Contact Performance in Relays


5.2 Contact Characteristics

The contact characteristics that affect switching performance are: 1. Electrical conductivity.
2. Thermal conductivity.
3. Hardness, limit of elasticity: Young's Modulus.
4. Resistance to 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 on 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.
6. Elastic or plastic deformation at the point of contact, which contributes to contact bounce.
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. 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 "coldwelding". The surface will absorb a monomolecular layer of volatile molecules in direct 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 sulfur, the halogens (fluorine, 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 (see Contact Activation, 5.4).

Some more active metals, either pure or in alloys, find special area 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.

Due to the above contact characteristics, total contact resistance is defined as:
Rtotal = Rbulk + Rconstriction + Rcontamination & film
The bulk resistance is the basic DC resistance of the metals. The constriction resistance is that caused by the "a" spot, where the contact makes. The contamination value is caused by surface films and other contaminants and oxides.