Phase Loss Protection; Thermistor Troubleshooting - Carrier 48HC 04-28 Manual

Phase Loss Protection

The phase loss protection option will monitor the three-phase electrical
system to provide phase reversal and phase loss protection.
Phase Reversal Protection
If the control senses an incorrect phase relationship, the relay (K1) will
be de-energized (opening its contact). If the phase relationship is
correct, the relay will be energized. The control has a self-bypass
function after a pre-set time. If the control determines that the three
phases stay in a correct relationship for 10 consecutive minutes, the
relay will stay energized regardless of the phase sequence of three
inputs as long as 24-vac control voltage is applied. This self-bypass
function will be reset if all three phases are restored in a phase loss
event.
Phase Loss Protection
If the reverse rotation board senses any one of the three phase inputs
has no AC voltage, the relay will be de- -energized (opening its
contact). This protection is always active as long as 24-vac control
voltage is applied, and is not affected by the self by-pass function of
the phase sequence monitoring function. However, in the event of
phase loss, the relay will be re-energized only if all three phases are
restored and the three phases are in the correct sequence.
A red LED is provided to indicate the function of the board. See
the table below.
LED STATUS
On Continuously Relay contact closed (normal operation).
Relay contact open (phase loss or phase
Blinking
reversal has occurred) — No power will be
supplied to the control system.
Off 24 vac control power not present (off).

Thermistor Troubleshooting

The electronic control uses thermistors to sense temperatures used
to control operation of the unit. Resistances at various temperatures
are listed in Table 17 and 18. Thermistor pin connection points are
shown in the Major System Components section. The general
locations of the thermistors are shown the Major System
Components section.
Air Temperatures
Air temperatures are measured with 10 kilo-ohm thermistors. This
includes supply-air temperature (SAT), outdoor-air temperature
(OAT), space temperature sensors (T55, T56, T58), and return air
temperature (RAT).
The supply air temperature (SAT) and outdoor air temperature
(OAT) thermistors use a snap-mount to attach through the unit
sheet metal panels. The snap-mount tabs must be flattened on the
tip end of the sensor to release for removal from the panel. (See
Fig. 16.) To reinstall, make sure the snap-mount tabs extend out.
Refrigerant Temperatures
Condenser coil temperatures are measured with 5 kilo-ohm
thermistors. These measurements provide an approximate saturated
condensing temperature for each circuit (SCT.A, SCT.B). Fig. 20- -21
show the factory locations for the SCT thermistors. Ensure that
thermistors are placed at the correct location and are snapped securely
over the return bend so that contact is made between the thermistor
and the tube.
FUNCTION
Fig. 16 - - SAT and OAT Thermistor Mounting
Thermistor/Temperature Sensor Check
A high quality digital volt-ohmmeter is required to perform this
check.
Connect the digital voltmeter across the appropriate thermistor
terminals at the J8 terminal strip on the Main Base Board (see
Major System Components section).
Using the voltage reading obtained, read the sensor temperature
from Table 17 and 18.
To check thermistor accuracy,
probe location with an
temperature-measuring instrument. Insulate thermocouple to avoid
ambient temperatures from influencing reading. Temperature
measured by thermocouple and temperature determined from
thermistor voltage reading should be close, within 5°F if care was
taken in applying thermocouple and taking readings.
If a more accurate check is required, unit must be shut down and
thermistor removed and checked at a known temperature (freezing
point or boiling point of water) using either voltage drop measured
across thermistor at the J8 terminal, or by determining the
resistance with unit shut down and thermistor
J8. Compare the values determined with the value read by the
control in the Temperatures mode using the Scrolling Marquee
display.
Sensor Trim
Corrective offsets can be applied to the space temperature and the
supply air temperature sensor readings. These corrections are set in
the Configuration→TRIM menu for the display, or in the
Maintenance→TRIM table for CCN. See the Indoor Air Quality
section for available adjustments to IAQ and OAQ sensor readings.
The space temperature may be corrected by entering either a
calibration temperature value in SPT.C, or an offset temperature
value in SPT.T. The supply-air temperature may be corrected by
entering either a calibration temperature value in SAT.C, or an
offset temperature value in SAT.T. If installed, the return air
temperature may be corrected by entering either a calibration
temperature value in RAT.C, or an offset temperature value in
RAT.T. Temperature corrections should only be made if sensor
readings are compared to an accurate reference temperature
measurement device.
46
C07015
measure temperature at
accurate thermocouple-type
disconnected from