EETC Power Distribution 4 — Questions and Answers
Question 1: In power distribution, 'load factor' is defined as:
- Peak demand divided by total connected load
- Average load divided by peak demand over a given period (Correct answer)
- Energy consumed divided by system capacity
- Installed capacity divided by actual demand
Correct answer: Average load divided by peak demand over a given period
Load factor = average load / peak load over a period, indicating how efficiently the system capacity is utilized; a higher load factor means more uniform loading.
Question 2: What is the typical voltage range for 'primary distribution' circuits in the United States?
- 120 V to 240 V
- 480 V to 600 V
- 2.4 kV to 35 kV (Correct answer)
- 69 kV to 345 kV
Correct answer: 2.4 kV to 35 kV
Primary distribution operates at medium voltages, typically 2.4 kV to 35 kV (most commonly 4, 13.2, 13.8, or 25 kV), between the substation and distribution transformers.
Question 3: Which type of fault produces the highest fault current on a three-phase power system?
- Single-line-to-ground fault
- Line-to-line fault
- Double-line-to-ground fault
- Bolted three-phase fault (Correct answer)
Correct answer: Bolted three-phase fault
A bolted (zero impedance) three-phase symmetrical fault produces the highest fault current because all three phases are involved symmetrically with no fault impedance.
Question 4: A distribution recloser is set for a '2 fast, 2 delayed' sequence. What does this mean?
- It opens twice quickly, then twice after a time delay, and locks out after the 4th operation (Correct answer)
- It closes twice quickly, waits, then closes twice more before alarming
- It uses fast curves for phase faults and delayed curves for ground faults only
- It automatically resets after 2 fast operations if the fault clears
Correct answer: It opens twice quickly, then twice after a time delay, and locks out after the 4th operation
The '2 fast, 2 delayed' sequence means the recloser trips twice on a fast (instantaneous) curve, then twice on a slower time-delay curve before locking out after the fourth operation if the fault persists.
Question 5: Underground residential distribution (URD) cables most commonly use which insulation material?
- Paper-lead (PILC)
- Cross-linked polyethylene (XLPE) or EPR (Correct answer)
- Polyvinyl chloride (PVC)
- Rubber with cotton braid
Correct answer: Cross-linked polyethylene (XLPE) or EPR
Modern URD cables use XLPE or EPR insulation for their superior electrical properties, moisture resistance, and long service life compared to older materials.
Question 6: What is the primary cause of 'ferroresonance' in distribution systems?
- Excessive harmonic distortion from nonlinear loads
- Interaction between transformer magnetizing inductance and system capacitance under light or no-load conditions (Correct answer)
- Loose connections causing series impedance resonance
- Overcorrection of power factor by capacitor banks
Correct answer: Interaction between transformer magnetizing inductance and system capacitance under light or no-load conditions
Ferroresonance occurs when the nonlinear magnetizing inductance of an unloaded or lightly loaded transformer resonates with cable or capacitor charging capacitance, producing overvoltages.
Question 7: According to the NESC, what minimum vertical clearance above ground is required for a 13.8 kV overhead distribution conductor crossing a public roadway (open country)?
- 15.5 feet
- 18 feet (Correct answer)
- 22 feet
- 25 feet
Correct answer: 18 feet
The NESC requires 18 feet of vertical clearance for conductors operating at 15 kV or less crossing roads accessible to truck traffic in open country.
EETC Power Distribution 4
In power distribution, 'load factor' is defined as: