28
HIGH PRESSURE SWITCH (HPS) — Each circuit is pro-
tected with a high pressure switch to prevent excessive con-
densing pressure. See the section Sensor Locations on page 32
for locations. The high pressure switch, P/N HK02ZZ001,
opens at 650 ± 10 psig (4482 ± 69 kPa) and closes at 500 ± 15
psig (3447 ± 103 kPa).
PRESSURE TRANSDUCERS — Each refrigerant circuit is
equipped with a suction and discharge pressure transducer. The
suction pressure transducers have a yellow body with a pres-
sure range of –6.7 to 420 psig (–46 to 2896 kPa) while the dis-
charge transducers have a red body with a pressure range of
14.5 to 667 psig (100 to 4599 kPa). These inputs connect to the
MBB (main base board) and are used to monitor the status of
the unit and to ensure the unit operates within the compressor
envelope. The transducers are used to protect the compressor
from operating at too low or too high of a pressure condition. In
some cases, the unit may not be able to run at full capacity. The
MBB will automatically reduce the capacity of a circuit as
needed to maintain specified maximum/minimum operating
pressures. Table 8 summarizes pressure transducer characteris-
tics.
Table 8 — Pressure Transducer Identification
THERMISTORS — The electronic control uses 3 to 7 therm-
istors to sense temperatures for controlling unit operation. See
Table 9. These sensors are outlined in the following sections.
Three different thermistor curves are utilized depending on the
thermistor and the configuration of the input. The three differ-
ent types are 5,000
:
at 77 F (25 C); 10,000
:
at 77 F (25 C);
and 86,000
:
at 77 F (25 C).
Return Air Temperature (RAT) — A return air temperature
sensor (Part No. 33ZCSENSAT) is required for certain control
types discussed in the section Capacity Control, beginning on
page 64. The sensor is field-installed in the indoor unit and
wired to the low voltage terminal (LVT) of the unit to measure
the air temperature entering the evaporator coil. Sensor wiring
should not be in conduit with other control voltages, or errone-
ous or erratic readings may result. The sensor should be
located directly in front of the evaporator coil after an outside
air intake. See Fig. 24.
The RAT sensor consists of a thermistor encased within a
stainless steel probe. See Fig. 25. The sensor probe is 6 in.
nominal length with 114 in. (2.9 m) of unshielded, 2-conductor,
18 AWG twisted-pair cables. The sensor temperature range is
–40 to 245 F (–40 to 118 C) with a nominal resistance of
10,000 ohms at 77 F (25 C). The sensor has an accuracy of
±0.36 F ( ±0.2 C). Refer to the 38AP Installation Instructions
for information on wiring.
In lieu of wiring a sensor to the 38AP controls, the MAT/
RAT sensor reading can be broadcast to the unit. It is recom-
mended that the broadcast frequency be at least once every 30
seconds. If a broadcast is not received for 3 minutes a thermis-
tor failure alert will be generated. The broadcast should write to
the CCN point, RETURN_T. Even though the MAT/RAT tem-
perature is being broadcast, a thermistor type
Configuration
o
OPT1
o
RAT.T
(RAT Thermistor Type) must be set to
0
(5,000
:
) or
1
(10,000
:
).
CB1
REMOTE
CONTROL
ENABLE
SCROLLING MARQUEE
DISPLAY
ENABLE/OFF/REMOTE
CONTACT SWITCH
EMERGENCY
ON-OFF SWITCH
SW1
OFF
OFF
ON
SW2
CB2
CB3
Fig. 23 — Scrolling Marquee, Enable/Off/Remote Contact Switch, and Emergency On/Off Switch Locations
LEGEND
CB
—
Circuit Breaker
SW —
Switch
TRANSDUCER
CARRIER
PART NUMBER
BODY
COLOR
PRESSURE
RANGE,
psi (kPa)
Discharge
HK05ZZ001
Red
14.5 to 667
(100 to 4599)
Suction
HK05SZ003
Yellow
–6.7 to 420
(–46 to 2896)
