Contact protection

Contact protection methods are designed to mitigate the wear and degradation occurring during the normal use of contacts within an electromechanical switch, relay or contactor and thus avoid an excessive increase in contact resistance or switch failure. A “contact” is a pair of electrodes (typically, one moving; one stationary) designed to control electricity. Electromechanical switches, relays, and contactors “turn power on” when the moving electrode makes contact with the stationary electrode to carry current. Conversely, they “turn power off” when the moving electrode breaks contact and the resulting arc plasma stops burning as the dielectric gap widens sufficiently to prevent current flow. Power relays and contactors have two primary life expectancy ratings: “mechanical life” is based on operating either without current or below the wetting current (i.e., “Dry”) and “electrical life” is based on operating above the wetting current (i.e., “Wet”). These different ratings are due to contacts being designed to compensate for the destructive arcing that naturally occurs between the electrodes during normal Wet operation. Contact arcing is so destructive that the electrical life of power relays and contactors is most often a fraction of their respective mechanical life. Every time the contacts of an electromechanical switch, relay or contactor are opened or closed, there is a certain amount of contact wear. If the contact is cycling without electricity (dry), the impact of the contact electrodes a slightly deformed by the resulting cold forging. When the contact is operating under power (wet), the sources of the wear are the result of high current densities in microscopic areas, and the electric arc. Contact wear includes material transfer between contacts, loss of contact material due to splattering and evaporation, and oxidation or corrosion of the contacts due to high temperatures and atmospheric influences. While a pair of contacts is closed, only a small part of the contacts are in intimate contact due to asperities and low-conductivity films.