CME CME Power Quality & Energy Efficiency 2 — Questions and Answers
Question 1: What does a displacement power factor of 0.95 lagging indicate about a facility's electrical load?
- The load is predominantly inductive, drawing reactive power from the utility (Correct answer)
- The load is purely resistive with no reactive component
- The load is capacitive and supplying reactive power to the grid
- The load has significant harmonic distortion offsetting true power factor
Correct answer: The load is predominantly inductive, drawing reactive power from the utility
A lagging displacement power factor indicates inductive loads (motors, transformers) consuming reactive power, which increases apparent power demand.
Question 2: Which energy measurement metric represents the actual work-producing power consumed by a load?
- Real power (kW) (Correct answer)
- Reactive power (kVAR)
- Apparent power (kVA)
- Complex power (kVA vector)
Correct answer: Real power (kW)
Real power (kilowatts) is the component of electrical power that performs actual work, such as driving motors or producing light and heat.
Question 3: A building energy audit identifies that replacing T12 fluorescent fixtures with LED equivalents will reduce lighting load by 40%. What additional NEC requirement applies to the new LED drivers?
- LED drivers must be listed for the specific luminaire per NEC 410.6 (Correct answer)
- LED drivers must be installed in weatherproof enclosures
- LED drivers require a dedicated 20A circuit each
- LED drivers must be rated for 277V minimum in commercial applications
Correct answer: LED drivers must be listed for the specific luminaire per NEC 410.6
NEC 410.6 requires that luminaires, lampholders, and associated drivers be listed for their intended use to ensure safety and compatibility.
Question 4: What is the primary cause of neutral conductor overheating in a three-phase wye system serving computer loads?
- Third-order (triplen) harmonic currents add in the neutral rather than cancel (Correct answer)
- Unbalanced phase voltages across single-phase loads
- Neutral conductor is undersized per NEC minimum requirements
- Phase currents exceed the neutral's rated ampacity at full load
Correct answer: Third-order (triplen) harmonic currents add in the neutral rather than cancel
Triplen harmonics (3rd, 9th, 15th) are zero-sequence and are additive in the neutral conductor instead of canceling as fundamental currents do.
Question 5: Which power quality phenomenon is best described as a momentary increase in RMS voltage between 110% and 180% of nominal lasting 0.5 cycles to 1 minute?
- Voltage swell (Correct answer)
- Voltage sag
- Voltage spike
- Transient overvoltage
Correct answer: Voltage swell
IEEE 1159 defines a voltage swell as an RMS increase to 110%–180% of nominal lasting 0.5 cycles to 1 minute, often caused by sudden load removal.
Question 6: Per NEC 210.19(A)(1), branch circuit conductors serving continuous loads must be sized to what percentage of the continuous load current?
- 125% (Correct answer)
- 100%
- 115%
- 110%
Correct answer: 125%
NEC 210.19(A)(1) requires branch circuit conductors to have an ampacity not less than 125% of the continuous load to prevent thermal degradation.
CME CME Power Quality & Energy Efficiency 2
What does a displacement power factor of 0.95 lagging indicate about a facility's electrical load?