ECS Electrical Supply Systems and Distribution 1 — Questions and Answers
Question 1: What is the standard nominal voltage for a single-phase domestic supply in the United States?
- 110V
- 120V (Correct answer)
- 208V
- 240V
Correct answer: 120V
The standard nominal voltage for a single-phase domestic supply in the US is 120V (nominally, though it may vary slightly between 114V and 126V).
Question 2: In a three-phase electrical supply system, what is the phase angle between each of the three phases?
- 90 degrees
- 120 degrees (Correct answer)
- 180 degrees
- 360 degrees
Correct answer: 120 degrees
In a balanced three-phase system, each phase is displaced by 120 degrees from the others, equally dividing the 360-degree cycle.
Question 3: What type of transformer connection is most commonly used to supply three-phase power to commercial buildings in the US?
- Delta-Delta
- Wye-Wye
- Delta-Wye (Correct answer)
- Wye-Delta
Correct answer: Delta-Wye
A Delta-Wye (Delta-Star) transformer configuration is most commonly used to step down transmission voltage for commercial distribution, providing both line-to-line and line-to-neutral voltages.
Question 4: What is the primary purpose of a main distribution board (MDB) in an electrical installation?
- To measure electricity consumption
- To distribute power to sub-circuits and provide overcurrent protection (Correct answer)
- To convert AC power to DC power
- To regulate voltage fluctuations on the supply
Correct answer: To distribute power to sub-circuits and provide overcurrent protection
A main distribution board distributes incoming electrical power to multiple outgoing circuits while housing protective devices such as circuit breakers and fuses.
Question 5: Which of the following best describes a 'radial circuit' in electrical distribution?
- A circuit that loops back to connect to the supply at both ends
- A circuit where power flows in one direction from the supply to the final outlet (Correct answer)
- A circuit that distributes power in a ring around the building
- A circuit designed only for three-phase loads
Correct answer: A circuit where power flows in one direction from the supply to the final outlet
A radial circuit runs from the distribution board in one direction to supply outlets or loads, with the cable terminating at the last point without returning to the board.
Question 6: What is the function of a neutral conductor in a single-phase AC supply?
- To carry fault current to earth during a short circuit
- To provide a return path for the load current back to the supply (Correct answer)
- To protect equipment from voltage surges
- To regulate the frequency of the supply
Correct answer: To provide a return path for the load current back to the supply
The neutral conductor provides the return path for current flowing through the load, completing the circuit back to the supply transformer.
Question 7: What does the term 'demand factor' refer to in electrical distribution planning?
- The ratio of actual power consumed to apparent power
- The ratio of maximum demand to the total connected load (Correct answer)
- The efficiency rating of the distribution transformer
- The percentage of voltage drop across the distribution cable
Correct answer: The ratio of maximum demand to the total connected load
Demand factor is the ratio of the maximum demand on a system at any given time to the total installed connected load, used to size supply equipment correctly.
ECS Electrical Supply Systems and Distribution 1
What is the standard nominal voltage for a single-phase domestic supply in the United States?