Carrier 69NT40-541-306 Operation And Service Download

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5–13

T-354

The actual temperatures used to make the “open” or “closed” determinations are dependent on the type of defrost
request made and the operator setting of configuration variable CnF41. Configuration variable CnF41may be fac-
tory set at the default value of 25.6



C (78



F) or a lower value of 18



C (64



F).

When a request for defrost is made by the use of the Manual Defrost Switch or Communications, the unit will enter
defrost when the reading at the defrost temperature sensor is at or below the CnF41 setting. Defrost will terminate
when the defrost sensor temperature reading rises above the CnF41 setting.

When a request for defrost is made by probe check, the unit will enter defrost when the defrost temperature sensor
reading is at or below 25.6



C (78



F). The unit will terminate defrost when the Defrost Temperature Sensor reading

rises above 25.6



C (78



When a request for defrost is made by demand defrost, the unit will enter defrost when the reading at the defrost
temperature sensor is at or below 18



C (64.4



F). Defrost will terminate when the defrost sensor temperature read-

ing rises above the CnF41 setting.

When a defrost has terminated, the defrost interval timer will begin counting when the reading at the defrost tem-
perature sensor is at or below 10



C (50



F). Once the timer has counted the required time, the unit will enter defrost

if the Defrost Temperature Sensor is at or below 25.6



C (78



F). Defrost will terminate when the defrost sensor tem-

perature reading rises above the CnF41 setting.

When the unit enters defrost, the controller opens contacts TC, TN, and TE (or TV) to de-energize the compressor,
condenser fan and evaporator fans (see

Figure 5.8

). The COOL light is also de-energized.

The controller then closes TH to supply power to the heaters. The defrost light is illuminated.

When the defrost temperature sensor reading rises to the applicable defrost termination thermostat “opening”
point, the de-icing operation is terminated.

If defrost does not terminate correctly and temperature reaches the set point of the heat termination thermostat
(HTT), the thermostat will open to de-energize the heaters. If termination does not occur within two hours, the con-
troller will terminate defrost. An alarm will be given of a possible DTS failure.

If probe check (controller function code CnF31) is configured to special, the unit will proceed to the next operation
(snap freeze or terminate defrost). If the code is configured to standard, the unit will perform a probe check. The
purpose of the probe check is to detect malfunctions or drift in the sensed temperature that is too small to be
detected by the normal sensor out of range tests. The system will run for eight minutes in this condition. At the end
of the eight minutes, probe alarms will be set or cleared based on the conditions seen.

5.11 EMERGENCY OPERATION (OPTION)

Operation by the refrigeration controller may be overridden by use of the EMERGENCY BYPASS or by use of the
EMERGENCY DEFROST switch. The EMERGENCY BYPASS switch functions to bypass the controller in the
event of controller failure. The EMERGENCY DEFROST switch functions to bypass the controller and place the
unit in the defrost mode.

5.11.1

Emergency Bypass Operation

To place the unit in the emergency bypass mode, cut the wire tie installed at the switch mounting and place the
EMERGENCY BYPASS switch in the BYPASS position. This will in turn activate the Emergency Bypass System
(EBS) control module.

To operate the fans only, the MODE switch must be in the FANS ONLY position and the EMERGENCY BYPASS
Switch must be in the ON position.

The EBS module uses the system's safety devices (high pressure switch, motor internal protectors, and heat termi-
nation thermostat) to protect the system while in Emergency Bypass Mode.

CAUTION

The unit will remain in the full cooling mode as long as the emergency bypass switch is in
the BYPASS position and the MODE SWITCH is in the FULL COOL position.

If the cargo may be damaged by low temperatures, the operator must monitor container tempera-
ture and manually cycle operation as required to maintain temperature within required limits.