CEP Troubleshooting & System Testing 4 — Questions and Answers
Question 1: 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:
- Sympathetic tripping
- Cascade tripping
- Selectivity failure or lack of coordination (Correct answer)
- Inverse-time overcurrent overlap
Correct 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.
Question 2: A power factor correction capacitor bank causes nuisance tripping of a nearby VFD when energized. The most likely culprit is:
- Voltage sag from capacitor inrush current
- Transient overvoltage from capacitor switching causing VFD DC bus overvoltage trip (Correct answer)
- Reduced power factor below VFD minimum requirement
- Reverse power flow through the VFD rectifier
Correct 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.
Question 3: An insulation resistance test (Megger) on a cable shows a polarization index (PI) of 1.1 after 10 minutes. This indicates:
- Excellent dry insulation with high PI
- Acceptable insulation, PI above minimum threshold
- Contaminated or wet insulation requiring investigation (Correct answer)
- The cable is fully serviceable with no action needed
Correct 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.
Question 4: During a short-circuit capacity test of a diesel generator, the generator output voltage collapses and does not recover. The most likely cause is:
- The excitation system (AVR) has failed or is disabled (Correct answer)
- The governor is set too low
- Fuel injection timing is advanced
- The generator is oversized for the load
Correct 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.
Question 5: 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:
- The VFD analog input is calibrated incorrectly
- A voltage drop across loop resistance due to a high-resistance connection or ground fault (Correct answer)
- The transmitter power supply is too high
- Normal variation within 4–20 mA loop tolerance
Correct 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.
Question 6: 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:
- Normal manufacturing variance within tolerance
- A shorted turn in the phase B winding
- A loose or high-resistance internal connection on phase B (Correct answer)
- Incorrect tap changer position on phase B
Correct 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.
Question 7: 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:
- 2nd harmonic, typically caused by transformer saturation
- 3rd harmonic, typically caused by single-phase nonlinear loads (Correct answer)
- 5th harmonic, typically caused by six-pulse converters
- Fundamental frequency sideband caused by modulation
Correct 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.
CEP Troubleshooting & System Testing 4
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: