Cable Testing and Diagnostics Flashcards

During a time-domain reflectometry (TDR) test on a shielded power cable, you observe a reflected pulse with the same polarity as the incident pulse. What type of fault does this indicate?

Answer: An open-circuit fault or high-impedance discontinuity

In TDR testing, a reflected pulse with the same polarity as the incident pulse indicates an open-circuit or high-impedance discontinuity. This occurs because the impedance at the fault is higher than the characteristic impedance of the cable, causing a positive reflection coefficient. A low-resistance short produces a negative (opposite polarity) reflection due to the impedance drop.

A 15 kV XLPE cable passes its DC hipot test at 125% of rated voltage but fails within 6 months of returning to service. Which phenomenon best explains this failure pattern?

Answer: Space charge accumulation and field enhancement accelerating treeing in XLPE insulation

DC hipot testing on XLPE cables is known to be unreliable and potentially damaging because DC voltage causes space charge accumulation within the cross-linked polyethylene insulation. When the cable is returned to AC service, the trapped space charges create localized electric field enhancements that can accelerate water treeing and electrical treeing, leading to premature failure. This is why NETA and IEEE standards now recommend VLF (Very Low Frequency) or tan delta testing for XLPE cables instead of DC hipot.

When performing a very low frequency (VLF) tan delta (dissipation factor) diagnostic test on a medium-voltage cable, a technician observes that the tan delta value increases significantly between the 0.5U₀ and 1.0U₀ test voltage steps but stabilizes at 2.0U₀. What is the most likely interpretation?

Answer: Localized defects or moisture ingress are causing partial discharge inception between those voltage levels

A significant increase in tan delta between 0.5U₀ and 1.0U₀ that stabilizes at higher voltages is characteristic of localized defects, moisture ingress, or partial discharge (PD) activity. PD inception voltage often falls in this mid-range for degraded cables. The tan delta 'tip-up' between voltage steps is a key diagnostic indicator in NETA and IEEE 400.2 protocols — a flat tan delta across all voltage steps indicates healthy insulation, while voltage-dependent rise signals deterioration requiring further investigation or rejection.

A 5 kV shielded cable circuit measures an insulation resistance of 2,500 MΩ immediately after applying test voltage, which increases to 18,000 MΩ after 10 minutes. The dielectric absorption ratio (DAR) is calculated at 1.6. What is the most accurate assessment?

Answer: The insulation is in good condition; the DAR above 1.25 confirms adequate dryness and cleanliness

The Dielectric Absorption Ratio (DAR) is the ratio of the 60-second to 30-second insulation resistance readings. A DAR of 1.6 exceeds the NETA acceptance criterion of 1.25 minimum, indicating good insulation quality — dry, clean, and free of significant contamination. Contaminated or wet insulation produces DAR values near 1.0 because conductivity remains constant throughout the absorption period. While the polarization index (10-min/1-min ratio) is more commonly used for rotating machinery, DAR is a valid and accepted metric for cable insulation assessment per NETA MTS standards.

During partial discharge (PD) mapping on a 35 kV cable system, a technician detects PD activity measured at 350 pC occurring at a phase angle of approximately 90° and 270° on the AC cycle (peaks of the voltage waveform). Which defect morphology does this phase-resolved PD pattern most strongly suggest?

Answer: Internal voids or delamination within the bulk insulation

Phase-resolved PD (PRPD) patterns are a powerful diagnostic tool. Internal voids within solid dielectric insulation characteristically produce PD events clustered near the positive and negative voltage peaks (90° and 270°), because the electric field across a void reaches its maximum at the voltage peaks. Surface tracking and corona discharges typically occur during the rising and falling edges of the voltage waveform (near 0° and 180° crossings), producing a distinctly different PRPD signature. This pattern recognition is essential for NETA-level cable diagnostic interpretation.

A 600 V, 500 kcmil copper conductor cable installed in an industrial facility measures a DC resistance of 0.034 Ω/1000 ft at 68°F during acceptance testing. The published resistance for 500 kcmil copper at 75°C is 0.0258 Ω/1000 ft. Correcting the field measurement to 75°C using the copper temperature coefficient (α = 0.00393/°C), what conclusion should the technician draw?

Answer: The conductor has significant cross-sectional area reduction indicating broken strands or manufacturing defect

Correcting 0.034 Ω/1000 ft from 20°C (68°F) to 75°C: R₇₅ = R₂₀ × [1 + 0.00393×(75−20)] = 0.034 × [1 + 0.2162] = 0.034 × 1.2162 ≈ 0.0415 Ω/1000 ft. The corrected value of 0.0415 Ω/1000 ft is approximately 60% higher than the published 0.0258 Ω/1000 ft for 500 kcmil. NETA MTS allows a maximum of 110–115% of the published resistance. A value this elevated strongly indicates significant loss of conductor cross-section from broken strands, improper conductor size, or a manufacturing defect — the cable should be rejected and investigated.