Here is a list of commercially available electrical contact materials having application in electrical engineering.
1. Copper:
Because of low cost, high electrical and thermal conductivities, it is generally used as contact material. Its drawbacks are poor resistance to oxidation and easy formation of chemical compounds (e.g., sulphides). Its typical applications are control relays, motor starter switches, tap changers.
2. Silver, Silver Alloys, and Silver Heterogeneous Mixture:
Silver has high resistance to oxidation, has low contact resistance and good current carrying capacity. Silver alloys are: cadmium silver (Ag plus up to 20% Cd), Palladium-silver (Ag plus upto 10% Pd). Silver and silver alloys contacts are used upto 600 V, 50 amp D.C. and 200 amp A.C. They are used for all types of industrial relays, automatic voltage and current regulators, light switches, generator cut-outs, thermal overload devices, domestic appliances, limit switches, thermostatic controls etc.
The important silver-refractory heterogeneous mixtures consist of silver and various percentages of tungsten, tungsten or molybdenum. These are used for medium and heavy duty (high current) interrupting apparatus and the contacts operate in air. They are highly resistant to corrosion.
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Some compositions are- Molybdenum-silver (Ag plus 40-50% Mo), Tungsten-silver (Ag plus 40-75%W), Tungsten carbide-silver (Ag plus 40-60% tungsten carbide. They are operated at voltages upto 300,000 volts, and current upto 10,000 amp. They are used as arcing tips for air type multistage circuit breakers, light and medium duty circuit-breakers, arcing contacts for oil circuit- breakers, and as contacts for heavy duty cut rollers.
Silver-semi refractory heterogeneous mixtures include Cadmium oxide silver (Ag plus upto 15% CdO), Nickel silver (Ag plus 10-15% Ni) and other. They are used at voltages upto 550 V.A.C. and 220 V.D.C and upto 3,00,000 amp as current carrying contacts for multistage breaker-maximum normal operating current upto 300 amp. Such contacts are used for light duty breakers, heavy duty motor starters and commercial relays.
3. Copper-Tungsten Heterogeneous Mixtures:
Copper tungsten (Cu plus 55-70% W) contacts can operate upto 3,00,000 volts, and upto 10,000 amp. Their typical applications are oil immersed circuit-breakers, transformer tap changers, and arcing tips on certain air breakers.
4. Platinum and Its Alloys:
Platinum has high melting point and provides good resistance to electrical erosion. Pure platinum is soft electrical contact material, so is alloyed in various percentages upto 35%. Common alloy elements being iridium, ruthenium, and osmium.
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They are used for lightly loaded contacts, used upto 500 volts A.C. or D.C. and upto 25 amp. Examples are: magneto ignition system, thermostatic relays, sensitive relays, galvanometer contacts, recording instruments, railway signal equipment, vibrator contacts, relays for the chemical industry etc.
5. Tungsten:
The extremely high melting point of tungsten, coupled with high hardness and resistance to abrasion establish tungsten as an outstanding material for certain applications. They operate under not too severe arcing. They are used upto 200 V.A.C. and D.C. and upto 15 amp. Its typical applications are: battery ignition system, magneto ignition systems, vibrators, electric razors, automotive horn contacts.
The sliding contact material used for wire wound resistors of manganin, constantan, or chrome-nickel alloys are bronze-nickel sliders, or hard platinum alloys.