Wet Joint and Single-Phase Earth Fault on an 11 kV XLPE Cable
An 11 kV, three-core, 95 sq mm XLPE cable runs 600 m from a ring main unit to a distribution transformer. After two days of heavy rain, the 11 kV feeder trips on earth fault. The cable is isolated, proved dead, discharged and earthed. A 5 kV insulation resistance test gives: red to earth 0.3 MΩ, yellow to earth 480 MΩ, blue to earth 520 MΩ; red to yellow and red to blue both above 1000 MΩ; yellow to blue above 1000 MΩ. A time-domain reflectometer is not available, but a cable route map shows a straight joint at 180 m and another at 410 m. The 180 m joint is in a shallow inspection pit that was full of muddy water. The cable sheath at that pit shows a small nick. The lineman has a heat-shrink straight-joint kit for 11 kV, a hacksaw, a cable knife, a propane torch, a megger and portable earthing leads.
1. Interpret the test results. What fault type and location do they indicate, and what should be done before digging?
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The red phase has 0.3 MΩ to earth, while yellow and blue have hundreds of megohms and the phase-to-phase readings are above 1000 MΩ. That is a single red-phase earth fault, not a phase-to-phase fault. The 180 m joint is the prime suspect because it sits in a water-filled pit and the sheath is nicked; water and sheath damage allow moisture to reach the insulation. The 410 m joint cannot be excluded, but it is less likely. Before digging, obtain the permit and cable route drawing, prove the cable dead and keep portable earths applied. Use a cable locator and hand-dig a trial pit; do not use a mechanical excavator until the cable is visibly located. If a TDR is unavailable, a surge or loop test can be arranged, but visual and insulation evidence may justify opening the 180 m pit first. Do not re-energize to burn the fault.
2. Describe the repair sequence at the joint, including how you would decide whether to cut back the cable.
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Pump out the pit, support the cable and cut out the faulty joint. Remove the sheath and insulation progressively until the insulation is dry and a repeat insulation test is acceptable; as a guide, cut back at least 0.5 m each side and more if moisture has travelled along the conductor. If water has entered the stranded conductor, replace the affected cable section rather than jointing wet conductor. Fit the heat-shrink straight joint strictly to the kit instructions: clean the insulation, apply stress control, centre the joint, shrink evenly with the torch, and bond the metallic sheath and earth. Protect the joint from mechanical damage and backfill with selected sand and a warning tile. Keep a fire watch with the propane torch and ventilate the pit.
3. What tests and records are required before re-energizing, and what residual risks remain?
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After the joint, repeat the 5 kV insulation resistance test on each phase to earth and between phases, check phase identification and continuity, and test the sheath and earth bonding. Follow the utility's high-voltage test procedure; if a DC or very-low-frequency test is required, apply it with the cable isolated and all ends earthed afterwards to discharge. Record ambient conditions, test voltages, readings and joint location. Re-energize only after the permit is cancelled and earths are removed. Residual risks remain: moisture may migrate from an unnoticed section, the new joint may have a hidden void, and the pit may flood again. Improve pit drainage, schedule an early recheck and thermal-scan the joint under load.