How to Make High-Voltage Cable Intermediate Joints More Reliable? Full-Process Specifications + Quality Control Points

Highvoltage cable intermediate joints are key connecting components in power transmission lines, undertaking core functions such as conductor connection, insulation restoration, and sealing protection. Their production quality directly determines the operational stability and safety of cable lines. In practical operation and maintenance, about 60% of high-voltage cable faults are related to improper production of intermediate joints. Common problems such as loose connection heating, insulation breakdown, and seal failure can easily lead to line power outages, and even cause major accidents such as fires and equipment damage. So, how to make highvoltage cable intermediate joints more reliable? Combining national standards and industry practical experience, this article provides a actionable standardized production guide from five core dimensions: production principles, pre-preparation, step-by-step processes, key quality control, and post-protection, helping practitioners avoid risks and improve the reliability of joint production.
I. First, Clarify: Core Production Principles for Reliable Intermediate Joints
The reliable production of high-voltage cable intermediate joints must firmly follow the four core principles of "reliable conductor connection, in-place insulation restoration, tight sealing protection, and uniform electric field distribution", which are the foundation for ensuring the long-term stable operation of joints:
Reliable Conductor Connection: The conductor joint must have low contact resistance and high mechanical strength to avoid local heating or even melting and ablation due to poor contact. The resistance at the joint is required to be no greater than 1.2 times the resistance of the conductor of the same cross-section cable.
InPlace Insulation Restoration: The restored insulation layer must have the same insulation performance as the original cable, with a thickness not less than the original insulation layer, and no defects such as bubbles, cracks, or impurities, which can effectively withstand rated voltage and overvoltage shocks.
Tight Sealing Protection: The entire joint must have good waterproof, moisture-proof, and corrosion-resistant performance to prevent the intrusion of moisture, dust, and corrosive media, and avoid moisture aging of the insulation layer and oxidation and rust of the conductor.
Uniform Electric Field Distribution: Eliminate the phenomenon of electric field concentration at the joint through reasonable shielding treatment and transition design, avoiding partial discharge caused by excessive local electric field strength, which may lead to insulation aging and breakdown.
II. Pre-Preparation: Build a Solid Foundation for Reliable Production
The adequacy of pre-preparation directly affects the production quality of intermediate joints, which must be strictly controlled from four dimensions: environment, materials, tools, and personnel:
1. Environmental Control: Create Clean and Dry Operating Conditions
The production of high-voltage cable intermediate joints has high requirements for the environment. Humid, dusty, and low-temperature environments will seriously affect the insulation adhesion and sealing effect:
The operating environment must be kept dry with a relative humidity not exceeding 60%. Outdoor operations are strictly prohibited in rainy, foggy, and windy and dusty weather; if operating outdoors, a temporary protective shed must be built, equipped with dehumidification and dust-proof equipment.
The ambient temperature must be controlled between 5℃ and 35℃. Too low temperature will make the insulation material brittle and reduce adhesion. The ambient temperature can be adjusted by heating equipment; too high temperature requires sun protection and deformation prevention measures for materials.
The operating area must be cleaned, away from flammable and materials. Warning signs of "High Voltage Danger, No Entry" shall be set up, and special personnel shall be arranged for on-site supervision to avoid interference from irrelevant personnel.
2. Material Verification: Ensure Suitable Selection and Qualified Quality
Materials are the core guarantee for reliable joints, and the accuracy of selection and compliance of quality must be strictly verified:
The intermediate joint body must be completely matched with the cable model and specifications (voltage level, conductor cross-section). Priority should be given to original factory supporting joints, and non-standard and low-quality joints are prohibited.
Core materials (insulating self-adhesive tape, cold-shrink/heat-shrink insulating sleeve, semi-conductive tape, sealant, connecting pipe, etc.) must have factory qualification certificates and test reports. Check that the appearance is free of damage, aging, and deterioration, such as no wrinkles on insulating tape and no caking on sealant.
Material storage must comply with requirements. Insulation materials and sealing materials must be stored in a dry and ventilated environment, avoiding moisture and direct sunlight. They must be taken out in advance to adapt to the ambient temperature (at least 2 hours) before use.
3. Tool and Protection Preparation: Ensure Accurate and Safe Operation
Prepare special tools: stripping tools (cable , utility knives, semi-conductive layer ), cleaning tools (anhydrous alcohol, cleaning cloths, sandpaper), crimping tools (hydraulic crimping pliers, crimping dies), testing tools (megohmmeters, multimeters, thermometers), etc. All tools must be calibrated qualified. Stripping tools must be sharp and burr-free to avoid damaging the cable insulation layer.
