Cable Testing and Diagnostics Flashcards

During a time-domain reflectometry (TDR) test on a shielded power cable, a reflected pulse arrives with the same polarity as the incident pulse. What does this indicate about the impedance at the discontinuity?

Answer: The discontinuity presents a higher impedance than the cable's characteristic impedance

In TDR analysis, a reflected pulse with the same polarity (positive reflection coefficient) indicates that the impedance at the discontinuity is higher than the cable's characteristic impedance — such as an open circuit, a high-resistance splice, or a connector with increased impedance. A lower impedance (short, water ingress, conductor damage) produces an opposite-polarity reflection.

A very low frequency (VLF) withstand test is being performed on a 15 kV shielded cable rated 133% insulation level. Per NETA MTS standards, what is the correct VLF test voltage to apply for a maintenance (in-service) acceptance test?

Answer: 2.0 × U0 (phase-to-ground voltage)

Per NETA MTS and IEEE 400.2, the VLF withstand test voltage for maintenance (after-service) acceptance testing of medium-voltage cable is 2.0 × U0, where U0 is the phase-to-ground voltage. New installation acceptance testing uses a higher voltage of 3.0 × U0. Using the higher voltage on aged in-service cable risks damaging serviceable insulation.

During a tan delta (dissipation factor) diagnostic test on an aged XLPE cable, the technician observes that the tan delta value increases significantly as the test voltage is stepped up from 0.5 × U0 to 2.0 × U0. This voltage dependency (ΔTD) is most indicative of which condition?

Answer: Water treeing or electrochemical degradation within the insulation bulk

A voltage-dependent increase in tan delta (high ΔTD) is the classic signature of water trees in XLPE insulation. Water trees are resistive defects that dissipate more energy as field strength increases, raising the dissipation factor at higher voltages. Voids and cavities trigger partial discharge activity (detected by a different ΔTD pattern with PD correlation), while thermal carbonization and termination problems produce different diagnostic signatures.

A technician performing a DC hipot test on a medium-voltage cable observes that the leakage current, after initially stabilizing, begins to steadily increase over the final minutes of the hold period at full voltage. The cable passes the maximum leakage current threshold. What is the correct interpretation and action?

Answer: The cable should be flagged for further diagnostic testing despite passing the threshold, as increasing current trend indicates insulation degradation

NETA and IEEE 400 standards emphasize that the leakage current TREND is as diagnostic as the absolute value. A steadily increasing leakage current trend during the hold period — even if below the maximum threshold — is a warning sign of advancing insulation breakdown or thermal runaway developing under stress. NETA MTS explicitly states cables exhibiting an upward current trend should be investigated further, as the cable may fail under service conditions even though it technically passed the go/no-go threshold.

When performing a partial discharge (PD) test on a cable system using the IEC 60270 method, a technician measures apparent charge in picocoulombs (pC). A consistent PD source registers 500 pC at 1.0 × U0. What additional measurement is essential to properly characterize the severity and location of this defect for a condemn/continue decision?

Answer: The inception voltage (PDIV) and extinction voltage (PDEV) at which the PD activity starts and stops

The inception voltage (PDIV) and extinction voltage (PDEV) are critical for severity characterization. A defect with a PDIV near or below operating voltage (U0) is far more dangerous than one requiring elevated test voltage to activate. PDEV below U0 means PD will persist after voltage reduction — a condemn indicator. The 500 pC magnitude alone cannot determine if the cable is safe at operating voltage without knowing at what voltage the PD activates and extinguishes.

A shielded 5 kV cable system has been de-energized and grounded for maintenance. Before performing an insulation resistance (IR) test, the technician must calculate the polarization index (PI). The 1-minute IR reading is 2,800 MΩ and the 10-minute IR reading is 3,100 MΩ. How should the technician interpret this PI result per NETA MTS acceptance criteria?

Answer: The PI of approximately 1.1 indicates questionable insulation that warrants additional diagnostic testing before returning to service

The PI = 3,100 / 2,800 ≈ 1.11. Per NETA MTS (and IEEE 43), a PI below 1.0 is dangerous/unacceptable, 1.0–2.0 is questionable/marginal, 2.0–4.0 is good, and above 4.0 is excellent. A PI of 1.1 falls in the 'questionable' band regardless of the high absolute values, indicating that moisture, contamination, or insulation degradation is limiting the normal rise of IR with time. Additional diagnostics (tan delta, PD, or VLF) are warranted before returning to service.