Relief Device; Control Circuit, 24-V - Carrier 48FC M20 Series Service And Maintenance Instructions

LOW PRESSURE SWITCH
The system is protected against a loss of charge and low evapora-
tor coil loading condition by a low pressure switch located on the
suction line near the compressor. The switch is stem-mounted.
Trip setting is 54 psig ± 5 psig (372 ± 34 kPa). Reset is automatic
at 117 ± 5 psig (807 ± 34 kPa).
EVAPORATOR FREEZE PROTECTION
The system is protected against evaporator coil frosting and low
temperature conditions by a temperature switch mounted on the
evaporator coil hairpin. Trip setting is 30°F ± 5°F (–1°C ± 3°C).
Reset is automatic at 45°F (7°C).
SUPPLY (INDOOR) FAN MOTOR PROTECTION
Disconnect and lockout power when servicing fan motor.
Supply fan motors contain a safety relay that opens in the event of
a fault. This relay protects the motor against certain supply power
conditions as well as over-temperature and over-current
protection. If the relay is open it will remove 24V to R on the UCB
and will also prevent cooling/heating operation to protect the unit
until the fault condition clears. Do not bypass this switch to correct
trouble. Determine the cause and correct it. (See Table 4.)
DESCRIPTION
No Error
NTC Over-Temperature Protection
Phase Fault
Over Current Protection
Locked Rotor Protection, Start-up
Locked Rotor Protection, Running
Over/Under Voltage
Current Sampling Error
Microelectronic (MCU) Fault
Table 4 — Supply Fan Motor Logic and Safety Relays
Automatic Reset – Motor starts 12 seconds after the temperature falls below reset limit.
Automatic Reset – Motor to start after 3 phases present.
Automatic Reset – If motor over-current protection trips, motor restarts after 20 seconds off time.
If over-current is detected 3 times consecutively, the motor is off for 3 minutes and restarts. Cycle starts
again after 20 seconds.
Automatic Reset – If motor detects locked rotor, it attempts to restart after 5 seconds.
If motor detects 3 consecutive faults, the motor waits 3 minutes and restarts.
Cycle starts again after 20 seconds.
Automatic Reset – Motor restarts as soon as input voltage is back within +/-10%.
Manual Reset – Power off and wait 2 minutes and restart motor.
Automatic Reset – Motor restarts 3 minutes after fault clears.
Manual Reset – Power off and wait 2 minutes and restart motor.
CONDENSER FAN MOTOR PROTECTION
The condenser fan motor is internally protected against
over-temperature.

Relief Device

A soft solder joint at the suction service access port provides pres-
sure relief under abnormal temperature and pressure conditions
(i.e., fire in building). Protect this joint during brazing operations
near this joint.

Control Circuit, 24-V

The control circuit is protected against over-current conditions
by a circuit breaker mounted on control transformer TRAN.
Reset is manual.
START DELAY
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