is energized with the compressor.
Fan motor #2 operation is controlled through the Pressure
Switch (PS) which will de-energize the motor when the dis-
charge pressure falls below 180 psi.
Safety Lockout
: The system controller (SC) has a lockout cir-
cuit to prevent compressor short-cycling on a safety control
with automatic reset. If the high or low refrigerant pressure
switches (HP or LP) open, the SC will enter lockout mode.
SC provides a 90 second bypass of the low pressure switch
LP to prevent nuisance lockouts during unit start-up.
A malfunction light (24V, 2 A max. resistive load) can be ener-
gized through SC, by connecting the light between terminals
X and B on TB1.
NOTE: To reset the unit after a lockout:
a. Turn the system switch on the thermostat to the “OFF”
position and back to the”COOL” position.
OR
b. Increase the set point of the room thermostat above the
temperature in the conditioned space and return it to its
original setting. If the unit continues to be shut down by
one of its safety controls, a service man should be called
to determine the cause of the problem. Repeated reset-
ting of the lockout circuit may damage the unit.
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