Material Selection & Properties Flashcards

HDPE electrofusion fittings require a specific minimum preheat temperature before fusion. What happens if fusion occurs below the specified temperature?

Answer: Incomplete fusion resulting in a weak joint with potential leak path

Below-temperature fusion prevents adequate polymer chain entanglement, producing a joint with insufficient tensile strength that can fail under operating pressure or ground movement.

For outdoor padmounted transformer enclosures in a residential neighborhood, which material is standard and why?

Answer: Heavy-gauge galvanized or painted steel with tamper-resistant hardware — vandal resistance and durability

Padmounted enclosures require robust steel construction with penta-head or hex-head bolts to deter unauthorized access and withstand weathering and physical abuse.

What does the 'W' designation in wire types like THWN indicate about the insulation material?

Answer: Rated for wet or damp locations

In NEC wire designations, 'W' signifies the insulation is suitable for wet locations, meaning it resists moisture intrusion in conduit or direct burial applications.

A BGE engineer selects API 5L Grade X52 pipe for a high-pressure gas transmission line. What does the '52' in the grade designation refer to?

Answer: Minimum yield strength of 52,000 psi

API 5L grade numbers represent the minimum yield strength in ksi (thousands of psi); X52 pipe must have a yield strength of at least 52,000 psi.

Which property of mica tape wrapping on generator stator windings makes it preferable over organic insulation at high temperatures?

Answer: Thermal stability above 155°C and resistance to partial discharge erosion

Mica is a Class F/H mineral insulation that maintains dielectric integrity at high temperatures and resists corona discharge erosion far better than organic polymer tapes.

When splicing underground medium-voltage (15kV) cables, which type of splice kit provides the best long-term moisture resistance?

Answer: Pre-molded cold-shrink or heat-shrink EPDM/EPR splice

Pre-molded cold-shrink or heat-shrink splice kits provide uniform dielectric enhancement and a sealed, waterproof joint that resists moisture ingress over the cable's service life.

What characteristic of silicon carbide (SiC) makes it effective in surge arresters protecting distribution equipment?

Answer: Non-linear voltage-current relationship — high resistance at normal voltage, low resistance during surges

SiC's non-linear (varistor) behavior allows it to block normal operating current but rapidly conduct and clamp surge voltages, protecting downstream equipment from transient overvoltages.