Model 422HESP is a three-stage surge protector that protects against lightning strikes and other types of voltage disturbances with three stages of protection for each line: a gas discharge tube followed by a series resistor and finally a Transient Voltage Suppresser (TVS). The surge protector is housed in a tough metal, panel mount case with a protected signal ground connection and dedicated chassis ground lug.In order to work properly, it is important to have a good connection between the #10 screws and a solid earth ground. Two terminal posts and two metal mounting brackets provide a good ground connection.