AEP Troubleshooting & Diagnostics 2 — Questions and Answers
Question 1: A three-phase motor runs but draws unbalanced current on all three phases. What is the most likely cause?
- Overloaded mechanical load
- Single-phasing condition
- Unbalanced supply voltage (Correct answer)
- Worn motor brushes
Correct answer: Unbalanced supply voltage
Unbalanced supply voltage causes unequal current draw across all three phases of a motor.
Question 2: Which instrument is best suited for locating a partial discharge site on a high-voltage cable?
- Clamp-on ammeter
- Oscilloscope with PD detection (Correct answer)
- Megohmmeter
- Power quality analyzer
Correct answer: Oscilloscope with PD detection
Partial discharge detection equipment using an oscilloscope captures high-frequency pulses that pinpoint defect locations.
Question 3: A distribution transformer hums louder than normal but output voltage is within spec. What should you suspect first?
- Faulty windings
- Loose core laminations (Correct answer)
- Overloaded secondary
- Failed tap changer
Correct answer: Loose core laminations
Loose core laminations vibrate at twice the power frequency, producing excessive humming noise.
Question 4: During a power factor test on a transformer, a high dissipation factor reading indicates:
- Healthy insulation
- Moisture or contamination in insulation (Correct answer)
- Low oil level
- Overcurrent condition
Correct answer: Moisture or contamination in insulation
A high dissipation factor (tan delta) indicates moisture, contamination, or deterioration in transformer insulation.
Question 5: A relay fails to trip during a fault simulation. After checking the relay element, what should you test next?
- Battery charger output
- Trip coil continuity and DC control voltage (Correct answer)
- CT secondary burden
- Relay firmware version
Correct answer: Trip coil continuity and DC control voltage
The trip coil and control voltage supply are the most common causes of a relay failing to trip after the relay element itself is verified.
Question 6: What does a high neutral-to-ground voltage reading (>5V) at a panel indicate?
- Normal system operation
- Overloaded neutral conductor (Correct answer)
- Properly bonded system
- Ground fault on a branch circuit
Correct answer: Overloaded neutral conductor
Excessive neutral-to-ground voltage suggests the neutral conductor is overloaded or has high impedance due to a loose connection.
Question 7: A capacitor bank causes nuisance tripping of a feeder breaker at energization. The most likely cause is:
- Low power factor on the feeder
- Inrush current exceeding relay pickup (Correct answer)
- Capacitor bank undersized
- Wrong capacitor kVAR rating
Correct answer: Inrush current exceeding relay pickup
Capacitor bank energization produces a high-magnitude, short-duration inrush current that can exceed overcurrent relay pickup settings.
AEP Troubleshooting & Diagnostics 2
A three-phase motor runs but draws unbalanced current on all three phases.
What is the most likely cause?