Underground transmission cable jointing in Korea covers jointing the underground transmission cables installed in ducts and tunnels, building the sheath earthing system and testing the cables. The competency unit applies to 154 kV and 345 kV cables, where a single poor joint can bring down the whole circuit, so every step includes checks on sealing and insulation. This guide covers three stages — jointing, earthing and bonding, and testing.
Contents
- Common Pre-Work Review
- 1. Cable Jointing
- 2. Earthing and Sheath Bonding
- 3. Cable Testing
- OF Cable vs XLPE (CV) Cable
- FAQ
- Reference Standards
- Summary
Common Pre-Work Review
Before work, review the design documents, method statement, construction specifications and KEPCO standards and field manuals, and check bridge weight limits and underpass height limits along the cable delivery route. Prepare pulling equipment (guide pipes, shovels, pulling eyes, jacks, chain blocks, winches, rollers) and jointing equipment for each cable type: compression, vacuum and plumbing (torch) equipment for oil-filled (OF) cable, and compression and housing assembly equipment for XLPE (CV) cable. Test equipment includes a partial discharge tester, AC withstand tester and high-voltage tester; measuring equipment includes a gas analyzer, earth resistance tester, insulation tester, multimeter and discharge device.
1. Cable Jointing
- Remove the anti-corrosion jacket to cut the cable’s metallic sheath.
- Make an offset so thermal expansion and contraction do not stress the joint.
- Carry out base plumbing to stop insulating oil leaking from the joint casing (OF cable).
- Expose the conductor and pencil the insulation to mark the jointing length.
- Compress the connector sleeve for each cable size using a compression pump.
- Reinforce joint insulation with paper wrapping and insert the housing.
- Seal the joint by plumbing, then carry out vacuum treatment and oil filling.
- Install earthing for safety and apply anti-corrosion treatment to protect the metallic sheath.
2. Earthing and Sheath Bonding
- Confirm material specifications and methods for the earthing system at each section and location from the installation drawings.
- To balance line constants, check cross-bonding lead connections, then apply anti-corrosion treatment after sleeve compression and bolt tightening.
- To protect insulating joints, check that sheath voltage limiters operate and fit them.
- Keep each earthing conductor clear of support hardware to secure insulation and safety.
Sheath voltage limiters protect the underground line from surges and abnormal voltages. Joints include straight-through, insulating (sectionalizing) and terminating types.
3. Cable Testing
- Wear safety equipment.
- Set the test zone and keep unauthorized people out.
- Carry out oil-flow resistance and gas constant tests for each section as specified.
- Measure insulation resistance before the AC withstand test to confirm soundness.
- Carry out the AC withstand test and an on-load partial discharge (PD) test to confirm insulation.
- Carry out line constant tests — conductor resistance, inductance and capacitance — as specified.
OF Cable vs XLPE (CV) Cable
| OF cable | XLPE (CV) cable | |
|---|---|---|
| Construction | Paper insulation impregnated with oil for insulation and cooling | Cross-linked polyethylene solid insulation |
| Jointing equipment | Compression, vacuum and plumbing equipment | Compression and housing assembly equipment |
| Jointing focus | Plumbing to prevent oil leaks; vacuum and oil filling | Housing insertion and insulation reinforcement |
| Tests | Oil-flow resistance, gas constant, insulation resistance, AC withstand, PD, line constants | Insulation resistance, AC withstand, PD, line constants |
Terms: pencilling marks the jointing length; base plumbing (좌연공) seals OF cable joints against oil leaks; snake width is the slack allowed in advance for thermal expansion.
FAQ
Why is cross bonding used?
Sheaths are transposed between sections to reduce circulating currents in the metallic sheath and balance line constants. After connection, check sleeve compression, bolt tightness and anti-corrosion treatment.
Why measure insulation resistance before the AC withstand test?
Confirming basic soundness first avoids unnecessary damage when high voltage is applied in the withstand test.
What does a partial discharge test show?
It measures partial discharges caused by small defects inside joints and cable insulation, confirming insulation condition.
Reference Standards
- Electricity Business Act (전기사업법) — law.go.kr
- Electrical Safety Management Act (전기안전관리법) — law.go.kr
- Electric Technology Management Act (전력기술관리법) — law.go.kr
- Electrical Construction Business Act (전기공사업법) — law.go.kr
- Occupational Safety and Health Act (산업안전보건법) — law.go.kr
- Korea Electro-technical Code (KEC, 한국전기설비규정) — law.go.kr
- KEPCO design standards; field practice manuals for transmission and distribution work; Korean Industrial Standards (KS)
- NCS competency unit Underground Transmission Cable Jointing (지중송전케이블접속공사, 1901070222_22v4; NCS is Korea’s National Competency Standards) — ncs.go.kr
Summary
The most common mistakes in cable jointing are poor control of moisture and contamination in the jointing environment and unchecked cross-bonding connections. Jointing depends on offset, sealing and insulation reinforcement; bonding on cross-bonding and clearances; and testing on the sequence insulation resistance → withstand → partial discharge → line constants. Duct and tunnel work is covered in Underground Transmission Civil Works. For pre-construction preparation, see Transmission Line Construction Planning. A Korean version of this guide is available here.
This article restructures the Korean NCS competency unit ‘Underground Transmission Cable Jointing’ (지중송전케이블접속공사, code 1901070222_22v4) as a practical reference. Actual work must follow applicable standards and the judgment of qualified professionals; always confirm specific values, deadlines and criteria against the latest regulations and client standards.
Written/reviewed by VOLT · Last updated 2026-10-25