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temperature on a system has dropped to 29°F (-2°C)
and that any further temperature reduction could cause
some icing of the evaporator tubes. Saturated suction
temperature is computed by the micro by converting
suction pressure to temperature.
For the first 3 minutes that the saturated suction tempera-
ture is at or below 29°F (-2°C) any further compressor
loading is inhibited to allow time for the temperature to
rise. If the condition persists for more than 3 minutes, a 5
minute timer is started. As this timer counts down to zero,
a 1 second unload pulse will be sent to the slide valve of
the affected compressor every 5 seconds as long as the
temperature is below 31°F (-1°C). If the temperature
rises above 31°F (-1°C), the micro will inhibit loading
for the remainder of the 5 minute period.
If after the 5 minute period the saturated suction tem-
perature is above 29°F (-2°C), the compressor is allowed
to reload, if required, to maintain leaving chilled water.
Otherwise, the micro will reset the 5 minute timer and
start the process over again. To assure that leaving
chilled water requirements are satisfied while one com-
pressor is under Suction Temperature Limiting control,
the micro will start or load other compressor(s) as
necessary.
2.5
UNIT FAULT STATUS MESSAGES
A Unit Fault will shut the entire chiller down when a
preset safety threshold is exceeded. The chiller will
automatically restart after the condition causing the
shutdown clears. Restart will occur only after anti-re-
cycle timers are satisfied and cooling demand requires
additional cooling. A reset hysteresis is built into each
safety so repetitive faulting and clearing will not occur
in a short time period.
Continuous monitoring by the microprocessor assures
that instantaneous reactions result. When the chiller
is shut down on one of these safeties, a message will
appear on the Status display informing the operator of
the problem as shown in the text that follows.
Any time that a Unit Fault occurs, the shutdown will be
logged into the HISTORY BUFFER.
Low Ambient Temperature Cutout:
U N I T
L O W
A M B I E N T
The Low Ambient Temperature Safety protects the
chiller from running in very low temperatures which
JOHNSON CONTROLS
F A U L T
T E M P
could cause damage due to low system pressures. This
feature is programmable and can also be used to shut
down the chiller at a temperature where continued run-
ning of the chiller is not economical compared to the
use of "free" cooling techniques (see also Section 8.2
/ Low Ambient Temperature Cutout [page 162]). The
fault will clear when ambient temperature rises 2°F
(1°C) above the cut-out.
High Ambient Temperature Cutout:
U N I T
H I G H
A M B I E N T
The High Ambient Temperature Safety protects the
chiller from running in ambients above 130°F (54°C)
where potential malfunction of system mechanical and
electrical components may result. The High Ambient
Cutout is programmable and can be set for lower limit
values if required (see also Section 8.2 / High Ambient
Temperature Cutout [page 162]). The fault will clear
when ambient temperature drops 2°F (1°C) below the
cut-out.
Low Leaving Chilled Liquid Temperature Cutout:
U N I T
L O W
L I Q U I D
The Low Water Temperature Safety assures that the
evaporator is not damaged from freezing due to im-
properly set control points. It also attempts to protect
the chiller from freezing, if the flow switch should fail.
Whenever the chilled liquid temperature drops below
the programmable cutout, the chiller will shut down (see
also Section 8.2 / Leaving Water Temperature Cutout,
page 162). The chiller fault will clear when temperature
rises 4°F (2°C) above the cut-out and cooling demand
exists.
115VAC Under Voltage Cut-Out:
U N I T
1 1 5 V A C
U N D E R
The Under Voltage Safety assures that the system is
not operated at voltages where malfunction of the mi-
croprocessor could result in system damage. Whenever
the microprocessor senses an on-board control power
supply failure while a compressor is running, the chiller
is shut down. The microprocessor circuitry is capa-
ble of operating at voltages 10% below the nominal
115VAC supply to the panel. Auto-restart of the chiller
occurs after a 2 minute start-up timer has elapsed from
the time when power is reapplied, if the AUTO RE-
START ON POWER FAILURE is enabled. Otherwise
FORM 201.18-NM7
F A U L T
T E M P
F A U L T
T E M P
F A U L T
V O L T A G E
137
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