Troubleshooting & System Testing Flashcards

A time-current coordination study shows the feeder breaker trips before the branch circuit breaker for a fault on the branch. This coordination failure is called:

Answer: Selectivity failure or lack of coordination

When an upstream protective device operates before or simultaneously with a downstream device for a fault in the downstream zone, it is a selectivity or coordination failure.

A power factor correction capacitor bank causes nuisance tripping of a nearby VFD when energized. The most likely culprit is:

Answer: Transient overvoltage from capacitor switching causing VFD DC bus overvoltage trip

Capacitor switching generates high-frequency transient overvoltages that can spike the VFD's DC bus above its overvoltage trip threshold.

An insulation resistance test (Megger) on a cable shows a polarization index (PI) of 1.1 after 10 minutes. This indicates:

Answer: Contaminated or wet insulation requiring investigation

A PI below 2.0 for most insulation types indicates moisture, contamination, or deteriorated insulation; values above 4.0 are considered excellent.

During a short-circuit capacity test of a diesel generator, the generator output voltage collapses and does not recover. The most likely cause is:

Answer: The excitation system (AVR) has failed or is disabled

The automatic voltage regulator (AVR) maintains terminal voltage by controlling field excitation; a failed AVR prevents voltage recovery under fault or heavy load conditions.

A 4–20 mA analog control signal to a VFD reads 3.8 mA at the drive input with the transmitter confirmed at 4 mA output. This discrepancy indicates:

Answer: A voltage drop across loop resistance due to a high-resistance connection or ground fault

A 4–20 mA current loop reading low at the receiver compared to the transmitter output indicates added loop resistance from a loose connection or ground fault causing a voltage drop.

A technician performs a winding resistance test on a three-phase transformer and finds phase B resistance is 15% higher than phases A and C. This most likely indicates:

Answer: A loose or high-resistance internal connection on phase B

A significantly elevated winding resistance on one phase compared to others indicates a high-resistance joint, loose connection, or partial open in that winding.

When using a spectrum analyzer during a power quality survey, a pronounced peak is observed at 180 Hz on a 60 Hz system. This is the:

Answer: 3rd harmonic, typically caused by single-phase nonlinear loads

180 Hz is three times the 60 Hz fundamental (3rd harmonic), commonly produced by single-phase nonlinear loads such as computers, lighting ballasts, and switch-mode power supplies.