Electrical Systems and Lighting Flashcards

Which formula gives the real power of a balanced three-phase load?

Answer: P = √3 × V × I × PF

Three-phase real power equals 1.732 times line voltage times line current times power factor.

Why are capacitor banks commonly installed in buildings with many large induction motors?

Answer: To correct a lagging power factor

Capacitors supply leading reactive power that offsets the lagging reactive power drawn by inductive motors.

A daylight harvesting control system saves energy by:

Answer: Using photosensors to dim electric lighting when enough daylight is present

Photosensors measure available daylight and dim or switch fixtures near windows to hold the target light level.

The efficacy of a light source is expressed in which unit?

Answer: Lumens per watt

Efficacy is light output (lumens) divided by electrical input (watts).

According to NFPA 101 Life Safety Code, battery-powered emergency egress lighting must operate for at least:

Answer: 90 minutes

NFPA 101 requires emergency lighting to provide illumination for a minimum of 1½ hours after loss of normal power.

In a three-phase, four-wire system serving many computers, LED drivers, and VFDs, what problem can harmonic currents cause?

Answer: Overheating of the shared neutral conductor

Triplen harmonics add together in the neutral rather than canceling, which can overload it.

An infrared thermography scan of an energized panel is mainly used to find:

Answer: Loose or overloaded connections showing up as hot spots

High resistance at loose or overloaded connections produces heat that an infrared camera can detect before failure.