Pressure switch
Also known as: pressostat, low-pressure switch, high-pressure switch
Short definitionA switch that opens and closes a contact according to pressure. The low-pressure switch stops the compressor when suction pressure falls; the high-pressure switch cuts the system out when condensing pressure climbs to a dangerous level.
The low-pressure switch is connected to the suction line. It stops the compressor when pressure falls below the setting and restarts it when pressure rises. In pump-down systems this is how the compressor normally stops; in a refrigerant leak it also keeps the compressor from running in a vacuum. The high-pressure switch sits on the discharge line and is a safety device. Combined models put both in one body.
Settings are taken from the pressure-temperature chart for the refrigerant and room temperature. The high-pressure cut-out lies between 20 and 28 bar depending on the refrigerant. This switch trips when the condenser is dirty or a fan stops; most models are reset by hand, and the system is not restarted until the cause is fixed. An electronic controller can do the same job with a pressure transmitter, yet the mechanical switch stays in circuit as an independent last line of safety.