JOHNSON CONTROLS
236
FORM 201.23-NM2
ISSUE DATE: 09/25/2020
SECTION 8 - MICROPANEL
High Discharge Pressure Cutout (HPCO)
(Hardware) Fault
The mechanical High Pressure Cutout protects the
system from experiencing dangerously high discharge
pressure. A system will fault and shut down immedi-
ately when the mechanical high pressure cutout con-
tacts open. The fault will occur immediately and not
wait 3 seconds, which is typical of most system faults.
The HPCO is wired in series with the VSD Run Signal
and will only be checked by the Chiller Control Board
when the system is running.
The mechanical cutout opens at 315 psig plus or minus
8 psig and closes at 230 psig plus or minus 10 psig. The
Status display fault message for this system is shown
below:
SYS X YYYYYYYY HPCO FAULT
X indicates the system and YYYYYYY indicates the
system is in a “FAULT” condition and will restart when
the 120 second anti-recycle timer times out or “LOCK-
OUT” and will not restart until the operator clears the
fault using the keypad.
Low Suction Pressure Cutout (Software) Fault
The programmable Low Suction Pressure Cutout is
a secondary back-up for the flow switch and protects
against operation with low refrigerant charge, which
helps protect the chiller from an evaporator freeze-up,
should the system attempt to run with a low refrigerant
charge. The Status display fault message for this cut-
out is shown below:
SYS X YYYYYYYY LOW SUCTION PRESSURE
X indicates the system and YYYYYYY indicates the
system is in a “FAULT” condition and will restart when
the 120 second anti-recycle timer times out or “LOCK-
OUT” and will not restart until the operator clears the
fault using the keypad. Typically, the cutout will be set
at 24 psig for chilled water applications.
The cutout is ignored for the first 30 seconds of system
run time. During the next 3 minutes of run time the
cutout point is linearly ramped from 10% of the cutout
value up to the programmed cutout point. If at any time
during the first 3 minutes of operation the suction pres-
sure falls below the ramped cutout point, the system
will shut down with a controlled ramped shutdown.
The cutout pressure during operating periods of 30 sec-
onds to 210 seconds is ramped and can be calculated
by:
Cutout = (Programmed Cutout x Run Time)–1.2 psig
200
After the first 3 minutes and 30 seconds of run time, if
the suction pressure falls below the cutout as a result
of a transient in the system, a transient timer is set at
30 seconds and a linearly ramped cutout is set start-
ing at 10% of the programmed cutout. If over the next
30 seconds, the suction pressure does not stay above
the ramped cutout, which ramps between 10% of the
cutout and the programmed cutout over the 30 second
period, the system will fault on low suction pressure.
Low Motor Current Cutout Fault
The Motor Current Cutout shuts the system down with
a controlled ramped shutdown when the microproces-
sor detects the absence of motor current (less than 10%
FLA), usually indicating that a compressor is not run-
ning. This safety is ignored for the first 10 seconds of
operation.
The status display fault message for this safety is
shown below:
SYS X YYYYYYYY LOW MOTOR CURRENT
X indicates the system and YYYYYYY indicates the
system is in a “FAULT” condition and will restart when
the 120 second anti-recycle timer times out or “LOCK-
OUT” and will not restart until the operator clears the
fault using the keypad.
High Differential Oil Pressure Cutout Fault
The High Differential Oil Pressure Cutout protects the
compressor from low oil flow and insufficient lubri-
cation, possibly from a dirty oil filter. A system will
fault and shut down with a controlled ramped shut-
down when its Discharge to Oil Differential Pressure
rises above the cutout of 65 psid. This safety is ignored
for the first 90 seconds of run time. This safety mea-
sures the pressure differential between discharge and
oil pressure, which is the pressure drop across the oil
filter. The Status display fault message for this safety
is shown below:
SYS X YYYYYYYY HIGH DIFF OIL PRESSURE
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