NETA Electrical Testing Procedures & Equipment Operation 3 — Questions and Answers
Question 1: The polarization index (PI) is calculated by dividing the insulation resistance measured at 10 minutes by the reading at:
- 30 seconds
- 1 minute (Correct answer)
- 5 minutes
- 15 minutes
Correct answer: 1 minute
PI = IR at 10 minutes divided by IR at 1 minute; a value of 2.0 or above generally indicates acceptable insulation condition.
Question 2: Which safety precaution is mandatory before connecting a megohmmeter to a large capacitive load such as a cable run?
- Short the leads together to zero the instrument
- Verify the load is de-energized and fully discharged (Correct answer)
- Set the instrument to AC mode
- Connect the guard terminal to ground
Correct answer: Verify the load is de-energized and fully discharged
Capacitive equipment stores dangerous charge; the equipment must be de-energized and discharged (verified with a voltmeter) before connecting the megohmmeter.
Question 3: In a transformer turns ratio (TTR) test, a ratio error exceeding what percentage typically triggers further investigation per NETA MTS?
- 0.1%
- 0.5% (Correct answer)
- 1.0%
- 5.0%
Correct answer: 0.5%
NETA MTS flags a turns ratio deviation greater than 0.5% from the nameplate ratio as requiring further evaluation for possible winding faults.
Question 4: When performing an AC high-potential (hipot) test on cable insulation, the test voltage is typically applied for:
- 10 seconds
- 1 minute (Correct answer)
- 5 minutes
- 15 minutes
Correct answer: 1 minute
The standard AC hipot withstand duration for cable insulation per NETA MTS is 1 minute at the specified test voltage.
Question 5: A very low frequency (VLF) test on medium-voltage cable uses a test frequency of:
- 60 Hz
- 50 Hz
- 0.1 Hz (Correct answer)
- 400 Hz
Correct answer: 0.1 Hz
VLF testing uses 0.1 Hz (or 0.01–0.1 Hz range) to stress cable insulation at a fraction of the normal power frequency, reducing the required kVA from the test set.
Question 6: During a sweep frequency response analysis (SFRA) test on a power transformer, the primary purpose is to detect:
- Insulation moisture content
- Winding deformation or displacement due to mechanical stress (Correct answer)
- Core loss and exciting current
- Bushing capacitance imbalance
Correct answer: Winding deformation or displacement due to mechanical stress
SFRA compares frequency response signatures to detect physical winding movement, deformation, or damage caused by through-faults or transportation.
Question 7: What test is performed to verify that a protective relay correctly operates at the designed current pickup level?
- Contact resistance test
- Insulation resistance test
- Secondary current injection test (Correct answer)
- Dielectric absorption test
Correct answer: Secondary current injection test
Secondary injection testing applies a calibrated current directly to the relay's current input terminals to verify pickup, timing, and trip function without energizing primary equipment.
NETA Electrical Testing Procedures & Equipment Operation 3
The polarization index (PI) is calculated by dividing the insulation resistance measured at 10 minutes by the reading at: