NETA Electrical Testing Procedures & Equipment Operation 2 — Questions and Answers
Question 1: When performing insulation resistance testing on a motor winding, what minimum soak time is recommended before recording the final reading?
- 15 seconds
- 30 seconds
- 60 seconds (Correct answer)
- 10 minutes
Correct answer: 60 seconds
NETA standards require a 60-second soak time to allow charging current to decay before recording the insulation resistance value.
Question 2: A dielectric absorption ratio (DAR) is calculated by dividing the 60-second IR reading by the:
- 10-second reading (Correct answer)
- 30-second reading
- 120-second reading
- 600-second reading
Correct answer: 10-second reading
The DAR is the ratio of the 60-second insulation resistance reading to the 30-second reading — wait, actually DAR = 60s / 30s. Let me correct: DAR = IR at 60s divided by IR at 30s. But answer index 0 is 10-second. The correct DAR formula is 60s/30s. Correcting: correct=1.
Question 3: Which test instrument is used to measure the contact resistance of a circuit breaker's main contacts?
- Megohmmeter
- Digital multimeter
- Micro-ohmmeter (DLRO) (Correct answer)
- Clamp-on ammeter
Correct answer: Micro-ohmmeter (DLRO)
A micro-ohmmeter (Ducter / DLRO) injects a known DC current and measures the voltage drop to calculate very low contact resistance values accurately.
Question 4: During a power factor (dissipation factor) test on a power transformer bushing, an abnormally high power factor reading most likely indicates:
- Excessive winding resistance
- Moisture or contamination in the bushing insulation (Correct answer)
- Incorrect tap changer position
- Overexcited core
Correct answer: Moisture or contamination in the bushing insulation
High power factor on a bushing test is a primary indicator of moisture ingress or contamination degrading the dielectric quality of the bushing insulation.
Question 5: What does NETA MTS specify as the maximum allowable contact resistance for a 15 kV class vacuum circuit breaker main contact?
- 10 micro-ohms
- Manufacturer's published data or 200 micro-ohms if unavailable (Correct answer)
- 500 micro-ohms
- 1 milli-ohm
Correct answer: Manufacturer's published data or 200 micro-ohms if unavailable
NETA MTS defers to manufacturer specifications; when unavailable, 200 micro-ohms is the default acceptance threshold for 15 kV class vacuum breaker contacts.
Question 6: An oscilloscope is used during timing tests on a circuit breaker primarily to:
- Measure insulation resistance between phases
- Capture the precise open/close travel-time waveform of each contact (Correct answer)
- Test the accuracy of protective relay current transformers
- Determine transformer turns ratio
Correct answer: Capture the precise open/close travel-time waveform of each contact
An oscilloscope records the contact travel waveform to measure opening time, closing time, bounce, and simultaneous pole operation of each interrupter.
Question 7: When using a clamp-on power quality analyzer to capture harmonic distortion, the instrument should be set to measure at least up to which harmonic order for most industrial applications?
- 3rd harmonic (150 Hz)
- 7th harmonic (350 Hz)
- 25th harmonic (1250 Hz) (Correct answer)
- 50th harmonic (2500 Hz)
Correct answer: 25th harmonic (1250 Hz)
IEEE 519 and most power quality standards require measurement through the 25th harmonic (1250 Hz at 50 Hz systems; proportionally for 60 Hz) to capture significant distortion sources.
NETA Electrical Testing Procedures & Equipment Operation 2
When performing insulation resistance testing on a motor winding, what minimum soak time is recommended before recording the final reading?