Power Cable Test Methods Flashcards

During a very low frequency (VLF) withstand test on a 15 kV shielded cable, the test is halted after 3 minutes when the leakage current suddenly drops to near zero. What is the most likely cause?

Answer: The cable has developed a complete dielectric breakdown, disconnecting the current path

A sudden drop of leakage current to near zero mid-test is a classic indicator of complete dielectric breakdown creating an open circuit condition. When the insulation fully ruptures, the fault carbonizes and the arc extinguishes, leaving an open gap. Capacitive stabilization would show a gradual taper, not a sudden drop. Partial discharge suppression is a different phenomenon that doesn't cause near-zero leakage. Current limiting would typically produce a plateau, not a drop to zero.

A technician performs tan delta (dissipation factor) testing on a 25 kV XLPE cable and obtains the following tip-up values: 0.5Uo = 0.8×10⁻³, 1.0Uo = 0.9×10⁻³, 1.5Uo = 1.4×10⁻³. What does the significant increase at 1.5Uo most likely indicate?

Answer: Water treeing activity in the insulation, with partial discharge onset above 1.0Uo

A tan delta tip-up — where the dissipation factor increases significantly between 1.0Uo and 1.5Uo — is a hallmark indicator of water tree degradation combined with partial discharge (PD) onset. Water trees create conductive paths that become more active under higher field stress. A flat or very slight increase would be expected in healthy XLPE. Thermal runaway would require sustained operation, not a brief test. Calibration drift would not selectively affect only the 1.5Uo reading.

When applying a DC high-potential test to an oil-paper (PILC) cable, NETA standards require that the test voltage be applied in steps. What is the primary technical reason for stepped voltage application rather than a single ramp to maximum voltage?

Answer: To allow absorption and polarization currents to decay so that each step's leakage current reading reflects true conduction current

DC testing of oil-paper cable involves significant capacitive and dielectric absorption currents that take time to decay. By holding at each voltage step and waiting for currents to stabilize, the technician ensures the measured leakage current reflects true conduction through the insulation rather than transient absorption/polarization components. This makes the readings diagnostically meaningful. Schering bridges are used for AC tan delta, not DC step testing. Thermal effects on terminations are a secondary concern, and utility relay timing is unrelated to diagnostic test protocol.

A technician is performing time domain reflectometry (TDR) on a 2,000-foot shielded power cable and observes a reflected pulse at 1,200 feet with a positive polarity reflection coefficient. What type of fault or anomaly does a positive reflection coefficient indicate?

Answer: A high-impedance discontinuity such as a poor splice, open circuit, or impedance increase

In TDR, the polarity of the reflected pulse indicates the nature of the impedance change. A positive reflection coefficient means the impedance at the fault location is higher than the characteristic cable impedance — indicating an open circuit, poor splice with increased resistance, or other high-impedance discontinuity. A negative reflection coefficient indicates a low-impedance fault (short circuit, ground fault). An impedance-matched termination produces zero reflection. Capacitive coupling from parallel cables produces characteristic sinusoidal artifacts, not a discrete reflected pulse.

During a 60-Hz AC withstand test on a newly installed 35 kV cable system, partial discharge (PD) measurements are taken simultaneously. The PD inception voltage (PDIV) is measured at 22 kV phase-to-ground. The cable's rated phase-to-ground voltage is 20.2 kV. What is the correct assessment?

Answer: The cable fails; PDIV should exceed 1.1× rated voltage, meaning PDIV must be at least 22.2 kV

NETA acceptance criteria for PD on medium-voltage cables requires that PDIV exceed 1.1× the rated phase-to-ground voltage (U0). For a 35 kV cable, U0 = 35/√3 ≈ 20.2 kV, and 1.1 × 20.2 = 22.2 kV. A measured PDIV of 22 kV falls just below this threshold, indicating marginally insufficient insulation quality — the cable fails. Simply exceeding U0 is not the criterion. The 1.5× factor applies to withstand voltage levels, not PDIV. PDEV is a supplementary metric, not the sole pass/fail parameter.

A 5 kV shielded cable fails a DC high-pot test at 80% of the applied maximum voltage. Post-failure analysis reveals that the breakdown occurred at the cable termination, not along the cable body. Which installation/design factor most likely contributed to this failure location?

Answer: Inadequate stress relief or improper installation of the termination stress cone, causing electric field concentration at the shield cutback

Cable terminations are the most common location for dielectric failure in medium-voltage shielded cable systems, and the leading cause is improper stress cone installation or inadequate stress relief at the shield cutback point. When the metallic shield is cut back, the electric field lines concentrate intensely at the shield edge (stress point). A properly installed stress cone (heat-shrink, cold-shrink, or hand-taped) redistributes this field. Incorrect positioning, air voids, or omission of the stress cone creates localized field enhancement exceeding dielectric strength. Bend radius and thermal delamination are cable body failure modes, and ground loop induction does not cause dielectric breakdown.