NETA Cable Testing and Diagnostics 1 — Questions and Answers

Question 1: A DC hi-pot test on a shielded power cable measures:

The AC dielectric strength under operating conditions
The cable's insulation integrity by applying DC overvoltage and monitoring leakage current (Correct answer)
The cable's impedance and capacitance per unit length
The continuity of the cable shield/sheath

Correct answer: The cable's insulation integrity by applying DC overvoltage and monitoring leakage current

DC hi-pot tests apply a DC overvoltage (per IEEE 400 or cable manufacturer specifications) to cable insulation and monitor leakage current. Excessive current, current instability, or sudden increases indicate insulation defects.

DC hi-pot testing of power cables applies a high DC voltage (typically 3 to 4 times rated DC equivalent) and measures leakage current. Test acceptance criteria include: stable or decreasing leakage current (a good dielectric shows decreasing current as polarization stabilizes), no sudden current jumps indicating insulation failure or ionization, and total current below manufacturer limits. However, DC testing of extruded XLPE and EPR cables has largely been replaced by AC VLF testing because DC testing can create space charge accumulation in XLPE that causes failures after the test.

Question 2: The VLF (Very Low Frequency) test for medium-voltage cables uses a test frequency of approximately:

60 Hz (same as power frequency)
0.1 Hz, which reduces the charging current requirement by a factor of 600 compared to 60 Hz testing (Correct answer)
1000 Hz for faster testing
DC with a ripple component at low frequency

Correct answer: 0.1 Hz, which reduces the charging current requirement by a factor of 600 compared to 60 Hz testing

VLF tests at 0.1 Hz (one-tenth of a cycle per second), which reduces the capacitive charging current by a factor of 600 compared to 60 Hz (since Ic = 2*pi*f*C*V). This makes portable test equipment practical for field cable testing.

Power cables have significant capacitance. At 60 Hz, the charging current can be tens of amperes for long cables, requiring very large test equipment. At 0.1 Hz, the charging current is reduced by the ratio 60/0.1 = 600 times, making compact portable test sets practical. IEEE 400.2 covers VLF testing of shielded power cables. VLF test voltages are typically 2 to 3 times rated phase-to-phase voltage. Unlike DC testing, VLF AC testing exercises the full AC stress cycle in the insulation, making it more effective at detecting moisture trees, oxidized treeing channels, and mechanical damage in XLPE cables.

Question 3: Time Domain Reflectometry (TDR) is used on power cables to:

Measure the insulation resistance along the cable length
Locate the position of faults, splices, and discontinuities by measuring the travel time of reflected pulses (Correct answer)
Test the cable's high-frequency attenuation characteristics
Measure the capacitance of the cable for energy calculation

Correct answer: Locate the position of faults, splices, and discontinuities by measuring the travel time of reflected pulses

TDR sends a fast electrical pulse down the cable and measures the time for reflections to return from impedance discontinuities (faults, splices, terminations). Using the cable's propagation velocity, the distance to each discontinuity is calculated.

Time Domain Reflectometry (TDR) injects a fast rise-time pulse into the cable and monitors reflections returning from impedance changes. The distance to an anomaly = (velocity x time)/2, where velocity for typical cable is 50 to 70% of the speed of light. Different types of discontinuities produce characteristic reflections: open circuits reflect positive pulses, short circuits reflect negative pulses, and splices show characteristic double reflections. TDR is the primary method for locating faults and mapping cable infrastructure.

Question 4: The dielectric loss tangent (tan delta) test on cables measures:

The electrical breakdown strength of the insulation at high voltage
The ratio of resistive (loss) current to capacitive current, indicating insulation quality and aging (Correct answer)
The capacitance of the cable per unit length
The corona inception voltage of the cable insulation

Correct answer: The ratio of resistive (loss) current to capacitive current, indicating insulation quality and aging

Tan delta (dissipation factor) measures the ratio of power loss (resistive current) to reactive power (capacitive current) in the insulation. Values increase with moisture contamination, chemical degradation, or water treeing.

The loss tangent (tan delta) of cable insulation measures the quality of the dielectric. For new XLPE or EPR insulation, tan delta is extremely low (less than 1 times 10^-4 at low voltage, relatively flat with voltage). As insulation degrades from water treeing, thermal aging, or moisture contamination, tan delta increases and becomes voltage-dependent (tip-up effect). IEEE 400.4 covers tan delta testing of shielded cables. Combined VLF tan delta testing has become the recommended diagnostic tool for assessing XLPE cable condition, replacing DC hi-pot for most medium-voltage cable testing.

Question 5: Cable jacket testing (sheath integrity testing) using DC current verifies:

The integrity of the phase conductor insulation
That the outer jacket/sheath has no breaches allowing moisture to reach the metallic shield (Correct answer)
The ampacity of the cable under burial conditions
The bond between jacket and insulation in the cable construction

Correct answer: That the outer jacket/sheath has no breaches allowing moisture to reach the metallic shield

DC sheath testing applies low DC voltage (typically 1 to 5 kV) between the metallic shield and the outer jacket, measuring insulation resistance to verify no holes, cuts, or abraded areas allow moisture ingress that could cause shield corrosion and insulation moisture treeing.

For shielded cables with an outer polymeric jacket, the jacket integrity test verifies the outer jacket has no damage exposing the metallic shield to moisture and soil. Test voltage (typically 1 to 5 kV DC per IEEE 400 and manufacturer recommendations) is applied between the metallic shield and ground or a water test pan, measuring current to the shield through any jacket defects. Damaged jackets allow moisture to reach the shield, causing corrosion and electrochemical degradation of the insulation-shield interface, accelerating water tree growth. Cable jacket integrity testing is performed annually on direct-buried cables and after any excavation near cable routes per NETA specifications.

Question 6: Partial discharge (PD) measurement on a medium-voltage cable system identifies:

The total insulation resistance of the cable run
Localized defects in the insulation or accessories (splices, terminations) where PD activity is concentrated (Correct answer)
The cable's rated voltage class and insulation level
Short-circuit current capacity of the cable

Correct answer: Localized defects in the insulation or accessories (splices, terminations) where PD activity is concentrated

PD measurement on cables detects and locates localized insulation defects — voids, contamination, water trees at critical size — primarily at terminations and splices where field enhancement occurs, before these progress to complete insulation failure.

Partial discharge measurement on cable systems uses high-frequency current sensors to detect the high-frequency current pulses generated by PD events in the insulation. PD is most commonly found at cable terminations, splices where factory insulation is disturbed during installation, and within the cable insulation at void or water tree sites. IEEE 400.3 covers PD testing of cable systems. Calibrated PD measurement allows comparison to baseline values and assessment of insulation condition and expected remaining life.

NETA Cable Testing and Diagnostics 1

A DC hi-pot test on a shielded power cable measures: