Troubleshooting & Diagnostics Flashcards

A three-phase motor runs but draws unbalanced current on all three phases. What is the most likely cause?

Answer: Unbalanced supply voltage

Unbalanced supply voltage causes unequal current draw across all three phases of a motor.

Which instrument is best suited for locating a partial discharge site on a high-voltage cable?

Answer: Oscilloscope with PD detection

Partial discharge detection equipment using an oscilloscope captures high-frequency pulses that pinpoint defect locations.

A distribution transformer hums louder than normal but output voltage is within spec. What should you suspect first?

Answer: Loose core laminations

Loose core laminations vibrate at twice the power frequency, producing excessive humming noise.

During a power factor test on a transformer, a high dissipation factor reading indicates:

Answer: Moisture or contamination in insulation

A high dissipation factor (tan delta) indicates moisture, contamination, or deterioration in transformer insulation.

A relay fails to trip during a fault simulation. After checking the relay element, what should you test next?

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.

What does a high neutral-to-ground voltage reading (>5V) at a panel indicate?

Answer: Overloaded neutral conductor

Excessive neutral-to-ground voltage suggests the neutral conductor is overloaded or has high impedance due to a loose connection.

A capacitor bank causes nuisance tripping of a feeder breaker at energization. The most likely cause is:

Answer: Inrush current exceeding relay pickup

Capacitor bank energization produces a high-magnitude, short-duration inrush current that can exceed overcurrent relay pickup settings.