AC compressor not engaging

The system uses a pressure switch to activate the compressor clutch.It has to have at least 50 psi of pressure for the compressor to run.This is a measure of safety so the compressor will get proper oiling while charging the system.Don't by pass the pressure switch,before refrigerant is added.The pressure switch should be located on or near the reciever dryer,on the high pressure side,of the system.