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10kV Tunnel Cable Fault Detection: Locating a 619m Fault in an Expressway Tunnel Cable

2026-09-21

Project Overview

This case records the fault detection of a 10kV three-core armored cable serving the Shiquan Expressway tunnels in Ankang, Shaanxi, China. The cable had been in service for more than ten years between the Tunnel 1 and Tunnel 2 distribution rooms when it developed an insulation fault. On 14–15 September 2026 the Xi’an Xu&Hui field team used insulation resistance testing, low-voltage pulse and 21kV flashover pre-location plus precise pinpointing, then verified the cable after the fault was cut out.

Project Background

Item	Detail
Project name	Ankang Shiquan Expressway Tunnel 10kV Cable Fault Detection
Cable name	Shiquan Expressway Tunnel 1 to Tunnel 2 distribution room
Cable structure	Three-core, armored
Total length	1,307 m
Start point	Shiquan Tunnel 1 distribution room
End point	Shiquan Tunnel 2 distribution room
Test end	Tunnel 1 distribution room
Detection date	14–15 September 2026
Detection unit	Xi’an Xu&Hui Electromechanical Technology Co., Ltd.

On-Site Situation

The cable route was essentially clear, running out of the distribution room along the tunnel roadbed. The distance between the two tunnel portals is around 200 m. The cable was an old cable that had been in service for more than ten years, and the exact location of its joints was not known.

Detection Procedure and Results

Step	Test item	Result
1	Cable main insulation resistance test	At the 2500V range, Phase A insulation measured 0.1 MΩ, Phase B measured 0.1 MΩ, and Phase C was normal
2	Low-voltage pulse pre-location on Phase B	Analysis of the collected low-voltage pulse waveform showed the total cable length to be about 1,307 m, but no fault distance could be identified
3	High-voltage flashover pre-location on Phase B	With the high-voltage pulse generator peak voltage at about 21kV, the fault point was fully broken down. Analysis of the flashover waveform indicated the fault point was about 619.19 m from the test end
4	Precise fault pinpointing	The fault was located in the area between Tunnel 1 and Tunnel 2, about two meters from the Tunnel 2 portal.
5	Fault excavation	An audible discharge could be heard at a depth of about one meter. After manual excavation, the fault was confirmed to be a joint-type fault.
6	Insulation test after cutting both cable ends	After the fault point was cut out, the cable insulation tested normal. The cable was stripped and tested toward both ends; the resistance values were normal, and no other fault points were found .

On the low-voltage pulse test, a suspected joint was visible at around 630 m. Because the fault lay beside the expressway roadbed under several tens of centimeters of concrete, excavation was difficult, so the fault point was cut out and the cable tested toward both ends.

Equipment Used

5kV insulation resistance tester
XHGG502A1 cable fault locator
Operation box and test transformer
40/6 capacitor
XHDD503E cable fault pinpointer
XHGX507

Detection Team

Personnel: Li Cheng, Gao Fei
Detection time: 14–15 September 2026
Detection unit: Xi’an Xu&Hui Electromechanical Technology Co., Ltd.

Conclusion

By combining insulation testing, low-voltage pulse and flashover pre-location with precise pinpointing, the team isolated a joint-type fault about 619 m from the test end, between the two expressway tunnels — consistent with the suspected joint seen at around 630 m on the low-voltage pulse waveform. After the fault was cut out and the cable tested toward both ends, the insulation was normal and no other fault points remained, confirming the cable could be restored to service.

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