EETC Electrical Equipment & Maintenance 5 ā Questions and Answers
Question 1: What is the primary reason for performing a core ground test on a power transformer?
- Verify the turns ratio between windings
- Detect multiple core ground points that cause circulating currents and overheating (Correct answer)
- Measure the magnetizing current of the core
- Check the mechanical integrity of the core clamping bolts
Correct answer: Detect multiple core ground points that cause circulating currents and overheating
A transformer core should be grounded at only one point; multiple ground paths create a closed loop where circulating currents cause localized overheating.
Question 2: When a soft starter is used with an induction motor, what happens to the starting torque compared to across-the-line starting?
- Starting torque increases proportionally with reduced voltage
- Starting torque is reduced because torque varies with the square of voltage (Correct answer)
- Starting torque remains the same regardless of applied voltage
- Starting torque doubles during the ramp-up period
Correct answer: Starting torque is reduced because torque varies with the square of voltage
Motor torque is proportional to VĀ², so reducing voltage to 50% reduces starting torque to 25% of full-voltage torque.
Question 3: In a battery backup system (UPS), what is indicated by excessive equalization charging voltage causing electrolyte to bubble vigorously?
- The battery is being properly desulfated
- Overcharging is occurring, causing water electrolysis and gassing (Correct answer)
- The battery has reached full charge and is ready for service
- An internal short circuit is present in one cell
Correct answer: Overcharging is occurring, causing water electrolysis and gassing
Excessive charge voltage electrolyzes water in the electrolyte into hydrogen and oxygen gases, causing vigorous bubbling and water loss.
Question 4: During scheduled maintenance, a technician finds that an air circuit breaker's trip time during testing is significantly longer than specified. The MOST likely cause is:
- The breaker has been upgraded to a higher interrupt rating
- Worn or sluggish trip mechanism components requiring lubrication or replacement (Correct answer)
- The test current was applied for an insufficient duration
- The breaker's frame rating has been exceeded
Correct answer: Worn or sluggish trip mechanism components requiring lubrication or replacement
Slow trip times indicate mechanical wear, corrosion, or dried lubrication in the trip mechanism that prevents rapid contact separation.
Question 5: What is the purpose of installing surge protective devices (SPDs) on motor control circuits?
- Limit the motor's maximum operating speed
- Protect sensitive control electronics from voltage transients and lightning surges (Correct answer)
- Reduce harmonic distortion in the motor supply
- Provide phase-loss protection for three-phase motors
Correct answer: Protect sensitive control electronics from voltage transients and lightning surges
SPDs clamp voltage transients from switching operations and lightning to safe levels, protecting PLCs, drives, and control components from damage.
Question 6: A technician measures rotor bar currents using a clamp-on ammeter around individual rotor bars on a wound-rotor motor. Unequal readings between bars indicate:
- Normal variation due to rotor position at measurement time
- Cracked or broken rotor bars causing unequal current distribution (Correct answer)
- The motor is operating below rated load
- Excessive slip due to high ambient temperature
Correct answer: Cracked or broken rotor bars causing unequal current distribution
Broken or cracked rotor bars have higher resistance, forcing adjacent bars to carry more current, resulting in unequal readings and increased vibration.
Question 7: When de-energizing high-voltage switchgear for maintenance, what is the correct sequence after opening the main breaker?
- Install grounds, verify absence of voltage, lock out/tag out
- Lock out/tag out, verify absence of voltage, install grounds (Correct answer)
- Verify absence of voltage, lock out/tag out, install grounds
- Install grounds, lock out/tag out, verify absence of voltage
Correct answer: Lock out/tag out, verify absence of voltage, install grounds
NFPA 70E and OSHA procedures require LOTO to be applied before testing, then voltage absence is verified, and finally protective grounds are installed.
EETC Electrical Equipment & Maintenance 5
What is the primary reason for performing a core ground test on a power transformer?