Branch cable coupling: connection diagram and necessary tools 

Branch Cable Joint: Critical Installation Considerations and Tool Selection

Installation of branch connections in medium and high voltage networks is not just a mechanical connection of conductors, but the creation of a reliable unit that can withstand thermal and electrodynamic loads for decades. Error at stagehigh voltage spliceor the wrong choice of tool can lead to insulation breakdown, local overheating and emergency shutdown of a network section. In our engineering practice, we have repeatedly encountered the consequences of careless installation: from partial discharges to complete failures of cable lines in the winter, when the load on the network reaches its peak.

This manual has been developed based on real experience in operating cable accessories in difficult climatic conditions. We will look at a detailed diagram for connecting branch couplings, a list of necessary professional tools and key mistakes that even experienced installers make. Particular attention is paid to technologies used by leading manufacturers such asShijiazhuang Hist Electric Co.,Ltd, whose solutions have proven their effectiveness in projects from Siberia to the Middle East.

Why standard twisting methods are not applicable for high-voltage lines

In low-voltage networks (up to 1000 V), electricians often use mechanical clamps or simple twisting followed by insulation. However, when moving to voltage classes of 6–35 kV and higher, the physics of the process changes dramatically. The main enemy is not only current, but also a high-intensity electric field. At the junction of the conductors, a concentration of the electric field occurs, which requires the use of special shielding elements and semiconducting layers to equalize the potential.

If you ignore the requirements for connection geometry and insulation quality, a “needle” effect occurs - microscopic air cavities inside the insulation begin to ionize. This leads to partial discharges, which gradually burn out the channels in the dielectric. The result is predictable: insulation breakdown after several months or years of operation. That's whyhigh voltage splicingrequires the use of certified couplings (heat-shrinkable or cold-shrinkable) and strict adherence to the technological process.

Necessary tools and materials for installing the branch coupling

The quality of installation of a branch coupling depends 80% on surface preparation and dimensional accuracy, and only 20% on the shrinking or crimping process itself. Having the right tool is not a matter of convenience, but rather a guarantee of safety. Below is a comprehensive list of equipment needed to get the job done professionally.

1. Cable cutting tool

Cable cutting is the most critical stage. Any scratch on the screen or damage to the residential insulation is unacceptable.

• Cable cutter or hacksaw:For straight cable cuts. It is important to use a tool that leaves minimal burrs. An uneven end will make it difficult to seal the coupling.
• Strippers for stripping:Special knives with adjustable cutting depth. The use of a conventional mounting knife (“shoe knife”) is strictly prohibited, since the risk of cutting the conductive wire is almost 100%. We recommend using strippers with a fixed blade configured for the specific type of insulation (XLPE, EPR or paper).
• Semi-conductive screen remover:For cables with extruded insulation, it is necessary to carefully remove the semiconducting shield layer without damaging the main insulation. Often special abrasive belts or scrapers are used for this.

2. Surface treatment tool

Adhesion of heat-shrinkable tubing or contact in cold-shrinkable sleeves requires perfect surface cleanliness.

• Lint-free wipes:Regular rags leave microfibers that can become puncture centers. Use only specialized non-woven napkins.
• Solvent for degreasing:Isopropyl alcohol or special cleaners recommended by the coupling manufacturer. Use acetone with caution, as it can be aggressive to some types of polymer insulation.
• Abrasive belts (P60–P120):Necessary for creating micro-roughness on the insulation surface before applying semi-conductive putty or installing tubes. This improves adhesion and eliminates air gaps.

3. Tool for connecting cores

The mechanical strength of the contact determines the ability of the coupling to withstand short-circuit currents.

• Hydraulic press:For crimping sleeves. Hand pliers are only suitable for small sections (up to 50 mm²). For power cables 70–240 mm² and larger, a hydraulic tool with dies corresponding to the type of sleeve is required.
• Set of matrices:Hexagonal or square dies depending on the type of connecting sleeves used. It is important to monitor the wear of the dies: a deformed sleeve profile will lead to uneven current distribution and overheating.

