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400V Low-Voltage Cable Fault Detection at Xianyang Normal University Using Impulse and Acoustic Pinpointing

2026-08-17

This case documents a 400V low-voltage power cable fault detection project completed at Xianyang Normal University on July 30, 2026. The cable under test was a YJV 3×120+1×95 armored low-voltage cable laid in 2003, routed from the box-type transformer low-voltage side to the main teaching building power distribution room via direct burial, iron conduit, and cable trench, with buried depth up to 2 m and a joint at an unknown location.

Project Information

Item	Details
Project Name	400V Low-Voltage Cable Fault Detection at Xianyang Normal University
Test Personnel	Lead: Li Hao; Member: Du Chaochao
Test Date	July 30, 2024
Cable Type	YJV 3×120+1×95, 400V armored low-voltage cable
Cable Length	Labeled 260 m; measured 253.6 m
Instruments Used	Distance measuring host 502 + 501A, HV impulse transmitter 510-12, pinpoint locator 503
Fault Type	Leakage low-resistance fault

Insulation Diagnosis

Using a 500V electronic megohmmeter, insulation resistance was measured between all phases (A, B, C), neutral (N) and ground. All three phases to neutral measured 0.1 MΩ (at 500V), while phase-to-phase and phase-to-ground insulation measured 0 MΩ (at 5V), confirming a leakage low-resistance fault.

Coarse Fault Location

Coarse measurement combined low-voltage pulse and high-voltage impulse flash methods. The pulse waveform measured the full cable length at 253.6 m, with a short circuit initially suspected around 150 m. The box-transformer-side impulse flash waveform fell in the blind zone, while the opposite-side (distribution room) waveform gave a fault distance of about 80 m.

Precise Fault Pinpointing

The cable path was clear, since the original laying personnel were on site. Main insulation fault location used audio and acoustic-magnetic synchronization methods.

Distribution room side: After moving the HV equipment to the distribution room, the fault distance became about 80 m. Clear discharge sound was heard at the stairway with the 503 locator, decreasing clearly ahead of and behind that point. The stairway cable was excavated and found inside iron conduit, so the fault could not be located quickly. An endoscope initially found no damage, but after re-pressurizing to about 4 kV, a clear discharge flash appeared about 1 m from the break. Re-excavation confirmed the fault point.
Box-transformer side: Re-pressurization required grounding improvements; after installing ground rods, voltage was raised to 12 kV. Acoustic-magnetic synchronization with the 503 and 503E, plus the 503E step-voltage method, located the fault under the box-transformer concrete foundation (suspected at the cable bend exiting the transformer). Excavation confirmed external force damage.

Key Takeaways

Cables with multiple fault points may require pressurizing from both sides, because the high-voltage breakdown signal cannot reach a more distant fault point, so only the nearest fault can be detected at one time.
For conduit cables, a ground wire or bare copper wire can be inserted into the pipe to check for breakdown marks, or an endoscope can inspect the cable inside the pipe.
Discharge sound from conduit cables is weak; use the discharge echo and sound intensity to judge the fault location.
When using HV equipment with on-site power, ensure no other weak-current equipment shares the same supply to avoid damage.