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 TM 55-1905-219-14-8
4-28. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
l. High and Low Pressure Control Switches.
(1)
The safety and operating functions of the high and low pressure control switches are given in
paragraph 4-28a(13).
(2)
The high pressure switch is actuated by discharge pressure and the low pressure switch by
suction pressure.
(3)
When the discharge pressure rises above a safe limit (cut-out setting), the high pressure switch
contacts open, the electrical circuit is interrupted, and the compressor stops. The contacts reclose
automatically when the discharge pressure drops to a safe level (cut-in setting).
(4)
When the suction pressure has been pumped down to the desired level (cut-out setting), the low
pressure switch contacts open and stop the compressor. When the pressure rises (cut-in setting), the switch
contacts close, the electrical circuit is completed, and the compressor starts.
(5)
Operation.
(a)
Separate high and low pressure control switches, similar in construction and operation, are
furnished.
(b) Pressure applied at the switch connection actuates a seamless metallic bellows power element which
operates the switch mechanism to open or close the electrical circuit. A permanent magnet in the switch
mechanism provides a positive snap-action on both the opening and closing cycles, preventing excessive
arcing at the contacts.
(6)
Adjustment.
(a)
The switch operating range and differential are both adjustable.
(b) The range adjustment screw located on the right side of the switch, outside the case, changes
both the cut-in and cut-out points by an equal amount. Turning the range screw clockwise raises both set points
and turning it counterclockwise lowers both set points.
4-859
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