4. Shrinking equipment (for heat-shrinkable sleeves)

If you are using heat shrink solutions, temperature control is critical.

• Soft flame propane torch:The harsh flame of a gas burner can burn through the insulation. It is necessary to use burners with a gas supply regulator and nozzles for uniform heat distribution.
• Pyrometer (infrared thermometer):Visual control is insufficient. The shrinkage temperature must be within a strict range (usually 120-140°C for most materials). Overheating above 160°C causes degradation of the polymer, and underheating below 110°C does not provide the necessary radial compressive force.

For cold-shrinkable couplings that are actively promotedShijiazhuang Hist Electric Co.,Ltdin its product line for difficult climatic conditions, gas equipment is not required. This reduces the risk of cable damage from fire and speeds up installation, especially in windy conditions or in confined spaces.

Step-by-step diagram for connecting a branch cable coupling

The installation process may vary depending on the type of coupling (single-phase or three-phase, thermo- or cold-shrinkable), but the general logichigh voltage spliceremains unchanged. Let's consider the algorithm using the example of installing a single-phase branch coupling on a cable with cross-linked polyethylene insulation (XLPE).

1. Cable preparation and marking

   The first step is to clean the outer sheath of the cable from dirt and moisture. Measure the groove length according to the coupling manufacturer's instructions. Typically it ranges from 300 to 500 mm for branch connections. Make circular cuts on the outer shell, screen and insulation, observing the depth of cut.Attention: the depth of the cut on the core insulation should not exceed 0.5–1 mm, so as not to damage the copper conductor.

2. Removing layers and cleaning

   Carefully remove the outer shell, metal shield (if present) and semi-conductive layer. The semiconducting layer must be removed to the length specified in the diagram (usually 20–30 mm from the edge of the main insulation). Carefully clean the surface of the main insulation with an abrasive tape to a matte state, then degrease with a lint-free cloth soaked in alcohol. Movements should only go from the insulation to the screen, so as not to carry graphite particles onto the clean insulation.

3. Installation of the connector (sleeve)

   At the ends of the connected conductors, remove the insulation to a length equal to half the length of the sleeve plus 5 mm of margin. Insert the cores into the connecting sleeve until they stop. Perform crimping with a hydraulic press, starting from the center of the liner to the edges. The number of crimps depends on the length of the sleeve (usually 2–4 crimps). After crimping, remove sharp edges and burrs on the sleeve with a file or needle file, then clean the joint.

4. Screen and insulation restoration

   This is a key step to ensure electrical strength.

   • For heat-shrinkable sleeves:Install the inner semi-conductive tube at the connection point. It should cover the bare core and part of the main insulation. Use a torch to heat evenly, moving from the center outward to force out air. Then install the outer insulating tube with adhesive layer. Monitor the temperature with a pyrometer.
   • For cold shrink couplings:Gently pull the elastomer tube over the connection. Thanks to its shape memory, the material tightly compresses the cable, creating constant contact pressure. This method eliminates the risk of thermal damage to the insulation, making the productShijiazhuang Hist Electric Co.,Ltdpreferable for installation in field conditions without access to gas equipment.
5. Sealing and protection

   Complete the installation by installing the outer protective covers or tapes. If the coupling is intended to be laid in the ground, use bitumen mastic or special sealing compounds to prevent moisture penetration. Make sure that all transitions between materials are smooth and have no steps that could create a concentrated electric field.

Typical installation errors and their consequences

Even with the right tools, installers often make system errors. Analysis of failures in our laboratories shows the following most common causes of defects:

One of our clients experienced serial branch coupling failures at a substation in a high humidity region. The investigation revealed that installers skipped the step of degreasing the insulation before installing the cold-shrink tubing. A microscopic film of silicone grease (residue from cable production) prevented the elastomer from sealing tightly. After implementing strict cleaning controls, the failure rate dropped to zero. This case highlights: technologyhigh voltage splicedoes not tolerate compromises in installation hygiene.

Choosing between heat-shrinkable and cold-shrinkable technologies

When purchasing equipment, the question often arises: which technology to choose? The answer depends not on price, but on operating conditions and personnel qualifications.

