157
JOHNSON CONTROLS
SECTION 7 – UNIT CONTROLS
FORM 150.67-NM1
ISSUE DATE: 4/28/2017
7
After the first 3 minutes, if the suction pressure falls
below the programmed cutout setting, a “transient
protection routine” is activated. This sets the cutout at
10% of the programmed value and ramps up the cut-
out over the next 30 seconds. If at any time during this
30 seconds the suction pressure falls below the ramped
cutout, the system will stop.
S Y S 1 M P / H P C O F A U L T
S Y S 2 M P / H P C O F A U L T
S Y S 1 M P / H P C O I N H I B
S Y S 2 M P / H P C O I N H I B
The Motor Protector/Mechanical High Pressure Cutout
protects the compressor motor from overheating or the
system from experiencing dangerously high discharge
pressure.
This fault condition is present when CR1 (SYS 1) or
CR2 (SYS 2) relays de-energize due to the HP switch
or motor protector opening. This causes the respective
CR contacts to open causing 0VDC to be read on the
inputs to the microboard. The fault condition is cleared
when a 30VDC signal is restored to the input.
The internal motor protector opens at 185 °F to 248
°F (85 °C to 120 °C) and auto resets. The mechanical
HP switch opens at 585 PSIG plus or minus 10 PSIG
(27.92 barg plus or minus .69 barg) and closes at 330
PSIG plus or minus 25 PSIG (22.75 barg plus or minus
1.72 barg).
The compressor is also equipped with a discharge
temperature sensor for the purpose of sensing inter-
nal scroll temperature. This sensor protects the scrolls
from overheating due to inadequate cooling that may
occur when refrigerant charge is low, or superheat is
too high.
When the sensor senses a high temperature, it opens
the motor protector circuit in the compressor causing
the compressor to shut down.
During the first two faults an MP/HP INHIBIT message
will be displayed and the system will not be locked out.
Only after the third fault in 90 minutes will the MP/
HPCO FAULT message be displayed.
System Safeties
System safeties are faults that cause individual systems
to be shut down if a safety threshold is exceeded for 3
seconds. They are auto reset faults in that the system
will be allowed to restart automatically after the fault
condition is no longer present. However, if 3 faults on
the same system occur within 90 minutes, that system
will be locked out on the last fault. This condition is
then a manual reset. The system switch (under OP-
TIONS key) must be turned OFF and then back ON
to clear the lockout fault. Fault messages will be dis-
played whenever a system is locked out.
S Y S 1 H I G H D S C H P R E S
S Y S 2 H I G H D S C H P R E S
The Discharge Pressure Cutout is a software cutout in
the microprocessor and is backed-up by a mechanical
high pressure cutout switch located in the refrigerant
circuit. It assures that the system pressure does not ex-
ceed safe working limits. The system will shutdown
when the programmable cutout is exceeded and will be
allowed to restart when the discharge pressure falls 40
PSIG below the cutout. Discharge transducers must be
installed for this function to operate.
S Y S 1 L O W S U C T P R E S S
S Y S 2 L O W S U C T P R E S S
The Suction Pressure Cutout is a software cutout that
helps protect the chiller from an evaporator freeze-up
should the system attempt to run with a low refrigerant
charge or a restriction in the refrigerant circuit.
Repeated starts after resetting a low
suction pressure fault will cause evapo-
rator freeze-up. Whenever a system
locks out on this safety or any safety,
immediate steps should be taken to
identify the cause.
At system start, the cutout is set to 10% of programmed
value. During the next 3 minutes the cutout point is
ramped up to the programmed cutout point. If at any
time during this 3 minutes the suction pressure falls
below the ramped cutout point, the system will stop.
This cutout is completely ignored for the first 30 sec-
onds of system run time to avoid nuisance shutdowns,
especially on units that utilize a low pressure switch in
place of the suction pressure transducer.
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