The low-pressure cutout switch is also known as a suction pressure control switch. It is the control that causes the compressor to go on or off as required for normal operation of the refrigeration plant. This switch is connected to the suction side of the compressor and is actuated by pressure changes in the suction line. When the solenoid valves in the lines to the various evaporators are closed so that the flow of refrigerant to the evaporators is stopped, the pressure of the vapor in the compressor suction line drops quickly. When the suction pressure has dropped to the set pressure, the low-pressure cutout switch causes the compressor motor to stop. When the temperature in the refrigerated spaces has risen enough to operate one or more of the solenoid valves, refrigerant is again admitted to the cooling coils, and the compressor suction pressure builds up again. At the desired pressure, the low-pressure cutout switch closes, starting the compressor again and repeating the cycle.

HIGH-PRESSURE CUTOUT SWITCH

A high-pressure cutout switch is connected to the compressor discharge line to protect the high-pressure side of the system against excessive pressure. The design of this switch is essentially the same as that of the low-pressure cutout switch. However, the low-pressure cutout switch is made to CLOSE when the suction pressure reaches its upper normal limit. The high-pressure cutout switch is made to OPEN when the discharge pressure is too high. As mentioned before, the low-pressure cutout switch is the compressor control for normal operation of the plant. The high-pressure cutout switch, on the other hand, is a safety device only and does not have control of compressor operation under normal conditions.

OIL FAILURE SWITCH

An oil failure switch is provided with high-speed compressors. This differential pressure switch is designed to prevent operation of the compressor in the event of low oil pressure. The switch has one bellows connected to the discharge oil line of the compressor oil pump and the other connected to the compressor crankcase suction refrigeration pressure. The switch is set to open the electrical circuit and to stop the compressor when the oil pressure drops to a low-pressure set point. The switch closes the electrical circuit and starts the compressor when the oil pressure reaches the reset set point.

To start the compressor after it has been stopped and the contacts of the oil failure switch have opened, a time delay mechanism works in conjunction with the compressor motor controller. The time delay switch should open 10 to 30 seconds after the compressor motor has started. The oil pressure will normally build up within this time interval. The oil pressure switch will have made contact to keep the compressor motor electrical circuit energized after the time delay switch opens. If the oil pressure has not built up within about 30 seconds after the compressor is started, the contacts of the oil pressure differential switch will not have closed. The compressor will stop because the time delay relay switch is open.