Contact Performance in Relays


5.4 Contact Activation

In atmospheres containing certain hydrocarbons, contact arcing can cause carbon deposits on contact surfaces increasing the duration of subsequent arcs with a resultant rapid increase in contact erosion. The phenomenon is referred to as "contact activation". Gold, silver, platinum, palladium, and copper are subject to activation in varying degrees; however, it does not occur on surfaces of tungsten, nickel, or molybdenum. Activation lowers the electrical field at which an arc will start and lowers the minimum arc current thus increasing the duration of the arc causing a significant increase in the erosion rate. Arcing on activated contacts produces a relatively uniform erosion over the contact surface rather than the pip and crater normally observed in non-activated arcing contact along with a black powder (carbon particles) over the contact surface and adjacent parts. Active arcs can result in a contact life of 1/4 to 1/8 that of non-active arcing.
Sources of the hydrocarbon vapors that cause contact activation are certain contact cleansers, paint and exhaust fumes, lotions and certain aerosol sprays, certain plasticizers, phenolic plastics, some magnet wire insulations, organic lubricants, etc. Erosion due to activation is a greater problem in sealed relays with internal hydrocarbon sources. The presence of air (as is the case with open or dust covered relays) inhibits contact activation by reducing the number of hydrocarbon molecules absorbed by the contact surfaces. Low duty cycles (long "off" times) allowing the absorption and formation of hydrocarbon based films, and intermediate current switching (0.1 to 0.5 A) providing enough energy to turn the film into carbon but not enough to burn off the carbon are the conditions most favorable to contact activation.