SECTION 7 - OPERATION
JOHNSON CONTROLS
125
Form 201.28-NM1.1
Issue date: 08/06/2021
7
Unit safeties
Unit safety operation
Unit faults are safeties that cause all running compres-
sors to be shut down, if a safety threshold is exceeded for
3 seconds. Unit faults are recorded in the history buffer
along with all data on the unit and system operating con-
ditions. Unit faults are auto reset faults where the unit will
be allowed to restart automatically after the fault condi-
tion is no longer present. The only exception is any of the
VSD related unit faults. If any 3 VSD unit faults occur
within 90 minutes, the unit will be locked out on the last
fault. A VSD lockout condition requires a manual reset
using the system switches. Both system switches must be
cycled OFF and ON to clear a VSD unit lockout fault. If
a unit safety is in effect, the message will be displayed to
the operator when the STATUS key is pressed.
In the descriptions of the fault displays that follow, the
fault message will show a YYYYYYY to indicate that a
system is in a “FAULT” condition and will restart when
the fault clears or LOCKOUT” and will not restart until
the operator clears the fault using the keypad.
If a control panel safety occurs after the VSD fault, but
before the fault is reset, the control panel fault is an
ALL FAULT of the VSD fault, meaning it will be reg-
istered as such in the History because it occurred while
the VSD was shutting down or while the systems were
shut down. All faults do not store operating data at the
time of the fault.
If a “VSD” fault occurs during the fault rampdown or
while the systems are shut down, the VSD fault will
be registered as a new fault. The reason for this is the
belief any VSD fault should be registered with a full
account of the systems data at the time of the fault.
High ambient temp fault
If the ambient temperature rises above 55°C (131°F),
the chiller will shut down with a controlled ramped
shutdown. Restart will automatically occur, if demand
allows, when temperature falls 1.1°C (2°F) below
the cutout (53.9°C [129°F]). This fault cannot cause
a lockout. The fault display message will be present
only during the time when the ambient temperature is
causing a fault condition. The following is a sample
display:
UNIT YYYYYYYY
HIGH AMBIENT TEMP
The unit will also be inhibited from starting any time
the temperature is above 53.9°C (129°F).
Low ambient temp fault
If the ambient temperature falls below the program-
mable Low Ambient Temp Cutout the chiller will
shut down with a controlled ramped shutdown. This
fault will only occur if the Low Ambient Cutout is
“ENABLED” under the OPTIONS key. Restart can oc-
cur, if demand allows, when temperature rises 1.1°C
(2°F) above the cutout. This fault cannot cause a lock-
out. The fault display message will be present only dur-
ing the time when the ambient temperature is causing
a fault condition. The following is a sample display:
UNIT YYYYYYYY
LOW AMBIENT TEMP
The unit is also inhibited from starting any time the
temperature is below the cutout plus 1.1°C (2°F).
Low leaving chilled liquid temp fault
The Low Leaving Chilled Liquid Temp Cutout helps to
protect the chiller from an evaporator freeze-up should
the chilled liquid temp drop below the freeze point.
This situation could occur under low flow conditions
or if the Micro Panel setpoint values are improperly
programmed. Any time the leaving chilled liquid tem-
perature (water or brine) drops below the program-
mable cutout point, the chiller will fault and shutdown
with a controlled ramped shutdown. Restart can occur,
if demand allows, when chilled liquid temperature rises
2.2°C (4°F) above the cutout. This fault cannot cause a
lockout. The following is a sample shutdown message:
UNIT YYYYYYYY
LOW LEAVING CHILLED LIQUID TEMP
The unit is inhibited from starting any time the chilled
liquid temperature is below the cutout plus 2.2°C (4°F).
VSD communications failure fault
The VSD Communications Failure is to prevent the
unit from trying to run, if the Chiller Control Board
never initializes communications with the VSD Logic
Board. The unit will also shut down with a controlled
ramped shutdown if the Chiller Control Board loses
communications with the VSD Logic Board while the
chiller is operating.
On power-up, the Chiller Microprocessor Board will
attempt to initialize communications with the VSD
Logic Board. The control panel will request data from
the VSD, which includes the number of compressors
and the VSD software version. Once these data points
have been received by the Chiller Control Board,
