AEP Material Selection & Properties 3 — Questions and Answers
Question 1: What is the primary advantage of using SF6 (sulfur hexafluoride) gas in high-voltage circuit breakers?
- It is non-toxic and environmentally safe
- It has excellent arc-quenching properties and high dielectric strength (Correct answer)
- It is inexpensive and abundantly available
- It improves the mechanical switching speed
Correct answer: It has excellent arc-quenching properties and high dielectric strength
SF6 gas has superior arc-quenching capability and dielectric strength approximately 2.5 times that of air, enabling compact high-voltage switchgear designs.
Question 2: In a copper-clad aluminum (CCA) conductor, what is the main functional role of the copper outer layer?
- To increase the overall tensile strength
- To carry the majority of current due to the skin effect at high frequencies
- To prevent oxidation at connection points and reduce contact resistance (Correct answer)
- To increase the conductor's thermal conductivity
Correct answer: To prevent oxidation at connection points and reduce contact resistance
The copper cladding prevents the formation of high-resistance aluminum oxide at termination points, ensuring reliable low-resistance connections.
Question 3: Which material property is defined as the ratio of stress to strain within the elastic limit?
- Yield strength
- Ultimate tensile strength
- Modulus of elasticity (Young's modulus) (Correct answer)
- Fracture toughness
Correct answer: Modulus of elasticity (Young's modulus)
Young's modulus (modulus of elasticity) is the ratio of tensile stress to tensile strain in the linear elastic region of a material's stress-strain curve.
Question 4: Why is annealed copper used for flexible wiring applications rather than hard-drawn copper?
- Annealed copper has higher electrical conductivity
- Annealed copper is softer and more ductile, providing greater flexibility (Correct answer)
- Annealed copper is more resistant to corrosion
- Annealed copper has a higher melting point
Correct answer: Annealed copper is softer and more ductile, providing greater flexibility
Annealing relieves internal stresses and reduces hardness, making copper more ductile and flexible for applications requiring repeated bending without fatigue failure.
Question 5: What is the main reason fiberglass reinforced polymer (FRP) is used for utility pole crossarms instead of traditional wood?
- Higher thermal conductivity
- Dimensional stability, non-conductivity, and resistance to rot and insects (Correct answer)
- Lower weight compared to wood
- Better fire resistance in all conditions
Correct answer: Dimensional stability, non-conductivity, and resistance to rot and insects
FRP crossarms are electrically non-conductive, dimensionally stable, immune to rot, insect damage, and weathering, offering a longer service life than wood.
Question 6: Which material phenomenon causes a conductor's effective resistance to increase at higher frequencies due to current crowding near the surface?
- Proximity effect
- Skin effect (Correct answer)
- Hysteresis effect
- Capacitive reactance
Correct answer: Skin effect
The skin effect causes alternating current to flow predominantly near the surface of a conductor, reducing the effective cross-sectional area and increasing AC resistance.
Question 7: For high-temperature applications in power plant environments, which alloy family is most commonly selected due to its oxidation and creep resistance?
- Carbon steel
- Nickel-based superalloys (Correct answer)
- Gray cast iron
- Aluminum alloys
Correct answer: Nickel-based superalloys
Nickel-based superalloys maintain high strength and oxidation resistance at temperatures exceeding 1000°C, making them essential for gas turbine blades and high-temp power plant components.
AEP Material Selection & Properties 3
What is the primary advantage of using SF6 (sulfur hexafluoride) gas in high-voltage circuit breakers?