Material Selection & Properties Flashcards

Which type of pipe joint is preferred for natural gas distribution mains because it eliminates mechanical connections that can leak over time?

Answer: Butt fusion or electrofusion weld (for PE pipe)

Fusion joints in PE pipe create a monolithic, homogeneous bond with no mechanical components, providing a leak-free joint that matches or exceeds the pipe's own strength.

In a utility substation, what material property is most important when selecting bus bar for high-current applications at 60 Hz?

Answer: High electrical conductivity combined with adequate tensile strength

Bus bars must carry thousands of amperes continuously, so high conductivity (to minimize resistive losses and heating) and sufficient mechanical strength (to withstand fault current forces) are paramount.

A field crew discovers a coated steel gas main showing disbonded coating with no cathodic protection current reaching the bare metal. What failure mechanism is occurring?

Answer: External corrosion (electrochemical) due to cathodic shielding by the disbonded coating

Disbonded coating traps moisture against bare steel while blocking cathodic protection current, creating an ideal environment for accelerated external corrosion beneath the coating.

What is the significance of a cable's 'ampacity' rating, and what two factors most directly limit it?

Answer: Maximum continuous current the cable can carry safely; limited by heat generated in the conductor and the surrounding thermal environment

Ampacity is the safe continuous current rating; it is limited by I²R heating in the conductor and the ability of the insulation and surrounding soil or conduit to dissipate that heat.

Which NEC Article governs the installation of electrical conductors and equipment operating at over 1000 volts (medium and high voltage)?

Answer: NEC Article 490

NEC Article 490 covers equipment and wiring operating at over 1000 volts, including special requirements for materials, clearances, and protective systems at medium and high voltage.

When comparing dry-type and liquid-immersed transformers for indoor substation use, what is the primary material advantage of dry-type units?

Answer: No flammable liquid — eliminates fire and spill containment requirements indoors

Dry-type transformers use air or solid epoxy insulation instead of oil, eliminating the fire risk and EPA spill containment requirements associated with mineral oil in indoor or environmentally sensitive locations.

A gas service regulator's diaphragm must maintain flexibility across Maryland's temperature range (−10°F to 105°F). Which material is most appropriate?

Answer: Buna-N (nitrile rubber, NBR)

Nitrile rubber (Buna-N) is resistant to natural gas and LPG hydrocarbons and remains flexible over a wide temperature range, making it the standard diaphragm material for gas pressure regulators.