NETA Cable Testing and Diagnostics 5 — Questions and Answers
Question 1: During a cable dielectric absorption (DA) test, a DA ratio less than 1.0 most likely indicates:
- Excellent insulation condition
- Insulation that is wet, contaminated, or severely degraded (Correct answer)
- Normal dielectric response
- Capacitive discharge artifact
Correct answer: Insulation that is wet, contaminated, or severely degraded
A dielectric absorption ratio below 1.0 means current increased over time rather than decreased, strongly indicating wet, conductive, or severely degraded insulation.
Question 2: Which cable diagnostic test method uses a sinusoidal voltage at 0.1 Hz and simultaneously measures the tan delta to assess insulation condition?
- DC hi-pot test
- VLF withstand test with tan delta monitoring (Correct answer)
- Impulse test
- Megohmmeter step voltage test
Correct answer: VLF withstand test with tan delta monitoring
VLF tan delta testing applies 0.1 Hz sinusoidal voltage while continuously measuring the dissipation factor to detect insulation aging and water treeing.
Question 3: When a cable section has been flooded with water, how does this primarily affect megohmmeter insulation resistance readings?
- IR increases significantly
- IR remains unchanged
- IR decreases dramatically, often below 1 MΩ (Correct answer)
- IR fluctuates randomly with no trend
Correct answer: IR decreases dramatically, often below 1 MΩ
Water creates conductive paths through or along insulation, drastically lowering insulation resistance and often producing readings well below 1 MΩ.
Question 4: According to NETA MTS, the temperature correction factor for insulation resistance tests is applied to normalize readings to which reference temperature?
- 0°C
- 20°C (Correct answer)
- 40°C
- 60°C
Correct answer: 20°C
NETA MTS specifies correcting insulation resistance measurements to a reference temperature of 20°C to allow valid comparisons between tests taken at different ambient conditions.
Question 5: What is the significance of a 'kink' or sudden step in the TDR trace at a location other than the far end of the cable?
- Normal splice attenuation
- A fault, splice, or impedance discontinuity at that distance (Correct answer)
- End-of-cable reflection
- Instrument calibration artifact
Correct answer: A fault, splice, or impedance discontinuity at that distance
A step or kink in the TDR waveform at a mid-cable location indicates a fault, splice, connector, or impedance change at the corresponding distance from the test point.
Question 6: A cable's polarization index (PI) is calculated as the ratio of insulation resistance at 10 minutes to insulation resistance at which other time interval?
- 30 seconds
- 1 minute (Correct answer)
- 5 minutes
- 15 minutes
Correct answer: 1 minute
The polarization index is the ratio of the 10-minute IR reading divided by the 1-minute IR reading, with a ratio above 2.0 generally indicating acceptable insulation.
Question 7: When performing a DC hi-pot test on a cable, why does NETA and IEEE 400 now discourage routine DC hi-pot testing on aged XLPE cables?
- DC tests take too long to perform
- DC voltage can cause space charge accumulation in XLPE insulation, worsening pre-existing water trees (Correct answer)
- DC equipment is more expensive than AC equipment
- DC tests cannot detect partial discharges
Correct answer: DC voltage can cause space charge accumulation in XLPE insulation, worsening pre-existing water trees
DC voltage applied to aged XLPE cables with water trees causes space charge buildup that can accelerate insulation damage and may cause failure shortly after test voltage is removed.
NETA Cable Testing and Diagnostics 5
During a cable dielectric absorption (DA) test, a DA ratio less than 1.0 most likely indicates: