Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 128

P120 (Circuit A Low Saturated Suction Temp, Comp Shutdown)
T120 (Circuit A Low Saturated Suction Temperature Alert)
A120 (Circuit A Low Saturated Suction Temperature Alarm)
P121 (Circuit B Low Saturated Suction Temp, Comp Shutdown)
T121 (Circuit B Low Saturated Suction Temperature Alert)
A121 (Circuit B Low Saturated Suction Temperature Alarm)
This alert/alarm is used to keep the evaporator coils from freez-
ing and the saturated suction temperature above the low limit
for the compressors.
When Temperatures  REF.T  SST.A
 REF.T  SST.B is less than 20°F for 4 minutes, less than 10°F
for 2 minutes, less than 0°F for 1 minute, or less than –20°F for
20 seconds continuously, a compressor of the affected circuit will
be shut down with a local pre-alert (P120, P121) and a 15-minute
timeguard will be added to the compressor. If the saturated suction
temperature continues to be less than 20°F for 4 minutes, less than
10°F for 2 minutes, less than 0°F for 1 minute, or less than –20°F
for 20 seconds continuously, another compressor of the affected
circuit, if it exists, will be shut down with a local pre-alert (P120,
P121) and a 15-minute timeguard will be added to the compressor.
This sequence will continue until the last compressor on the circuit
is shut down, at which time the circuit will be shut down with alert
(T120, T121).
This failure follows a 3-strike methodology. When the circuit is
shut down entirely, an alert (T120, T121) is generated and a
strike is logged on the circuit. The Alert and strikes logged will
automatically reset if the saturated suction temperature remains
above 29.4°F for 15 minutes. On the third strike, alarm (A120,
A121) will be generated which will necessitate a manual reset
to get the circuit back running. It is important to note that a
strike is called out only if all compressors in the circuit are off
at the time of alert.
To prevent nuisance alerts, P120 and P121 show up in the
alarm history and locally at the display, but are never broadcast
to the network. To recover from these pre-alerts, both a 15-
minute hold off timer and saturated suction temperature rising
above 29.4°F must occur. If recovery occurs, staging will be al-
lowed on the circuit once again. A strike is tied to the circuit
going off entirely, not reducing capacity and recovering. There-
fore, it is possible that multiple P120 and P121 alerts may be
stored in alarm history but not broadcast.
T122 (Circuit A High Saturated Suction Temperature)
T123 (Circuit B High Saturated Suction Temperature)
This alert is for display purposes only. No action is taken by
the control when the alert occurs.
When Temperatures  REF.T  SST.A
 REF.T  SST.B is greater than 70°F for Configuration
 COOL  H.SST minutes, local alert T122 or T123 will occur.
The alerts automatically reset when the corresponding saturated
suction temperature drops below 70°F.
P126 (Circuit A High Head Pressure, Comp Shutdown)
T126 (Circuit A High Head Pressure Alert)
A126 (Circuit A High Head Pressure Alarm)
P127 (Circuit B High Head Pressure, Comp Shutdown)
T127 (Circuit B High Head Pressure Alert)
A127 (Circuit B High Head Pressure Alarm)
This alert/alarm is used to keep the saturated condensing tem-
perature below the compressor operating envelope outlined in
Fig. 17. This alert/alarm also attempts to prevent the saturated
condensing temperature from reaching the high pressure
switch trip point by reducing the upper horizontal portion of
the compressor operating envelope to a level slightly below the
saturated condensing temperature recorded upon a high pres-
sure switch trip (T057, T058).
When Temperatures  REF.T  SCT.A
 REF.T  SCT.B rise above the compressor operating enve-
lope for the corresponding Temperatures  REF.T  SST.A or
Temperatures  REF.T  SST.B, a compressor of the affected
circuit will be immediately shut down with pre-alert (P126,
P127) and a 10-minute timeguard will be added to the com-
pressor. If the saturated condensing temperature remains above
the envelope for 5 more seconds, another compressor of the af-
fected circuit, if it exists, will be shut down with pre-alert
(P126, P127) and a 10-minute timeguard will be added to the
compressor. This sequence will continue until the last com-
pressor on the circuit is shut down, at which time the circuit
will be shut down with alert (T126, T127).
This failure follows a 3-strike methodology. When the circuit is
shut down entirely, an alert (T126, T127) is generated and a
or Temperatures
strike is logged on the circuit. On the third strike, alarm (A126,
A127) will be generated which will necessitate a manual reset
to get the circuit back running. It is important to note that a
strike is called out only if all compressors in the circuit are off
at the time of the alert.
To prevent nuisance alerts, P126 and P127 show up in the
alarm history and locally at the display, but are never broadcast
to the network. To recover from these alerts, both a 10-minute
hold off timer and saturated condensing temperature returning
under the compressor envelope must occur. If recovery occurs,
staging will be allowed on the circuit once again. Again, a
strike is tied to the circuit going off entirely, not reducing ca-
pacity and recovering. Therefore it is possible that multiple
P126 and P127 alerts may be stored in alarm history but not
broadcast.
If an excessive number of head pressure Pre Alerts, Alerts, or
Alarms are occurring and the causes identified in Table 72
(Cooling Service Analysis) have not been effective in correct-
ing the situation, it may be necessary to raise the EXV mini-
mum position (EX.MN) from its default setting of 20%. Note
that raising this configuration may result in an increase in the
number of flooding alerts.
T128 (Digital Scroll A1 High Discharge Temperature Alert)
A128 (Digital Scroll A1 High Discharge Temperature Alarm)
This alert/alarm is for units with a digital scroll compressor on-
ly. The digital scroll compressor is equipped with a tempera-
ture thermistor that is attached to the discharge line of the com-
pressor.
The alert occurs when the discharge temperature thermistor has
measured a temperature above 268°F or the thermistor is short
circuited. The digital scroll compressor will be shut down and
alert T128 will be generated. The compressor will be allowed
to restart after a 30-minute delay and after the thermistor tem-
perature is below 250°F.
If 5 high discharge temperature alerts have occurred within 4
or Temperatures hours, alarm A128 will be generated which will necessitate a
manual reset to start the compressor.
A140 (Reverse Rotation Detected)
This alarm performs a check for correct compressor rotation
upon power up of the unit. The method for detecting correct ro-
tation is based on the assumption that there will be a drop in
suction pressure upon a compressor start if the compressor is
rotating in the correct direction.
A test is made once, on power up, for suction pressure change
on the first compressor of the first circuit to start.
Reverse rotation is determined by measuring suction pressure
at 3 points in time:
•
•
•
The rate of suction pressure change from 5 seconds prior to
compressor start to compressor start (rate prior) is compared to
the rate of suction pressure change from compressor start to 5
or Temperatures
seconds after compressor start (rate after).
If (rate after) is less than (rate prior minus 1.25), alarm A140 is
generated.
128
5 seconds prior to compressor start.
At the instant the compressor starts.
5 seconds after the compressor starts.