Carrier 19XL - Manual (Page 81)

Shunt Trip
The shunt trip function of the PIC is a safety trip. The shunt trip is wired from an output on the
SMM to the motor circuit breaker. If the PIC tries to shut down the compressor through normal
shutdown procedure but is unsuccessful for 30 seconds, the shunt trip output is energized and
causes the circuit breaker to trip off. If ground fault protection has been applied to the starter,
the ground fault trip will also energize the shunt trip to trip the circuit breaker.
Default Screen Freeze
Whenever an alarm occurs, the LID default screen will freeze displaying the condition of the
machine at the time of alarm. Knowledge of the operating state of the chiller at the time an
alarm occurs is useful when troubleshooting. Current machine information can be viewed on the
Status tables. Once all existing alarms are cleared (by pressing the RESET softkey), the default
LID will return normal operation.
Motor Cooling Control
Motor temperature is reduced by refrigerant entering the motor shell and evaporating. The
refrigerant is regulated by the motor cooling relay. This relay will energize when the compressor
is running and motor temperature is above 125 F (51.7 C). The relay will close when motor
temperature is below 100 F (37.8 C). Note that there is always a minimum flow of refrigerant
when the compressor is operating for motor cooling; the relay only controls additional
refrigerant to the motor.