JOHNSON CONTROLS
103
SECTION 7 – UNIT CONTROLS
FORM 150.27-NM1
ISSUE DATE: 09/30/2015
7
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 suc-
tion pressure fault will cause evaporator
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 these 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.
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 cutout
over the next 30 seconds. If at any time during these
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.
S Y S 1 P U M P I N G D O W N
S Y S 2 P U M P I N G D O W N
The PUMPING DOWN message indicates that a com-
pressor in the respective system is presently in the pro-
cess of pumping the system down. When pumpdown is
initiated on shutdown, the liquid line solenoid or EEV
will close and a compressor will continue to run. When
the suction pressure decreases to the suction pressure
cutout setpoint or runs for 180 seconds, whichever
comes first, the compressor will cycle off.
Fault Safety Status Messages
Safety Status messages appear when safety thresholds
in the unit have been exceeded. Safeties are divided
into two categories – system safeties and unit safeties.
System safeties are faults that cause the individual sys-
tem to be shut down. Unit safeties are faults that cause
all running compressors to be shut down. Following
are display messages and explanations.
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 sys-
tem will be locked out on the last fault. This condi-
tion is then a manual reset. The system switch (under
OPTIONS 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.
