Electrical Supply Systems and Distribution Flashcards
7 cards from real ECS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical Supply Systems and Distribution flashcards as text
What is the maximum permitted voltage drop from the supply terminals to any point in a final circuit for lighting loads under NEC guidelines?
Answer: 3%
The NEC recommends a maximum voltage drop of 3% for branch circuits (final circuits), with a combined drop of no more than 5% from the service to the final outlet.
In electrical distribution, what is meant by 'prospective short circuit current' (PSCC) at a given point?
Answer: The maximum current that would flow if a zero-impedance short circuit occurred at that point
Prospective short circuit current is the theoretical maximum fault current that would flow if a bolted (zero impedance) short circuit occurred, used to select appropriately rated protective devices.
What is the purpose of power factor correction capacitors in an electrical distribution system?
Answer: To reduce reactive power and improve the power factor of the supply
Power factor correction capacitors supply reactive power locally, reducing the reactive current drawn from the supply and improving the overall power factor, which reduces distribution losses.
Which instrument would an electrician use to measure the impedance of an earth fault loop at a point in an installation?
Answer: An earth fault loop impedance tester
An earth fault loop impedance tester injects a test current around the fault loop path and measures the resulting impedance, used to verify protective devices will operate within required disconnection times.
What is the significance of the 'short circuit capacity' (SCC) rating marked on a circuit breaker?
Answer: The maximum fault current the breaker can safely interrupt without damage
The short circuit capacity (also called interrupting or breaking capacity) is the maximum prospective fault current the circuit breaker can safely break without being damaged or causing an arc flash.
In a three-phase balanced load, if the line current is 20A, what is the total three-phase apparent power (VA) supplied at 208V line-to-line voltage?
Answer: 7,205 VA
Three-phase apparent power = √3 × V_line × I_line = 1.732 × 208V × 20A ≈ 7,205 VA.
What is the primary reason for balancing loads across the three phases of a three-phase distribution system?
Answer: To minimize neutral current and prevent overheating of the neutral conductor
In a balanced three-phase system, the neutral current is theoretically zero because the three phase currents cancel each other out; unbalanced loads result in neutral current that can cause overheating.