Heat-shrinkable sleevestraditionally cheaper and widely distributed. They require a fire source (gas burner) and a qualified operator who can control the temperature. Their advantage is high mechanical strength after shrinkage and good adhesion of the adhesive layer. However, in conditions of strong wind or in enclosed spaces with limited ventilation, the use of open flames may be prohibited or difficult.

Cold shrink couplings, such as those produced using technologyShijiazhuang Hist Electric Co.,Ltd, are free from these shortcomings. Installation is faster (no time for heating and cooling), and the risk of human error associated with overheating is eliminated. Elastomeric materials provide constant radial pressure, compensating for thermal expansion of the cable (“cable breathing”). This makes them ideal for branch connections in hard-to-reach areas, on power transmission towers or in shafts.

From an economic perspective, although the cost of the cold shrink coupling itself may be 15-20% higher, the total cost of ownership (TCO) is often lower due to reduced installation time and the elimination of the need for expensive gas equipment and maintenance.

Quality control and testing after installation

Installation does not end with the installation of the last tube. Before applying voltage, it is necessary to perform visual and instrumental inspection.

• Visual inspection:Check for wrinkles, bubbles, cracks or damage to the outer shell. Make sure all markings are installed correctly.
• Insulation resistance measurement:Use a megohmmeter for a voltage of 2.5 kV or 5 kV (depending on the voltage class of the cable). The resistance must comply with PUE or local standards (usually at least 1000 MOhm km for new cables).
• Constant high voltage test:For critical components, it is recommended to carry out direct current tests with a voltage 2–3 times higher than the rated voltage for 15 minutes. No breakdown and stable leakage current confirm the qualityhigh voltage splice.

CompanyShijiazhuang Hist Electric Co.,Ltdis equipped with one of the largest electrical laboratories in Northern China, where each batch of products is tested in the range of up to 1000 kV. This level of control makes it possible to ensure that each coupling that reaches the installer already has a safety margin that is many times higher than the operating loads. This is especially important for projects in the CIS and Asia, where climate fluctuations can be extreme.

Frequently Asked Questions

Is it possible to use branch couplings to connect cables of different sections?

Yes, this is possible, but requires the use of special adapter sleeves or connectors. In this case, it is necessary to carefully follow the instructions for equalizing the electric field in the diameter transition zone. You cannot simply connect cores of different diameters with a regular sleeve without filling the voids with a special compound or tape, otherwise corona will occur.

What is the service life of a quality branch coupling?

With proper installation and compliance with operating conditions, the service life of modern polymer couplings is at least 30 years. Products that have been certified according to GOST or IEC standards, for example from trusted manufacturers, retain their properties even under cyclic loads and temperature changes from -60°C to +90°C.

Is it necessary to ground the shield of the branch coupling?

Necessarily. The coupling shield must be continuously connected to the main cable shield and reliably grounded. A broken shield or lack of grounding will result in a dangerous potential appearing on the surface of the coupling, creating a risk of electric shock to personnel and causing surface discharges.

What is the main advantage of cold-shrinkable couplings over heat-shrinkable ones?

The main advantage is independence from the heat source and eliminating the risk of thermal damage to the insulation. Cold shrink technologies provide more consistent installation quality in the field, especially in low temperatures or high winds where using a gas torch is ineffective or dangerous.

Conclusion

The branch cable joint is a critical element of the power system that requires careful selection of materials and tools. Successfulhigh voltage splicingis achieved not only by the quality of the product itself, but also by strict installation discipline: cleanliness, dimensional accuracy and control of each stage. The use of modern tools and technologies, such as cold-shrinkable systems, allows us to minimize risks and ensure an uninterrupted supply of electricity to consumers.

When choosing a supplier of cable accessories, pay attention to the availability of its own testing facilities and engineering support. Companies with deep expertise such asShijiazhuang Hist Electric Co.,Ltd, offer not just products, but comprehensive solutions adapted to real operating conditions. This ensures the longevity of your networks and the safety of your personnel.

Contact us today to receive technical advice and select the best solutions for your project.