EETC Power Distribution 5 — Questions and Answers
Question 1: What is the function of a 'keep-away' fuse used in power distribution?
- A fuse designed to interrupt fault current without expelling gases toward adjacent conductors (Correct answer)
- A fuse used solely on capacitor banks to prevent overcurrent damage
- A current-limiting fuse rated for transformer primary protection only
- A fuse that automatically resets after a fault clears
Correct answer: A fuse designed to interrupt fault current without expelling gases toward adjacent conductors
Keep-away (or non-venting) fuses are designed to interrupt fault current while directing expulsion gases safely, preventing flashover to adjacent phases or equipment.
Question 2: When sizing a distribution transformer for a commercial load, which factor accounts for the non-simultaneous operation of all connected equipment?
- Power factor
- Demand factor (Correct answer)
- Load factor
- Transformer impedance
Correct answer: Demand factor
Demand factor accounts for the fact that not all connected loads operate simultaneously, allowing the transformer to be sized smaller than the total connected load.
Question 3: Which grounding method connects the system neutral directly to earth without any impedance?
- Resistance grounding
- Reactance grounding
- Solid (effective) grounding (Correct answer)
- Resonant (Petersen coil) grounding
Correct answer: Solid (effective) grounding
Solid grounding, also called effective grounding, connects the neutral directly to earth, resulting in high ground fault currents that can be easily detected and interrupted.
Question 4: A distribution system operates at 12,470Y/7,200 V. What does '7,200 V' represent?
- The voltage between any two phase conductors
- The voltage from phase conductor to neutral (phase-to-neutral voltage) (Correct answer)
- The voltage at the secondary terminals of distribution transformers
- The open-circuit voltage of the substation transformer
Correct answer: The voltage from phase conductor to neutral (phase-to-neutral voltage)
In the designation 12,470Y/7,200 V, 12,470 V is the phase-to-phase voltage and 7,200 V is the phase-to-neutral voltage (12,470 / √3 ≈ 7,200).
Question 5: What is the purpose of installing a line voltage regulator on a distribution feeder?
- To disconnect the feeder automatically under high-load conditions
- To maintain constant voltage at a set point as load and source voltage vary (Correct answer)
- To limit fault current on long rural feeders
- To convert single-phase sections to three-phase service
Correct answer: To maintain constant voltage at a set point as load and source voltage vary
A voltage regulator uses a tap-changing autotransformer to hold the output voltage within ±10% of set point regardless of load current and source voltage fluctuations.
Question 6: Which test is performed on a distribution transformer to determine its winding resistance and verify it has not changed due to thermal damage or loose connections?
- Power factor (dissipation factor) test
- Turns ratio test
- DC winding resistance test (Correct answer)
- Dissolved gas analysis (DGA)
Correct answer: DC winding resistance test
The DC winding resistance test measures resistance of each winding using a low-current DC source; changes from factory values indicate damaged conductors, loose connections, or shorted turns.
Question 7: In the context of distribution automation, what does a SCADA system primarily enable utility operators to do?
- Generate real-time electricity bills for customers
- Monitor and control distribution equipment remotely in real time (Correct answer)
- Automatically repair damaged overhead conductors
- Calculate transformer inrush current during energization
Correct answer: Monitor and control distribution equipment remotely in real time
SCADA (Supervisory Control and Data Acquisition) systems allow operators to monitor system status and remotely operate switches, reclosers, and other devices from a central control room.
EETC Power Distribution 5
What is the function of a 'keep-away' fuse used in power distribution?