EETC Electrical Equipment & Maintenance 2 — Questions and Answers
Question 1: What is the primary purpose of a megohmmeter (megger) in electrical maintenance?
- Measure current flow in circuits
- Test insulation resistance of cables and motors (Correct answer)
- Check voltage at distribution panels
- Verify continuity of fuses
Correct answer: Test insulation resistance of cables and motors
A megohmmeter applies high DC voltage to measure insulation resistance, detecting degraded or damaged insulation before failures occur.
Question 2: When performing a polarization index (PI) test on a motor winding, a PI ratio below 1.0 indicates:
- Excellent insulation condition
- Acceptable insulation with minor moisture
- Severely contaminated or damaged insulation (Correct answer)
- Normal aging of the winding
Correct answer: Severely contaminated or damaged insulation
A PI ratio below 1.0 means insulation resistance decreased during the test, indicating moisture saturation, contamination, or serious deterioration.
Question 3: Which type of capacitor failure mode causes an electrolytic capacitor to vent or bulge?
- Dielectric absorption
- Excessive ripple current causing internal heating (Correct answer)
- Reverse polarity momentarily applied
- Open-circuit internal lead failure
Correct answer: Excessive ripple current causing internal heating
Excessive ripple current heats the electrolyte, generating gas pressure that bulges or ruptures the vent on an electrolytic capacitor.
Question 4: In a three-phase induction motor, what condition is indicated by high vibration at twice line frequency (120 Hz in a 60 Hz system)?
- Mechanical unbalance of the rotor
- Loose stator laminations or electromagnetic looseness (Correct answer)
- Bearing defect on the drive end
- Misalignment of the motor coupling
Correct answer: Loose stator laminations or electromagnetic looseness
Vibration at 2× line frequency (120 Hz) is a classic signature of electromagnetic looseness, often caused by loose stator core laminations.
Question 5: A transformer's dissolved gas analysis (DGA) shows high levels of acetylene (C₂H₂). What fault does this indicate?
- Overloaded core with thermal deterioration
- High-energy arcing or electrical discharge (Correct answer)
- Partial discharge in oil-impregnated paper
- Moisture contamination of insulating oil
Correct answer: High-energy arcing or electrical discharge
Acetylene is produced only at very high temperatures (above 700°C), making it a definitive marker of high-energy arcing inside the transformer.
Question 6: What is the correct procedure when re-energizing a motor that has been stored for an extended period?
- Apply full voltage immediately to check operation
- Perform a megger test and dry out windings if resistance is low before energizing (Correct answer)
- Lubricate bearings and start at half voltage
- Inspect terminal connections and energize without testing
Correct answer: Perform a megger test and dry out windings if resistance is low before energizing
Long storage allows moisture to accumulate in windings; a megger test verifies insulation integrity and drying out is required if resistance is below minimum acceptable values.
Question 7: Which maintenance technique uses ultrasonic detection to identify problems in electrical equipment?
- Thermographic scanning for hotspots
- Detecting partial discharge, corona, and arcing in switchgear (Correct answer)
- Measuring harmonic distortion in power systems
- Testing ground fault interrupter response time
Correct answer: Detecting partial discharge, corona, and arcing in switchgear
Ultrasonic detectors pick up high-frequency sound emissions from corona, partial discharge, and arcing that are inaudible to the human ear.
EETC Electrical Equipment & Maintenance 2
What is the primary purpose of a megohmmeter (megger) in electrical maintenance?