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Standard Controls
High Pressure Switches
The high pressure switch (HP1-HP4) is a single pole pres-
sure activated device that opens on a pressure rise. When the
switch opens it de-energizes the compressor circuit, shutting
down the compressor. The MicroTech II controller will dis-
play an alarm condition. Once the cause of the fault has been
identified and corrected the unit may be manually reset
through the MicroTech II keypad/display interface. The high
pressure switch is located at the top of the compressor on the
discharge connection. To check the control, shut off water
flow to the condensers and observe the cutout point on a high
pressure gauge. The high pressure control should open at
360 psig and close at 300 psig. After testing the high pres-
sure control, check the pressure relief device for leaks.
Low Pressure Switches
The low pressure switch (LP1 -LP4) is a single pole pressure
activated device which closes on a pressure rise. It senses
evaporator pressure and is factory set to close at 60 psig and
open at 35 psig. Compressor operation is not allowed until
the switch closes. The low pressure switch is an automatic
reset control. If the condition occurs on any one compressor
three times in a 24-hour period, the alarm will have to be
manually reset through the MicroTech II keypad/display
interface to restart the compressor. The low pressure switch
is located at the bottom of the compressor on the suction
connection.
Compressor Motor Protector
All compressors are thermally protected. All 10 horsepower
and larger compressors use a solid state protection device
(MP1 - MP4) located in the compressor junction box. When-
ever the protection system opens the compressor is shut
down for a period of four minutes and an alarm indication is
made at the MicroTech II controller. All 5 horsepower com-
pressors have in-line protection. The control automatically
resets when the alarm condition is removed and the time
delay is satisfied. If the condition occurs on any one com-
pressor three times in a 24-hour period, the alarm will have
to be manually reset through the MicroTech II keypad/dis-
play interface to restart the compressor.
30
Proof of Airflow Switch
A positive proof of airflow switch (PC7) is provided with all
units. The switch is factory set to close at 0.2 inches of H
The switch has a field adjustable set point range of 0.17 to
5.0 inches of H
O. Turn adjustment screw clockwise to
2
decrease differential pressure setting. Turn adjustment screw
counterclockwise to increase differential pressure setting. In
a constant volume system, if the fan system is energized and
the minimum pressure setting of the switch has not been
reached, the unit will be shut down and a loss of airflow
alarm indicated at the MicroTech II controller. For variable
air volume units, the unit will shutdown due to loss of air-
flow only if the airflow switch is open and the duct static
pressure is less than half the duct static pressure setpoint.
Once the reason for the fault has been corrected, the unit can
be manually reset through the MicroTech II keypad/display
interface. PC7 is located in the coil section on the back side
of the control panel.
Frost Protection Switches
A frost protection switch (FP1 -FP4) is used on each refrig-
erant circuit to protect against evaporator coil freeze up. The
frost protection switches are normally closed and open on a
drop in temperature. When a frosting condition is sensed the
compressor circuit is shutdown until the condition has been
removed. The frost protection control is an automatic reset-
control. If the condition occurs on any one compressor three
times in a 24 hour period, the alarm will have to be manually
reset through the MicroTech II keypad/display interface to
restart the compressor. The MicroTech II control will indi-
cate a warning when a frost condition exists. The tempera-
ture sensors are located on a return bend for each refrigerant
circuit.
Clogged Filter Switch
A clogged filter switch (PC5) is provided to indicate when
the unit filters are to be changed. The switch is factory set to
close at 0.6 inches of H
O. The switch has a field adjustable
2
set point range of 0.17 to 5.0 inches of H
screw clockwise to decrease differential pressure setting.
Turn adjustment screw counterclockwise to increase differ-
ential pressure setting. When the filter pressure differential
exceeds the switch setpoint, a clogged filter indication is
made at the MicroTech II controller. The unit is allowed to
continue operation. PC5 is located in the coil section on the
back side of the control panel.
O.
2
O. Turn adjustment
2
IM709
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