Equip with safety protective equipment: insulating gloves, insulating boots, safety helmets, voltage testers, grounding wires, etc. Ensure that the protective equipment is intact and effective. Before operation, the cable must be powered off, tested for voltage, and grounded.
4. Personnel Qualifications: Ensure Professional and Standardized Operation
Operators must have the qualification for making high-voltage cable joints, be familiar with the technical requirements of the cables and joints being operated, and master the standardized operation process; technical disclosure must be carried out before operation to clarify national standards (such as GB 50168-2018 "Code for Construction and Acceptance of Cable Lines of Electrical Installation Engineering") and key quality control points. Unqualified personnel are prohibited from operating.
III. Step-by-Step Implementation: Standardized Production Process of High-Voltage Cable Intermediate Joints
Taking the widely used cold-shrink intermediate joint as an example, the standardized production process is detailed. For heat-shrink joints, the heating link can be adjusted with reference to the core steps:
1. Cable Pretreatment: Accurate Stripping to Avoid Damage
Cable Positioning and Fixing: Place the two sections of cable to be connected straight, align the axes, and fix them firmly with cable brackets to avoid cable offset and torsion during construction.
Outer Sheath Stripping: According to the size required by the joint manual, strip the cable outer sheath with a cable . The stripped part must be flat and burr-free. The stripping length must be 5-10cm longer than the designed length of the joint to facilitate subsequent operations.
Armor Layer Treatment: For armored cables, after stripping the outer sheath, cut the armor layer with wire cutters, keep 5-8cm of armor layer at both ends for grounding, and remove the rest; polish the cut of the armor layer with sandpaper to remove burrs and avoid scratching the inner sheath.
Inner Sheath and Semi-Conductive Layer Stripping: Strip the inner sheath to expose the semi-conductive layer. Strip the semi-conductive layer according to the size required by the joint manual. A special semi-conductive layer must be used during stripping to ensure a flat cut, no steps or residual semiconductive particles at the transition with the insulation layer, and avoid damaging the main insulation layer.
Insulation Layer Trimming: Gently polish the surface of the main insulation layer with sandpaper to remove the oxide layer and impurities. The polishing length must cover the coverage area of the joint insulation sleeve. After polishing, wipe it clean with a cleaning cloth dipped in anhydrous alcohol, and set it aside to dry. Do not touch the surface of the insulation layer directly with hands.
2. Conductor Connection: Ensure Reliability and Reduce Contact Resistance
Conductor Arrangement: Strip the conductor insulation layer of the two sections of cable, and the exposed conductor length must match the length of the connecting pipe (generally 1-2mm shorter than the connecting pipe). Arrange the conductor strands to ensure no looseness or bending, and remove the oxide layer on the conductor surface.
Connecting Pipe Selection and Installation: Select a connecting pipe matching the conductor cross-section, check that the connecting pipe is free of deformation and rust, insert the two sections of conductor into both ends of the connecting pipe respectively, ensure that the insertion depth of the conductor is consistent, and the connecting pipe is placed in the center.
Crimping Operation: Select a matching crimping die, align the hydraulic crimping pliers with the connecting pipe, and crimp from the middle of the connecting pipe to both ends in sequence. The number of crimps must meet the requirements (generally 3-5 times). The crimping depth is based on the fit of the crimping die to ensure tight crimping and no looseness; after crimping, polish the burrs at the crimping place with sandpaper to make it flat.
Resistance Testing: Use a multimeter to measure the contact resistance at the connecting pipe to ensure it is no greater than 1.2 times the resistance of the conductor of the same cross-section cable. If the resistance exceeds the standard, re-crimp or replace the connecting pipe.
3. Insulation Restoration: Accurate Adhesion to Ensure Insulation Performance
Semi-Conductive Layer Restoration: Select a semi-conductive tape matching the original cable semi-conductive layer, start winding from the cut of the cable semi-conductive layer. During winding, the overlapping width is 1/2 of the tape width, and the stretching force is moderate to ensure that the tape is closely attached to the insulation layer and the connecting pipe. Wind to the cut of the semi-conductive layer at the other end to form a continuous semi-conductive shielding layer and avoid electric field concentration.
Insulation Layer Restoration:
Cold-Shrink Joint: Put the cold-shrink insulation sleeve into the cable, move it to the central position of the joint, first tear off the release rope at one end of the sleeve, let the sleeve shrink and fit the cable insulation layer under the action of elasticity, then slowly tear off the release rope at the other end to ensure that the sleeve shrinks evenly without bubbles or gaps; if there is a gap at the joint, fill an appropriate amount of insulating filling glue before putting in the sleeve.
