EETC Power Distribution 3 — Questions and Answers
Question 1: What is the standard color code for the high-voltage 'B' phase conductor in a North American overhead distribution system?
- Black
- Red
- Blue (Correct answer)
- White
Correct answer: Blue
In the standard A-B-C phase color convention, A=black, B=red, and C=blue for medium-voltage overhead distribution, though utility practices may vary.
Question 2: Which protection scheme is most appropriate for detecting a single-line-to-ground fault on an ungrounded distribution system?
- Overcurrent relays (51)
- Ground fault overcurrent relays (51G)
- Zero-sequence voltage (59N) or ground fault detectors (Correct answer)
- Differential relays (87)
Correct answer: Zero-sequence voltage (59N) or ground fault detectors
On an ungrounded system, fault current is very small and does not flow back through neutral, so zero-sequence voltage detection or capacitively coupled ground fault detectors are used.
Question 3: The 'demand factor' of an electrical installation is defined as:
- The ratio of actual energy consumed to the maximum possible energy over a period
- The ratio of the maximum demand to the total connected load (Correct answer)
- The ratio of peak load to average load over a billing period
- The number of hours per day the connected load operates at full capacity
Correct answer: The ratio of the maximum demand to the total connected load
Demand factor equals maximum demand divided by total connected load, indicating that not all connected loads operate simultaneously at full rated capacity.
Question 4: Which NEC article primarily governs the installation of service-entrance conductors for a residential customer?
- Article 220
- Article 230 (Correct answer)
- Article 240
- Article 250
Correct answer: Article 230
NEC Article 230 covers services, including service conductors, service entrance equipment, and service disconnects.
Question 5: A power distribution transformer is rated 500 kVA, 13,200/480V. What is the rated full-load secondary current?
- 37.9 A
- 601 A (Correct answer)
- 1,042 A
- 760 A
Correct answer: 601 A
Full-load secondary current = kVA × 1000 / (√3 × V) = 500,000 / (1.732 × 480) ≈ 601 A.
Question 6: What is the purpose of a neutral grounding resistor (NGR) connected in the neutral of a distribution transformer?
- To reduce transformer magnetizing inrush current
- To limit ground fault current while allowing fault detection (Correct answer)
- To balance phase voltages under normal load
- To prevent ferroresonance in unloaded cable systems
Correct answer: To limit ground fault current while allowing fault detection
An NGR limits ground fault current to a safe level (typically 5–400 A) while still allowing enough current to operate protective relays and detect faults.
Question 7: Which type of distribution substation configuration provides the highest reliability by allowing continuous operation even if one transformer fails?
- Single transformer radial
- Primary selective with automatic transfer
- Secondary selective network (Correct answer)
- Simple loop
Correct answer: Secondary selective network
A secondary selective network with multiple transformers and normally open tie breakers allows immediate load transfer to alternate transformers if one fails, ensuring uninterrupted service.
EETC Power Distribution 3
What is the standard color code for the high-voltage 'B' phase conductor in a North American overhead distribution system?