HVAC Pressure Controls, Switches: One safety feature for air-conditioning units with a compressor and condenser is a pressure-controlled switch. This switch is wired into the circuit to protect the system in case the system develops a leak. If a leak develops, it is possible to draw in moisture and air and damage the whole system. If the pressure builds too high, it can cause a rupture of any of the joints or weaker points in the system.
A low-voltage (24-V) relay is wired into the 240-V line that supplies the compressor motor. The relay contacts are wired into the supply line for the motor . The solenoid of the relay is wired in series with two pressure-operated switches. If the pressure builds too high, the high switch will open and cause the solenoid to de-energize. If this happens, it causes the contacts of the relay to open. This removes power from the compressor motor. If the low-pressure switch opens, it will do the same thing. This way the compressor is protected from both high- and low-pressure causing damage to the system.