York YCAV0157 Installation Operation & Maintenance page 217

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 temperature (water or
brine) drops below the programmable cutout point, the
chiller will fault and shutdown with a controlled ramped
shutdown. Restart can occur, if demand allows, when
chilled liquid temperature rises 4°F above the cutout.
This fault cannot cause a lockout. A sample shutdown
message is shown below:
UNIT YYYYYYYY
LOW LEAVING CHILLED LIQUID TEMP
The unit is inhibited from starting any time the chilled
liquid temperature is below the cutout + 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, and
has been successfully initialized, the Chiller Control
Board will not request them again. If the comms con-
nection fails to occur, the Chiller Control Board will
prevent the chiller from operating and a fault message
will be displayed.
During normal operation, if the control panel Chiller
Control Board receives no valid response to messages
for 8 seconds, the unit will shut down all compressors on
a Comms fault. The Chiller Control Board will continue
to send messages to the VSD while faulted. The unit
will be inhibited from starting until communications is
established. The fault will automatically reset when the
JOHNSON CONTROLS
UNIT SAFETIES (CON'T)
Chiller Control Board receives a valid response from the
VSD for a data request. Shown below is an example of
a Comms Failure fault message:
UNIT YYYYYYYY
VSD COMMUNICATIONS FAILURE
FORM 201.21-NM1 (1223)
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