215
JOHNSON CONTROLS
FORM 201.21-NM4 (616)
215
JOHNSON CONTROLS
8
SYSTEM SAFETIES (FAULTS)(CON'T)
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 immediately
when the mechanical high pressure cutout contacts 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 297+/-8 PSIG
and closes at 230 +/-10 PSIG. The Status display fault
message for this system is shown below:
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 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. 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
-
SYS X YYYYYYYY LOW SUCTION PRESSURE
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
seconds 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 starting
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 microprocessor
detects the absence of motor current (<10%FLA), usu-
ally indicating that a compressor is not running. 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.
