Priniciples of Electromechanical Relay Operation


3.14 Contact Characteristics

Contact characteristics that affect switching performance are:
1) Electrical conductivity
2) Thermal conductivity
3) Hardness, limit of elasticity: Young's Modulus
4) Resistance to are erosion, welding or electrical sticking, cold welding, mechanical wear, oxidation, atmosphere contamination (chemically active).
5) Tendency to bounce on impact, gaseous absorption, catalytic polymerization of hydrocarbons, metal transfer at contact closure and arcing at opening.
Besides the physical and chemical properties of the metal, there are some geometrical and dynamic considerations:
1) Shape of contacts
2) Force between contacts
3) Amount of slide or "wipe"
4) Amount of rolling or twisting motion
5) Resiliency of the supporting structure and its tendency to enhance or inhibit bounce or chatter.

When contacts meet, the metal at the point of the contact deforms until the actual touching area supports the contact force and provides metal-to-metal contact unless some foreign material interferes. Deformation is at the point of contact, which can be either in the elastic or plastic modes. This is one of several factors that contribute to the amount of contact bounce. On a microscopic scale, many actual points of contact (often referred to as a-spots) form the electrical conductor and carry the current. The contact interface is also subject to mechanical abrasion and metal "galling" as it rubs, and "cold welding". The surface will absorb a monomolecular layer of volatile molecules in direction proportion to the molecular weight and concentration of the volatile material and the ambient pressure and inversely proportional to the temperature. (Water vapor is also a particularly common substance forming very thin absorbed layers).

Each metal has its own pertinent chemical properties. Silver and silver alloys, which have excellent electrical and thermal characteristics, tend to combine chemically with gaseous compounds of sulphur, the halogens (flourine, chlorine, bromine, and iodine), and silicones to form high resistance, usually hard coatings. Unlike other "noble" metals (gold, platinum, rhodium, iridium, palladium, and ruthenium, all of which are used in contacts), silver has no measurable catalytic effect (polymerization) in the sense of changing, under sliding pressure, the absorbed hydrocarbon molecules into some solid hydrocarbon material. Arcing, however, can accomplish the precipitation of solid carbon or carbonaceous products, usually in a ring around the actual point of contact.

Some more active metals, either pure or in alloys, find special areas of usefulness due to particular mechanical properties. Molybdenum, tungsten, nickel and mercury, for example, are used alone or as alloying or sintering ingredients. Cadmium oxide, tungsten carbide, tin, magnesium, and carbon are sometimes added to silver to inhibit sticking or welding particularly in high current relays or contactors. When contacts are surrounded by an inert gas, like nitrogen, consideration can be given to contact materials that could not be used in open style relays.