NETA Cable Testing and Diagnostics 4 — Questions and Answers
Question 1: During a time-domain reflectometry (TDR) test, a reflected pulse with the same polarity as the incident pulse indicates which type of fault?
- Open-circuit fault (Correct answer)
- Short-circuit fault
- Impedance match
- Capacitive coupling fault
Correct answer: Open-circuit fault
An open-circuit fault reflects the pulse with the same polarity because current cannot flow past the open, causing the voltage to double at that point.
Question 2: What is the primary purpose of performing a partial discharge (PD) test on medium-voltage cables?
- Measure cable capacitance
- Detect internal voids and insulation defects before failure (Correct answer)
- Verify conductor resistance
- Test shield continuity
Correct answer: Detect internal voids and insulation defects before failure
Partial discharge testing identifies internal voids, contamination, and insulation degradation that can lead to eventual dielectric breakdown.
Question 3: According to NETA standards, what minimum insulation resistance value (in MΩ) is generally considered acceptable for a 15 kV cable under NETA MTS acceptance criteria?
- 100 MΩ
- 500 MΩ
- 1,000 MΩ
- The standard references a comparison trend, not an absolute minimum (Correct answer)
Correct answer: The standard references a comparison trend, not an absolute minimum
NETA MTS recommends evaluating insulation resistance trends and comparisons rather than relying on a single absolute minimum value, as cable IR varies with temperature, length, and age.
Question 4: A tan delta (dissipation factor) test result that increases significantly with voltage indicates what cable condition?
- Healthy insulation with no defects
- Moisture ingress or aged/degraded insulation (Correct answer)
- Excessive conductor resistance
- Improper shield grounding
Correct answer: Moisture ingress or aged/degraded insulation
A rising tan delta with increasing voltage (tip-up effect) indicates moisture ingress, water treeing, or degraded insulation that increases dielectric losses under stress.
Question 5: When performing a very low frequency (VLF) hi-pot test on a cable, what frequency is typically used?
- 0.1 Hz (Correct answer)
- 1 Hz
- 10 Hz
- 60 Hz
Correct answer: 0.1 Hz
VLF testing typically uses 0.1 Hz, which reduces the reactive current demand compared to 60 Hz, making it practical for testing long cable runs with portable equipment.
Question 6: In a shield resistance test on a shielded cable, a high resistance reading compared to manufacturer specifications most likely indicates:
- Insulation degradation
- Conductor overheating
- Corrosion, damage, or a break in the metallic shield (Correct answer)
- Low dielectric constant of the insulation
Correct answer: Corrosion, damage, or a break in the metallic shield
Elevated shield resistance indicates corrosion, physical damage, or continuity breaks in the metallic shield layer, which can compromise fault current return paths.
Question 7: What does a negative reflected pulse polarity in a TDR trace indicate when compared to the incident pulse?
- Open-circuit fault
- Short-circuit or low-impedance fault (Correct answer)
- Normal cable termination
- Ground fault in the shield only
Correct answer: Short-circuit or low-impedance fault
A reflected pulse of opposite polarity to the incident pulse indicates a short-circuit or low-impedance anomaly, where the impedance drops below the characteristic cable impedance.
NETA Cable Testing and Diagnostics 4
During a time-domain reflectometry (TDR) test, a reflected pulse with the same polarity as the incident pulse indicates which type of fault?