JOHNSON CONTROLS
110
FORM 150.72-ICOM7
ISSUE DATE: 10/22/2020
SECTION 7 – UNIT CONTROLS
This safety will shut down a system if either suction
temperature or suction pressure sensors read out of
range high or low. This condition must be present for
3 s to cause a system shutdown. The safety locks out a
system after the first fault and will not allow automatic
restarting.
Unit Safeties
Unit safeties are faults that cause all running compres-
sors to be shut down. Unit faults are auto reset faults
in that the unit will be allowed to restart automatically
after the fault condition is no longer present.
U N I T F A U L T :
L O W A M B I E N T T E M P
The Low Ambient Temp Cutout is a safety shutdown
designed to protect the chiller from operating in a low
ambient condition. If the outdoor ambient temperature
falls below the programmable cutout, the chiller will
shut down. Restart can occur when temperature rises
2°F above the cutoff.
U N I T F A U L T :
L O W L I Q U I D T E M P
The Low Leaving Chilled Liquid Temp Cutout protects
the chiller form an evaporator freeze-up should the
chilled liquid temperature drop below the freeze point.
This situation could occur under low flow conditions or
if the micro panel set point values are improperly pro-
grammed. Anytime the leaving chilled liquid tempera-
ture (water or glycol) drops below the cutout point, the
chiller will shutdown. Restart can occur when chilled
liquid temperature rises 2°F above the cutout.
U N I T F A U L T :
1 1 5 V A C U N D E R V O L T A G E
The Under Voltage Safety ensures that the system is
not operated at voltages where malfunction of the mi-
croprocessor could result in system damage. When
the 115 VAC to the micro panel drops below a certain
level, a unit fault is initiated to safely shut down the
unit. Restart is allowed after the unit is fully powered
again and the anti-recycle timers have finished count-
ing down.
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 min will the MP/HPCO
FAULT message be displayed.
Whenever the motor protector or discharge sensor
shuts down a compressor and the system, the internal
compressor contacts will open for a period of 30 min to
ensure that the motor or scroll temperatures have time
to dissipate the heat and cool down. The MP/HP IN-
HIBIT message
will be displayed while these con-
tacts are open or when the HPCO is open. While
this message is displayed, the compressors will
not be permitted to start.
After 30 min, the contacts will close and the system
will be permitted to restart. The microprocessor will
not try to restart the compressors in a system that shuts
down on this safety for a period of 30 min to allow the
internal compressor to time out.
During the 30 min timeout, the MP/HPCO INHIB mes-
sage will be displayed. The MP/HPCO fault will only
be displayed after 3 shutdowns in 90 min, indicating
the system is locked out and will not restart.
S Y S 1 H I G H M T R C U R R
S Y S 2 H I G H M T R C U R R
When the System Current Feedback option is installed
and selected (Option 11 under OPTIONS key Current
Feedback), this safety will operate as follows. If the
actual feedback voltage of the system proportional to
currents exceeds the programmed trip voltage for 5
seconds, the system will shutdown.
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