Medium voltage cable connection (MV connection): factory instructions 

High Voltage Splicing: Critical Reliability Requirements for Medium Voltage Connections

The reliability of power supply to industrial facilities and city networks directly depends on the quality of insulation at the cable connection points.High voltage splicingis not just a mechanical connection of two conductors, but a complex electrical assembly that must withstand rated voltage, thermal loads and mechanical stress for decades. An error when installing a coupling or the use of low-quality materials leads to insulation breakdown, emergency shutdowns and colossal financial losses for the operating organization.

In our practice, we regularly encounter the consequences of unqualified installation. One of our clients in Siberia lost more than 48 hours of production line operation due to a breakdown of a coupling installed in violation of the technology for stripping the semiconducting layer. This incident highlights a central truth: even the most expensive coupling is not guaranteed to be safe if the processhigh voltage splicemade without strict adherence to the manufacturer’s instructions and GOST or IEC standards.

This manual was developed based on the experience of engineers of Shijiazhuang Hist Electric LLC, a manufacturer with almost 20 years of history of supplying cable fittings to the CIS and Asian markets. We will analyze the step-by-step algorithm for preparation and installation, typical mistakes that installers make, and the criteria for accepting work. This instruction is intended for chief engineers, site managers and qualified electricians responsible for the safety of cable lines with a voltage of 6–35 kV.

Preparatory stage: tools, materials and environmental conditions

The success of a medium voltage cable joining operation begins before the first package of coupling components is opened. Ignoring the preparatory stages is the cause of 70% of all premature failures of cable lines. Before starting work, you must ensure that three key conditions are met: climatic parameters, the availability of specialized tools and proper preparation of the cable ends.

Climatic restrictions and working area

Installation of heat-shrinkable and cold-shrinkable sleeves requires control of humidity and temperature. Work is prohibited during precipitation (rain, snow, hail) and relative air humidity above 80%, unless a special protective tent with microclimate control is used. Dust and moisture on the insulation surface create conductive bridges, which lead to surface discharges after voltage is applied.

Temperature conditions are also critical. For most polymer materials, the lower limit for installation temperature is -10°C. At lower temperatures, polymers lose their elasticity, which makes quality compression or shrinkage impossible. If work is carried out outdoors in winter, the cable and coupling components must first be kept in a warm room for at least 24 hours. Trying to warm the cable with an open fire or a hair dryer is unacceptable - this causes local thermal stress in the insulation.

Necessary tools and consumables

For quality performancehigh voltage spliceA standard set of screwdrivers is not enough. The following list of equipment is required:

• Hydraulic or mechanical presswith matrices corresponding to the cross-section of the core. The use of a hammer and chisel to crimp the tips is strictly prohibited, as this disrupts the structure of the metal and reduces the contact pressure.
• Specialized stripping knives. The blade must be perfectly sharp and adjusted to a cutting depth that does not exceed the thickness of the insulating layer. Damage to the screen tape or semi-conductive screen is not acceptable.
• Gas burner with soft flame(for heat-shrinkable couplings). The flame must be adjusted to avoid direct fire contact with the material. It is recommended to use splitter attachments.
• Lint-free wipes and cleaning solvent(isopropyl alcohol or special cleaners). Cotton wool and rags leave microfibers that become ionization centers.
• Measuring tool: caliper, tape measure, megohmmeter for preliminary checking the condition of the cable.

The Shijiazhuang Hist Electric LLC company completes its couplings with all the necessary consumables, including pastes for leveling the electric field and abrasive tapes. However, the availability of additional professional tools remains the responsibility of the installation team. Check the integrity of the packaging of the components: any scratches or dirt on the silicone tubes are grounds for replacing the part.

Step-by-step instructions for installing the coupling

The process of connecting medium voltage cables (6–35 kV) requires a strict sequence of actions. Below is a universal technique adapted for modern heat-shrinkable and cold-shrinkable systems. Deviation from steps can lead to uneven distribution of the electric field and breakdown.

1. Cutting cable ends and checking geometry.

   Cut the cable ends so that they overlap at the center of the connection to the required length (usually 200–300 mm, depending on the coupling model). Remove the outer shell, armor and inner cushion, observing the dimensions indicated in the product data sheet. It is important to maintain a sealed sheath cut to prevent moisture from penetrating along the cable. After removing the armor, be sure to ground the metal braid. At this stage, check the ovality of the cores: if the cable has not been stored correctly, the cores may be deformed, which will make it difficult to install the connecting sleeves.