Heat-Shrink Joint: Put the heat-shrink insulation sleeve into the cable, position it to the central position of the joint, heat evenly from the middle of the sleeve to both ends with a hot air , the heating temperature is controlled at 120-160℃ to make the sleeve shrink and fit. During the heating process, move the hot air continuously to avoid local overheating damaging the sleeve, ensuring that the sleeve has no bubbles or wrinkles.
Insulation Layer Inspection: Press the insulation sleeve by hand to ensure it is closely attached to the cable insulation layer and the connecting pipe without looseness; check that the surface of the insulation layer is free of damage and cracks, and the thickness is uniform.
4. Sealing and Outer Sheath Restoration: Tight Protection to Resist Environmental Impact
Sealing Treatment: Wind the sealant strip at the junction of both ends of the insulation sleeve and the cable outer sheath, then apply sealant to ensure tight sealing; for outdoor or humid environments, a waterproof seal box can be added to further improve waterproof performance.
Outer Sheath Restoration: Select an outer sheath sleeve (cold-shrink/heat-shrink) matching the original cable outer sheath, put it into the joint, and shrink and fix it according to the corresponding operation requirements. Ensure that the outer sheath sleeve covers the entire joint area and 5-8cm of the outer sheath of the cable at both ends, and fits closely without looseness.
Armor Grounding Treatment: Firmly connect the armor layers of the two sections of cable with a grounding wire. The grounding wire must be a multi-strand copper core wire with a cross-section not less than 16mm². The connection must be fixed with a crimp terminal to ensure reliable grounding and avoid induced voltage generation.
IV. Key Quality Control Points: Avoid Common Problems and Improve Reliability
The following quality control points must be focused on during production to avoid joint failure due to detailed omissions:
Stripping Link: Strictly control the stripping size to avoid excessive or insufficient stripping; when stripping the semi-conductive layer and insulation layer, it is forbidden to damage the main insulation layer. If the insulation layer is scratched (depth ＞0.5mm), it must be repaired by winding insulating tape, and the cable must be cut and re-stripped in severe cases.
Cleaning Link: All surfaces in contact with the insulation layer and conductor must be kept clean and dry. After wiping with anhydrous alcohol, they must be completely dried to avoid moisture, oil stains, and impurities affecting adhesion and insulation performance.
Crimping Link: The connecting pipe must be matched with the conductor and crimping die. The crimping sequence and number of times must be standardized to avoid loose crimping or excessive crimping leading to conductor damage; the contact resistance must be tested in time after crimping, and rework is required if it is unqualified.
Insulation Restoration Link: The cold-shrink/heat-shrink sleeve must be matched with the cable specifications, and accurate positioning is required during installation to avoid offset; uniform heating (heat-shrink) or shrinkage (cold-shrink) is required during the process without bubbles or gaps, otherwise partial discharge will be caused.
Sealing Link: High-quality products must be selected for sealant and sealant strips, and applied/wound evenly to ensure no gaps at the joints; additional waterproof protection must be done for outdoor joints to avoid rainwater infiltration.
V. Post-Protection and Acceptance: Ensure Long-Term Stable Operation
1. Finished Product Inspection and Acceptance
Appearance Acceptance: Check that the overall appearance of the joint is flat, free of damage, cracks, and bubbles, the outer sheath fits closely, and there is no looseness or leakage at the sealing place.
Insulation Performance Testing: Use a megohmmeter to measure the insulation resistance at the joint. For 10kV cables, it is not less than 1000MΩ; for 35kV cables, it is not less than 2500MΩ; for important lines, a DC withstand voltage test is required. The test voltage is 3-5 times the rated voltage, lasting for 15 minutes. No breakdown and no obvious fluctuation of leakage current are qualified.
Power-On Trial Operation: After the test is qualified, remove the grounding wire and restore power supply. Conduct a trial operation for 1-2 hours, and monitor the temperature at the joint with a thermometer. No abnormal heating (temperature not exceeding ambient temperature +30℃) and no abnormal noise are qualified.
2. Post-Maintenance Protection
Identification Management: Hang an identification plate at the joint, indicating the joint position, production time, operator and other information to facilitate subsequent inspections.
Daily Inspection: Regularly check the appearance of the joint for damage, leakage, and heating; for joints in outdoor, humid, and corrosive environments, the inspection frequency must be increased, and surrounding debris must be cleaned in time.
Environmental Protection: Avoid mechanical collision and rolling at the joint. Outdoor joints must be protected from sun, rain, rats, and ants to ensure a stable operating environment for the joint.