What Are Contacts?

Contacts are conductive interface elements that establish electrical continuity between two conductors by physical touch, allowing current to flow between circuit components, devices, or conductors that must occasionally be separated. They appear in switches, relays, connectors, semiconductor devices, and power equipment, and their reliability directly determines the reliability of the larger system in which they operate. The discipline of electrical contacts draws on materials science, tribology, and electromagnetic theory to characterize and improve the performance of these interfaces under mechanical, thermal, and environmental stress.

Contact Resistance and Ohmic Contacts

Contact resistance is the opposition to current flow at a conductive interface, distinct from the bulk resistance of the materials on either side. It arises from two sources: constriction resistance, caused by current crowding through the small true contact area (the a-spots defined by surface asperities), and film resistance, caused by oxide layers or contaminants on the contact surfaces. For most metal contacts, resistance is approximately proportional to the inverse square root of the contact force, as described by the Holm contact model. Ohmic contacts are a specific category: interfaces between a metal and a semiconductor in which the current-voltage relationship is linear rather than rectifying. Achieving an ohmic contact requires careful selection of metal work function relative to semiconductor band structure, and often requires annealing to promote interdiffusion and reduce barrier height. The IEEE Holm Conference on Electrical Contacts is the principal international forum for research on both bulk and semiconductor contact phenomena.

Electrical and Sliding Contacts

Electrical contacts in switches and relays must carry rated current for thousands or millions of operating cycles without failing open or welding shut. Silver and silver-based alloys dominate relay and switch contact materials because silver oxide, unlike copper oxide, is electrically conductive and dissociates at temperatures below those reached during normal arcing. Gold is preferred for low-current signal contacts because it forms no oxide at all, preserving contact resistance in the milliohm range over decades. Sliding contacts transmit current between a stationary conductor and a moving one, as in slip rings and commutators. Brush contact systems use a spring-loaded carbon or metal-graphite brush pressed against a rotating ring; the IEEE standards on rotating machinery address brush current density, pressure, and allowable wear rate. Tribological wear determines brush service life: the friction coefficient and wear rate depend on the combination of brush material, ring material, surface finish, and the humidity and temperature of the operating environment.

Brushes and Contact Materials

Brushes in electrical machines are composite blocks formulated from carbon, graphite, metal powder, or a combination, chosen to balance conductivity, hardness, and self-lubricating properties. Hard-carbon grades suit high-current DC machines; electrographitic grades suit traction motors subjected to wide temperature swings. Contact material selection for power switching must account for arc erosion: the energy deposited in an arc during opening can vaporize contact material and deposit it in the gap or on insulating surfaces. Tungsten-copper composites resist arc erosion in vacuum interrupters and high-voltage circuit breakers, while silver-cadmium oxide (now largely replaced by silver-tin oxide for environmental reasons) was the standard for medium-voltage relay contacts. The IEC 60947 series specifies test methods for low-voltage switchgear contacts, including endurance cycling, temperature rise, and short-circuit withstand.

Applications

Contacts have applications in a wide range of disciplines, including:

• Power distribution: circuit breakers and disconnect switches that interrupt fault currents of tens of kiloamperes
• Automotive systems: relay contacts controlling starter motors, fuel injectors, and battery management electronics
• Semiconductor fabrication: ohmic contacts on transistors, diodes, and integrated circuits enabling predictable device characteristics
• Telecommunications: gold-plated connector contacts in high-frequency coaxial and RF assemblies requiring low and stable insertion loss
• Railway traction: pantograph-catenary sliding contacts supplying continuous power to electric locomotives and multiple-unit trains