York YCAV0157-0267 Installation Operation & Maintenance page 171

Chiller Run Contact
The Chiller Run dry contact is closed whenever any
system is running. It is open when all systems are shut
off.
Unit Switch
A double pole single throw ON/OFF rocker switch on
the front of the control panel is used to turn the entire
chiller on and off. When the switch is placed in the OFF
position, the entire unit shuts down immediately. One
pole of the UNIT Switch contacts is wired to the Sys
1/3 and the other to Sys 2/4 VSD Run Signal input and
the Chiller Control Board "Unit Switch X" digital input
(X = System 1 or 2). Separate System Fuses are also
wired in series with each set of UNIT Switch contacts.
If either fuse is pulled or blown, only the system with
the good fuse (Input is high) will run. When both inputs
are high, the entire chiller will be enabled to run. When
both inputs are low, the chiller will be disabled as a Unit
Switch OFF Shutdown.
JOHNSON CONTROLS
OPERATING CONTROLS (CON'T)
LD10605
UNIT
SWITCH
FORM 201.21-NM1 (616)
The Unit Switch should never be used
to shut down the chiller except in an
emergency. When the switch is thrown,
the compressors will immediately shut
down. Since the compressors are not
permitted to come to a controlled stop,
the rotors may back-spin, which may
result in some unusual compressor
noise. The back-spin will not hurt the
compressors, but should be avoided.
It is suggested that the System Switches
on the keypad be used whenever pos-
sible to turn a system off and allow the
compressor to complete a controlled
shutdown.
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