Material requirements for electrical contact applications (switches, brushes, relays) Which combination of properties is most desirable for electrical contact materials used in practical devices?

Introduction / Context:
Contacts in switches, brushes, and relays experience local hot spots, arcing, and mechanical wear. Selecting suitable materials is crucial for reliability and safety. This question emphasizes thermal and thermomechanical considerations that dominate contact performance.

Given Data / Assumptions:

• Contacts carry appreciable current and may break/make under load.
• Joule heating and possible arcing occur at asperities.
• We focus on thermal properties and melting behavior; mechanical hardness and oxidation resistance also matter but are secondary here.

Concept / Approach:
High thermal conductivity spreads heat away from the microscopic junction quickly, limiting temperature rise and reducing welding. A high melting point resists softening and erosion during arcing. Common solutions include silver, copper, and refractory composites (e.g., silver–tungsten), which combine excellent conductivity with high softening temperatures and arc-erosion resistance.

Step-by-Step Solution:
Identify failure modes: local overheating, metal transfer, and contact welding.Mitigation: maximize thermal conduction to sink heat; select materials that remain solid and structurally sound at high temperatures.Therefore choose materials with both high thermal conductivity and high melting point.

Verification / Alternative check:
Catalogs list Ag, Cu, Ag-W, Ag-CdO (legacy), and newer Ag-SnO2 as contact materials, reflecting the combined need for thermal performance and arc endurance.

Why Other Options Are Wrong:
Low thermal conductivity promotes hot spots. Low melting point fosters welding and rapid wear. Mixing high conductivity with low melting point fails under arcing conditions; conversely, high melting point with low conductivity still overheats locally.

Common Pitfalls:

• Overlooking oxidation/tarnish behavior, which also affects contact resistance.
• Assuming mechanical hardness alone ensures durability; thermal factors are equally critical.

Final Answer:
High thermal conductivity and high melting point