EETC Electrical Equipment & Maintenance 3 — Questions and Answers
Question 1: When replacing a failed contactor, which specification is MOST critical to match for motor protection?
- Coil voltage rating only
- Physical mounting dimensions
- Continuous current rating and interrupting capacity (Correct answer)
- Number of auxiliary contact positions
Correct answer: Continuous current rating and interrupting capacity
The continuous current rating must handle full-load amperes and the interrupting capacity must safely break fault currents without damage.
Question 2: In a variable frequency drive (VFD), what is the function of the DC bus capacitor bank?
- Convert AC input to DC supply voltage
- Filter and smooth the rectified DC voltage for the inverter section (Correct answer)
- Control the switching frequency of the IGBTs
- Provide regenerative braking energy storage
Correct answer: Filter and smooth the rectified DC voltage for the inverter section
DC bus capacitors smooth the pulsating rectified voltage into steady DC, reducing ripple and providing energy storage for the inverter section.
Question 3: What does a high dielectric absorption ratio (DAR) value indicate about transformer oil?
- Oil is heavily contaminated with moisture or carbon
- Oil is in good condition with high dielectric strength (Correct answer)
- Oil requires immediate replacement due to oxidation
- Oil has excessive dissolved gas content
Correct answer: Oil is in good condition with high dielectric strength
A high DAR (ratio of 60-second to 30-second insulation resistance) indicates clean, dry insulating oil with good dielectric properties.
Question 4: A power factor correction capacitor bank is found to have one failed (open) capacitor unit. What is the immediate effect on the bank?
- The bank output becomes zero
- Remaining units in that phase see increased voltage stress (Correct answer)
- Overall power factor improves slightly
- The bank trips due to overcurrent
Correct answer: Remaining units in that phase see increased voltage stress
With one capacitor open in a series-parallel bank, remaining parallel capacitors in that section carry the full voltage, increasing stress and risk of cascading failure.
Question 5: During preventive maintenance on switchgear, vacuum bottle integrity is verified by:
- Visual inspection of the bottle exterior for cracks
- Applying a high-potential (hi-pot) test across open contacts (Correct answer)
- Measuring contact resistance with a micro-ohmmeter
- Checking the spring charge pressure gauge
Correct answer: Applying a high-potential (hi-pot) test across open contacts
A high-potential test across the open vacuum interrupter verifies that the vacuum is intact and can withstand the required dielectric strength.
Question 6: What is the purpose of a load bank test on a standby generator?
- Verify fuel system capacity and transfer switch operation
- Exercise the generator at rated load to prevent wet-stacking and verify performance (Correct answer)
- Test battery backup systems for the control panel
- Check governor response at no-load conditions
Correct answer: Exercise the generator at rated load to prevent wet-stacking and verify performance
Load bank testing runs the generator at rated capacity, preventing wet-stacking in diesel engines and confirming the unit can carry its full rated load.
Question 7: When using thermal imaging (infrared) on electrical panels, a three-phase load imbalance typically appears as:
- All three phases equally hot compared to ambient
- One phase significantly hotter than the other two (Correct answer)
- The neutral bar overheating uniformly
- Random hotspots on all breakers simultaneously
Correct answer: One phase significantly hotter than the other two
An overloaded or unbalanced phase shows elevated temperature compared to the other two balanced phases due to higher I²R losses.
EETC Electrical Equipment & Maintenance 3
When replacing a failed contactor, which specification is MOST critical to match for motor protection?