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When a 35kV cable feeder tripped in Ruicheng County, Shanxi, the customer's fault recorder pointed to phase B. A field team used a cable fault tester to confirm the fault type and pre-locate the fault distance on site. This case study walks through the diagnosis, the measurement methods used and the conclusions drawn.

The customer reported that the fault recorder showed a phase B fault. To confirm the condition of the cable before energization, the team first measured the phase B phase-to-earth insulation resistance with a 5kV-range electronic megohmmeter. The reading was 2.5 GΩ at 5005V, which indicated that the phase B insulation was still normal — so an insulation test alone could not expose the fault, and a withstand (pressure) test was required.

A 5/50 operating-box test transformer was then used to carry out a DC withstand test. When phase B was raised to DC 40kV, a flashover breakdown discharge occurred. Based on this behaviour, the fault on phase B was judged to be a flashover high-resistance fault.

For pre-location, two complementary methods were applied: the low-voltage pulse method and the high-voltage direct flash method.

• Low-voltage pulse method: the full length of the cable was measured at 1896.2 m, producing a full-length waveform that served as the reference for the propagation path.
• High-voltage direct flash method: when the test voltage was raised to 40kV, the fault point formed a flashover discharge, and the resulting waveform gave a fault distance of 1629.8 m.

Analysing the two waveforms together made it possible to confirm both the total length of the feeder and the location of the fault, which is essential when the insulation resistance still reads normal.

• For very high flashover discharge faults, sampling with the direct flash method is recommended, because insulation and low-voltage measurements alone may not reveal the fault.
• Cable faults often occur at cable joints, especially joints that have been in service for more than five years.
• If the cable route is not clearly known, the route must be traced before fault location to avoid wasted time on site.