Carrier TRANSICOLD 69NT20-274 Operation And Service page 41

2.4.3 Defrost (See Figure 2-3.)
Refer to section 1.13.4 (Code 26) for description of
the defrost interval selector and automatic defrost
initiation.
When the defrost mode is initiated, the controller
relay contacts (TH) close to supply power to the heat
relay (HR) and in turn, energizes the defrost heaters. The
defrost light is illuminated.
TC opens to de-energize the compressor contactor
and cool light. Also (TN) relay opens to de-energize the
condenser fan motor contactor (CF).
Also (TE & TV) relays open to stop the evaporator
fan motors.
The in-range light remains illuminated during
defrost.
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= Energized Circuit
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When the coil temperature reaches 24_C (75_F)
defrost termination thermostat (DTT) opens to interrupt
the defrost cycle and the unit returns to its normal
function.
The 54.5_C (130_F) heat termination thermostat
(HTT) will open the circuit if the defrost mode does not
terminate at 24_C (75_F). The controller will terminate
defrost if termination does not occur within 1.0 to 1.5
hours depending on voltage. An alarm will be given of a
possible DTT failure.
When the return air falls to 7_C (45_F), the
controller checks to ensure defrost termination
thermostat (DTT) closed properly. If not, a DTT failure
alarm is given and the defrost mode is operated off of
return temperature sensor (RTS). The defrost mode will
be terminated after one hour by the controller.
TO CONTROL TRANSFORMER
Figure 2-3. Defrost
2-5
= De-energized Circuit