2. Stripping the insulation and forming cones.

   This is the most critical stagehigh voltage splice. Remove the main insulation from the cores, leaving the necessary overlap for installing the connecting sleeve. Then carefully remove the semiconductive screen. The boundary between the insulation and the removed screen must be smooth, without steps or burrs. Use fine sandpaper (P120-P240 grit) to sand the surface of the insulation. The surface should become matte and smooth. Any scratch left by a knife will become a source of partial discharges. After sanding, thoroughly clean the surface with solvent, moving from the middle of the core to the edge so as not to push the dirt back.

3. Installation of connecting sleeves.

   Place the connecting sleeves onto the cleaned ends of the wires. Press the sleeves with a press, starting from the center outward to force out air and ensure tight contact. The number of crimps is regulated by the instructions for the sleeve (usually 4–6 points). After firing, remove the burrs on the sleeve with a file and clean the transition from the sleeve to the insulation. Apply conductive paste, if provided for by the coupling design, to equalize the potential in the contact area.

4. Restoration of screen connection and insulation.

   Install shielding tubes over the core junction. For heat-shrinkable sleeves, start shrinking from the center, heating the surface evenly in a circle, avoiding overheating at one point. For cold shrink systems, carefully pull out the support helical cord, allowing the tubing to press the connection tightly. It is important to ensure overlap with the semiconducting cable screen by at least 50 mm. Violation of this overlap will lead to the concentration of the electric field at the edge of the screen and inevitable breakdown.

5. Installation of the outer shell and sealing.

   Install the outer protection tube or coupling body. Pay special attention to sealing the ends. Use the mastics and adhesive strips included in the kit. When heated, the glue should protrude from under the edges of the tube, indicating complete sealing. If the glue does not come out, it means the temperature was insufficient or the surface was dirty. Complete the installation by installing mechanical protection (if required) and marking the phases.

Each step must be recorded in the installation log. Photographing the steps, especially the insulation stripping area, is best practice for later analysis of possible problems. Engineers at Shijiazhuang Hist Electric LLC recommend using couplings with indicators of correct shrinkage, which change color when the required temperature is reached, which minimizes the human factor.

Typical errors during high-voltage splicing and methods for preventing them

Even experienced installers make mistakes that only appear after several months of operation. Analysis of returns and emergency cases allows us to identify three critical groups of errors.

Damage to the semiconducting screen

The most common mistake is making a deep cut in the insulation when removing the screen. Installers often use a regular mounting knife, which easily penetrates deeper than the thickness of the screen. This creates microcavities filled with air. Air has a much lower dielectric constant than insulation, which leads to partial discharges. Over time, these discharges “eat through” the insulation, leading to a short circuit.

Solution:Use only special knives with cutting depth limiters. After removing the screen, be sure to check the insulation surface by touch and visually at an angle. Any suspicious areas should be sanded and checked with a megohmmeter.

Insufficient surface cleaning

Leftover fingerprints, dust or microfibers from wipes act as conductive inclusions. In a high voltage field, these inclusions distort the field geometry, causing local overheating and aging of the insulation. We have seen cases where the coupling failed after 3 months because the installer wiped the insulation with a cotton cloth, leaving unnoticeable lint.

Solution:Use only lint-free wipes and clean gloves. Clean the surface immediately before installing the component, without touching the cleaned area with hands or tools.

Violation of the temperature regime of shrinkage

When using gas torches, installers often point the flame directly at the material, causing it to char. The charred carbon becomes a conductor, creating a path for current to flow across the surface of the coupling. In addition, uneven heating leads to the formation of air bubbles inside the insulation layers.

Solution:Keep the burner 15–20 cm from the surface, constantly moving it in a spiral. Use a blue flame (complete gas combustion). If the material has turned black, this area must be removed and the component replaced, since restoration of properties is impossible.

Quality control and testing after installation

Completion of installation does not mean the end of the work. Systemhigh voltage splicemust undergo a series of tests before being included in the network. Neglecting this step is tantamount to playing Russian roulette with the enterprise's energy system.

Primary control includes visual inspection: absence of cracks, bubbles, glue protrusions and correct geometric dimensions. This is followed by measuring the DC resistance of the contact connections. The resistance should not exceed the resistance of a solid core of the same length by more than 20%. Excessive resistance indicates poor crimping of the sleeve, which will lead to overheating and destruction of the contact under load.

