A switch in the high pressure line disengages the compressor if the pressure in the high-pressure side of the system (between the compressor and the inlet to the expansion valve) gets either too high or too low. Abnormally high pressures can be caused by blockages in the system, malfunctioning expansion valve, condenser fan not operating or too much refrigerant in the system. Abnormally low pressure almost always is the result of too little refrigerant in the system.
Also, a high-side pressure switch actuates the cooling fan if the high-side pressure rises above a specified point.