NCEES Electrical Engineering PE Exam 3 — Questions and Answers
Question 1: On the NCEES PE Electrical exam, what is 'harmonic distortion' in a power system and why is it a concern?
- Normal variation in 60 Hz frequency; concern for synchronous motor speed
- Non-fundamental frequency currents generated by nonlinear loads that cause overheating, interference, and metering errors (Correct answer)
- Voltage imbalance between phases; concern for motor insulation
- Electromagnetic interference from lightning surges
Correct answer: Non-fundamental frequency currents generated by nonlinear loads that cause overheating, interference, and metering errors
Harmonics are multiples of the 60 Hz fundamental generated by nonlinear loads (VFDs, UPS systems); they cause extra heating in neutrals, transformers, and motors, and can cause metering and communication errors.
Question 2: In the NCEES PE Electrical exam, what is the 'demand factor' used for in electrical load calculations?
- The ratio of maximum demand to installed load capacity, used to size service entrance conductors (Correct answer)
- The ratio of reactive power to real power
- The diversity factor for transformer sizing
- The multiplier for motor locked-rotor current
Correct answer: The ratio of maximum demand to installed load capacity, used to size service entrance conductors
Demand factor accounts for the fact that not all connected loads operate simultaneously at full capacity, allowing service conductors and switchgear to be sized more economically.
Question 3: For the NCEES PE Electrical exam, what is the significance of 'interrupting rating' on a circuit breaker?
- The continuous current the breaker can carry without tripping
- The maximum fault current the breaker can safely interrupt without damage (Correct answer)
- The number of operations before maintenance is required
- The minimum trip current setting
Correct answer: The maximum fault current the breaker can safely interrupt without damage
The interrupting rating (in kA rms) is the maximum symmetrical fault current the breaker can safely clear; if fault current exceeds this rating, the breaker may explode or fail to interrupt.
Question 4: On the NCEES PE Electrical exam, what is 'load flow analysis' (power flow study) used to determine?
- The fault current at each bus during a short circuit
- Bus voltages, phase angles, and real/reactive power flows throughout a power network under normal conditions (Correct answer)
- Transient stability during generator trip events
- Protection relay coordination settings
Correct answer: Bus voltages, phase angles, and real/reactive power flows throughout a power network under normal conditions
Load flow (power flow) analysis solves for steady-state bus voltages, angles, and branch power flows using Newton-Raphson or Gauss-Seidel iterative methods, used for system planning and operation.
Question 5: In the NCEES PE Electrical exam, what does 'capacitor bank power factor correction' achieve in an industrial facility?
- Reduces real power consumption of motors
- Supplies reactive power locally, reducing reactive current drawn from the utility and lowering demand charges (Correct answer)
- Filters harmonic currents from VFDs
- Provides backup power during outages
Correct answer: Supplies reactive power locally, reducing reactive current drawn from the utility and lowering demand charges
Power factor correction capacitors supply leading reactive power (VAR) that offsets lagging reactive demand of inductive loads, reducing total current, line losses, and utility reactive demand charges.
Question 6: For the NCEES PE Electrical exam, what is the 'sequence impedance' concept used for in fault analysis?
- Determining conductor size for balanced three-phase loads
- Resolving unbalanced fault conditions into positive, negative, and zero sequence networks for analysis (Correct answer)
- Calculating harmonic impedances in filter design
- Measuring transformer winding resistance
Correct answer: Resolving unbalanced fault conditions into positive, negative, and zero sequence networks for analysis
Symmetrical components decompose unbalanced phasors into positive, negative, and zero sequence sets, allowing unbalanced fault types (SLG, LL, LLG) to be solved using simple balanced sequence networks.
NCEES Electrical Engineering PE Exam 3
On the NCEES PE Electrical exam, what is 'harmonic distortion' in a power system and why is it a concern?