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Project Overview

This report documents a 110kV underground power cable outer sheath fault diagnostic and location testing project executed in Pune, Maharashtra, India on January 30, 2024. The field engineering team — Ye Peng, Li Yifu, and Yang Liuen — carried out the investigation employing industry-standard diagnostic protocols and advanced fault location equipment to precisely identify and verify an outer sheath insulation failure.

Site Conditions & Cable Specifications

Insulation Resistance Baseline & Testing Equipment

Initial measurements were performed using a 2.5kV electronic megohmmeter (Model XHMR-5000V). Phase B registered 0 MΩ to the outer sheath — a solid metallic short circuit confirmed by DC multimeter at 0 Ω. Phases A and C measured 16 MΩ each, within acceptable operational margins, isolating Phase B as the sole fault-bearing conductor. The complete equipment suite deployed included: XHMR-5000V Megohmmeter (insulation testing), Model 523 & Model 512-10 (high-voltage sources), Burn-Through Bridge (pre-location and withstand testing), Model 501A Host, and Pipeline Locator with A-Frame accessory (pinpointing).

Diagnostic Procedure & Findings

Step-Voltage Survey

Initial fault pinpointing via the Model 523 step-voltage method yielded limited results — signal intensity was pronounced before the first drilling point but diminished sharply thereafter with no reverse polarity. A subsequent A-Frame survey showed signal strength dropping to near-zero beyond the third drilling point, preventing definitive fault isolation through surface potential gradient measurements.

High-Voltage Acoustic Discharge Detection

The Model 512-10 high-voltage source was applied to Phase B under sustained elevated potential. Acoustic discharge emissions were successfully detected at approximately 37 meters from the test terminal, providing the first actionable fault location indication.

Bridge Pre-Location & Post-Repair Verification

The Burn-Through Bridge estimated the fault at approximately 47 meters from the test terminal. Following fault identification and excavation, a voltage withstand test was performed on Phases A and C outer sheaths, with both phases passing successfully.

Key Engineering Insights

• Zero-resistance faults suppress surface potential gradients, rendering step-voltage and A-Frame methods ineffective. Alternative excitation strategies are essential for such fault conditions.
• High-voltage acoustic discharge is a viable detection alternative: Sustained high-potential excitation successfully induced audible discharge at the fault location, even under a zero-ohm fault condition.
• Multi-method corroboration ensures confidence: Despite a 10-meter divergence between bridge pre-location (47 m) and acoustic pinpointing (37 m), the convergence of independent methods provided sufficient certainty for targeted excavation.
• Selective sheath integrity was validated: Post-repair withstand testing confirmed the fault was isolated to Phase B alone, ruling out systemic insulation degradation.

Conclusion

The 110kV cable outer sheath fault in Pune, Maharashtra was successfully located at approximately 37 meters from the test terminal using a combined high-voltage acoustic discharge and bridge-based pre-location approach. The zero-resistance fault condition was overcome through sustained high-potential excitation, and Phases A and C retained full sheath insulation integrity. The engineering team recommends the high-voltage acoustic discharge method as a standard supplementary procedure for all future zero-resistance outer sheath fault investigations.