High-Pressure Cutout Switch
A high-pressure cutout switch is a pressure-actuated refrigerant controller connected to the pilot (low) voltage circuit of the main contactor in a refrigeration system (see Figure 6-68). These switches are installed in all refrigerating units over 1 hp to provide protection against dangerously high head pressures. These excessive and unsafe pressures develop as a result of a number of different abnormal operating conditions, including (1) air in the refrigerant lines, (2) excessive refrigerant charge, (3) dirty condenser, (4) faulty or inoperative condenser fan, and (5) insufficient water in a water- cooled condenser.
As shown in Figure 6-69, the high-pressure cutout switch is connected to the so-called high side of the system. When the head pressure exceeds the control setting, the high-pressure cutout switch opens the compressor power circuit and prevents excessive head pressures from building in the condenser by shutting off the com- pressor motor.
If automatic reset is used, the compressor motor will start again when the pressure returns to normal. More specifically, the contacts of the high-pressure cutout switch close automatically and reestablish the compressor power circuit when the head pressure drops the amount of control differential.
Automatic reset allows the equipment to cycle off the high- pressure control and continue to provide some degree of cooling while the high-pressure condition is being corrected. However, if the high-pressure condition is not corrected and the equipment continues to cycle over an extended period of time, damage may be caused to the motor or compressor. For obvious reasons, this can- not occur when a manual reset high-pressure cutout switch is used.