NETA Power Cable Test Methods 3 — Questions and Answers
Question 1: What does a Dielectric Absorption Ratio (DAR) value greater than 1.25 typically indicate for cable insulation?
- Questionable insulation condition
- Good insulation condition (Correct answer)
- Failed insulation
- Saturated moisture ingress
Correct answer: Good insulation condition
A DAR above 1.25 indicates the insulation resistance is still rising after 60 seconds, suggesting acceptably dry and clean insulation.
Question 2: When using the step voltage test on a cable, what does a sharp increase in leakage current at a specific voltage step indicate?
- Normal capacitive charging behavior
- Incipient or existing insulation weakness at that stress level (Correct answer)
- Proper cable shielding
- Acceptable surface leakage
Correct answer: Incipient or existing insulation weakness at that stress level
A disproportionate current rise at a particular voltage step indicates the insulation has a defect that becomes active at that stress level.
Question 3: Which test is considered non-destructive and is recommended by IEEE 400 as a preferred diagnostic for medium-voltage XLPE cables?
- DC hipot
- 60 Hz AC withstand
- VLF tan delta (dissipation factor) (Correct answer)
- Megohm IR test only
Correct answer: VLF tan delta (dissipation factor)
VLF tan delta (loss angle) measurement is non-destructive and detects water treeing and insulation aging in XLPE cables without risk of damage.
Question 4: Before performing any hi-pot test on a cable, what must always be done first?
- Apply the full test voltage immediately
- Perform an insulation resistance test and verify cable is de-energized and isolated (Correct answer)
- Ground only one end of the cable
- Remove the cable shield completely
Correct answer: Perform an insulation resistance test and verify cable is de-energized and isolated
Verifying the cable is de-energized, isolated, and performing a preliminary IR test establishes baseline condition and ensures safety before applying high voltage.
Question 5: A shielded 15 kV XLPE cable fails a VLF withstand test. What is the most likely root cause indicated by this result?
- Conductor resistance is too high
- Insulation has degraded below minimum withstand strength (Correct answer)
- The cable is too long for VLF testing
- The shield continuity is open
Correct answer: Insulation has degraded below minimum withstand strength
Failure of a VLF withstand test means the insulation cannot sustain the required dielectric stress, indicating degradation such as water trees or thermal damage.
Question 6: What effect does increasing cable length have on the charging current required during a 60 Hz AC hipot test?
- No effect; charging current is independent of length
- Charging current decreases proportionally
- Charging current increases proportionally (Correct answer)
- Charging current becomes DC-like
Correct answer: Charging current increases proportionally
Cable capacitance increases proportionally with length, so longer cables require higher charging current from the 60 Hz test set.
Question 7: When testing an unshielded medium-voltage cable with a megohmmeter, which measurement correction factor is most important to apply?
- Length correction
- Temperature correction to 20°C (Correct answer)
- Frequency correction
- Phase angle correction
Correct answer: Temperature correction to 20°C
Temperature significantly affects insulation resistance readings and must be corrected to the standard 20°C baseline for meaningful comparison.
NETA Power Cable Test Methods 3
What does a Dielectric Absorption Ratio (DAR) value greater than 1.25 typically indicate for cable insulation?