A mandatory step is testing with increased voltage of direct or alternating current. For 10 kV cables, the test voltage is usually 2.5–3 Un for 15 minutes. Monitor the leakage current during the test. Its stability and low absolute value confirm the quality of the insulation. An increase in leakage current over time indicates the presence of defects, such as wet pores or air inclusions.

For critical infrastructure, partial discharge (PD) tests are recommended. This method allows you to identify hidden insulation defects that do not appear during standard voltage tests. The PD level should not exceed the values ​​​​established by standards (for example, IEC 60502-4 or GOST R 54538). Shijiazhuang Hist Electric LLC conducts internal tests of each batch of products in its own laboratory up to 1000 kV, which guarantees compliance with the declared characteristics, but the final check of installation quality lies with the contractor.

Choosing a manufacturer of cable accessories: what to look for

The cable fittings market is oversaturated with offers, but not all manufacturers provide the same level of reliability. When selecting components forhigh voltage spliceIt is necessary to evaluate not only the price, but also the technological maturity of the plant.

The key criterion is the availability of its own testing facility. Manufacturers who send products to third-party certification centers only on a selective basis cannot guarantee consistency of quality from batch to batch. Shijiazhuang Hist Electric Co., Ltd., based in Shijiazhuang, is investing heavily in developing one of the largest electrical engineering laboratories in Northern China. The ability to conduct full-scale testing up to 1000 kV allows the company to verify each new coupling design and control input raw materials.

The second important factor is engineering support. Complex projects require an individual approach: calculation of transient resistances, selection of materials for specific climatic conditions (for example, permafrost or tropics). The presence of qualified engineers and scientists on staff (as is the case with Histe Electric's 256 specialists, including PhDs) provides a depth of technical expertise not available to reseller trading companies.

The third aspect is logistics and inventory. Emergency repairs require prompt delivery. The presence of three specialized warehouses and a well-functioning logistics infrastructure allows us to supply products to the CIS countries, the Middle East and Africa in a short time, minimizing network downtime.

Frequently Asked Questions

What is the service life of a properly installed coupling?

If the installation technology is followed and there is no external mechanical damage, the service life of modern heat-shrinkable and cold-shrinkable couplings is at least 30 years. However, this indicator is only valid for products that have been certified to IEC or GOST standards and installed by qualified personnel. The use of cheap analogues without UV stabilizers can reduce the service life to 5–7 years due to cracking of the outer shell.

Is it possible to perform high-voltage splicing at subzero temperatures?

Yes, but with restrictions. Installation is possible down to -10°C for most standard kits. At lower temperatures, it is necessary to use special winter versions of couplings with modified polymers that maintain elasticity, and also to heat the cable and components in a warm tent before installation. Ignoring these requirements will lead to brittle failure of the insulation during shrinkage.

What is the difference between heat shrink and cold shrink sleeves?

Heat-shrinkable sleeves require heat to secure, which provides high mechanical strength and tightness, but depends on the quality of the installer's work with the torch. Cold-shrinkable couplings are installed without heating, due to the shape memory of silicone, which eliminates the risk of thermal damage to the insulation and speeds up installation. The choice depends on site conditions: in gas-contaminated or explosive areas, cold-shrinkable solutions are preferred, while in standard conditions, heat-shrinkable couplings are often cost effective.

How to check the quality of crimping of the connecting sleeve?

Visual inspection is not sufficient. It is necessary to measure the DC resistance of the connection section and compare it with the resistance of a whole core of the same length. The difference should not exceed 20%. It is also possible to perform radiographic crimp inspection to identify internal voids, although this is rarely used in practice due to cost. The main method is strict adherence to the number and places of crimping specified in the instructions, using a calibrated tool.

Conclusion: safety as an investment priority

Investments in high-quality cable accessories and professional installation pay off many times over due to the absence of emergency downtime.High voltage splicingis a technology where savings on materials or personnel qualifications are unacceptable. Every element of the system, from the semiconducting tape to the outer shell, plays a role in ensuring the long-term reliability of the network.

Shijiazhuang Hist Electric LLC is ready to provide not only products that meet international standards, but also complete technical documentation confirming the characteristics of each product. Our engineers are ready to advise on the selection of couplings for your specific operating conditions.

To receive technical advice, request certificates or calculate the cost of a batch, contact our specialists. We provide transparency of transactions and support at all stages of cooperation.

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