A pressure switch is an electrical switch operated by pressure. The low-pressure (LP) switch is connected to the suction line: when the pressure falls below the setting it stops the compressor, when it rises it starts it. In pump-down systems this is what normally stops the compressor; in a refrigerant leak, as suction pressure falls, it protects the compressor from running in a vacuum.
The high-pressure (HP) switch is on the discharge line and is a safety device: when the condenser gets dirty, a fan fails or air flow is blocked, condensing pressure rises; the switch stops the compressor at the set value (20–28 bar depending on refrigerant). Most models have a manual reset; the system must not be restarted until the cause is removed. Combined (LP/HP) switches bring both functions together in one body.
Settings are determined by refrigerant and application; with R404A for a 0 °C room, for example, an LP setting of cut-out at about 1.5 bar and cut-in at 3 bar. Electronic controllers do the same job with a pressure transmitter and pass the values to remote monitoring; the mechanical switch nevertheless stays as an independent final safeguard. Our refrigerant pressure–temperature chart shows which pressure corresponds to which temperature.