BGE Material Selection & Properties 2 — Questions and Answers
Question 1: Which conductor material is most commonly used for overhead transmission lines due to its high strength-to-weight ratio?
- Copper
- ACSR (Aluminum Conductor Steel Reinforced) (Correct answer)
- Solid aluminum
- Galvanized steel
Correct answer: ACSR (Aluminum Conductor Steel Reinforced)
ACSR combines aluminum's conductivity with a steel core for tensile strength, making it ideal for long-span overhead transmission lines.
Question 2: What is the primary reason polyethylene (PE) piping is preferred over steel for new underground natural gas distribution mains?
- Higher pressure rating
- Better thermal conductivity
- Corrosion resistance and ease of installation (Correct answer)
- Lower cost of raw materials
Correct answer: Corrosion resistance and ease of installation
Polyethylene does not corrode, requires no cathodic protection, and can be fused in the field, significantly reducing long-term maintenance costs.
Question 3: A utility crew needs to connect a copper busbar to an aluminum conductor. What material concern must be addressed?
- Magnetic interference
- Galvanic corrosion at the bimetallic junction (Correct answer)
- Difference in insulation class
- Ampacity mismatch
Correct answer: Galvanic corrosion at the bimetallic junction
Copper and aluminum have different electrochemical potentials; direct contact in the presence of moisture causes galvanic corrosion that degrades the connection.
Question 4: Which insulation class is rated for continuous operation at 90°C for underground residential distribution (URD) cables?
- Class A (60°C)
- Class B (75°C)
- Class C (90°C) — XLPE or EPR (Correct answer)
- Class H (180°C)
Correct answer: Class C (90°C) — XLPE or EPR
Cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) are standard 90°C-rated insulations used in URD cable installations.
Question 5: For gas service risers transitioning from underground PE to aboveground steel, what fitting is required at the transition point?
- Compression coupling
- Electrofusion saddle
- Anodeless riser or PE-to-steel transition fitting (Correct answer)
- Flanged union
Correct answer: Anodeless riser or PE-to-steel transition fitting
An anodeless riser or factory-made PE-to-steel transition fitting safely joins the two incompatible materials while maintaining pressure integrity and corrosion protection.
Question 6: What property of SF6 (sulfur hexafluoride) gas makes it valuable in high-voltage switchgear?
- High thermal conductivity
- Superior dielectric strength — about 2.5× that of air (Correct answer)
- Low density enabling lightweight equipment
- Non-flammability only
Correct answer: Superior dielectric strength — about 2.5× that of air
SF6's exceptional dielectric strength allows switchgear to interrupt fault currents and withstand high voltages in a much smaller physical footprint than air-insulated equipment.
Question 7: When selecting wire for a motor control center (MCC) operating at 600V, what minimum insulation voltage rating is typically required by the NEC?
- 300V rated THHN
- 600V rated wire (e.g., THWN-2 or XHHW-2) (Correct answer)
- 1000V rated only
- No rating required below 480V
Correct answer: 600V rated wire (e.g., THWN-2 or XHHW-2)
The NEC requires conductors in 600V systems to have insulation rated for at least 600V; THWN-2 and XHHW-2 are common choices that also handle moisture.
BGE Material Selection & Properties 2
Which conductor material is most commonly used for overhead transmission lines due to its high strength-to-weight ratio?