MV connection: step-by-step installation instructions from factory engineers 

High Voltage Splicing: Critical Requirements for Safety and Reliability of Connections

The reliability of power supply to industrial facilities and distribution networks directly depends on the quality of each element of the cable line. Among themhigh voltage splicingrepresents the most vulnerable node, where the concentration of the electric field is maximum, and the risk of insulation breakdown increases many times over. Errors at the installation stage of couplings or the wrong choice of materials lead not just to local accidents, but to long production downtime, fines from network companies and significant financial losses. In our engineering practice, we have repeatedly encountered situations where saving on high-quality components or neglecting cable preparation technology resulted in the failure of the entire power transmission line during the first year of operation.

The process of connecting power cables with voltages above 1 kV requires strict compliance with regulatory documents, such as PUE (Electrical Installation Rules) and GOST R 50571.3-2012. However, theoretical knowledge often diverges from actual field conditions. Humidity, low temperatures, air pollution and mechanical stress dictate the need for solutions tested in extreme conditions. Shijiazhuang Hist Electric LLC, based in Hebei province, has been solving these problems for almost two decades, supplying the markets of the CIS, Middle East and Africa with cable accessories that have passed rigorous tests in its own laboratory with a range of up to 1000 kV. Our experience shows that the durability of a connection is 80% determined by the quality of surface preparation and compliance with the sequence of operations, and only 20% by the properties of the coupling material itself.

In this guide, we will analyze in detail the step-by-step algorithm for installing couplings, based on real cases of our engineers. We will not retell general truths, but will focus on those details that are usually omitted from standard manufacturers’ instructions, but they are the ones that cause accidents. You will learn how to properly prepare insulation, why controlling the tightening force of bolted connections is important, and how to avoid common mistakes when working with heat-shrinkable and cold-shrinkable materials. This information is intended for chief engineers, installation site foremen and technical specialists responsible for the smooth operation of energy infrastructure.

Preparatory stage: cable diagnostics and selection of coupling type

Any work on high-voltage splicing begins not with unpacking the tool, but with a thorough diagnosis of the cable ends being connected. Before installation, you must ensure that the cable meets the design specifications for cross-section, insulation type (XLPE, paper, rubber) and rated voltage. The use of a coupling designed for a cable with XLPE insulation on a cable with paper insulation without special transition elements is strictly prohibited. This leads to uneven distribution of the electric field and rapid aging of the dielectric.

The most important step is to measure the insulation resistance with a megohmmeter. If the values ​​are below standard values, installation cannot be carried out until the reasons for the deterioration in performance are clarified. Often moisture penetrates the cable cores through unsealed ends during storage in a warehouse. In the practice of Shijiazhuang Hist Electric LLC, we recommend that you always check the presence of moisture in the insulation by heating the sample or using special indicator tubes. The presence of even microscopic drops of water in the contact zone of current-carrying conductors causes the effect of “electrothermal wood”, which destroys the insulation from the inside within a few months.

The choice of coupling type depends on the operating conditions and voltage class. For indoor applications, heat-shrinkable sleeves are more often used, requiring an open flame or an industrial hair dryer for shrinkage. For difficult climate conditions, including low temperatures and high humidity, cold-shrinkable couplings are preferred. They are made of special silicone rubber with a high coefficient of elasticity and are installed without heating, which eliminates the risk of thermal damage to the cable insulation. Our experts note that in regions with variable climates, such as most of Russia and Kazakhstan, cold-shrinkable solutions demonstrate a 40% lower failure rate in the first five years of service.

Before starting work, it is also necessary to prepare the workplace. It must be dry, protected from wind and dust. At ambient temperatures below +5°C, outdoor work requires the creation of a temporary heated shelter. A cold cable becomes rigid, and the coupling materials lose elasticity, which complicates installation and reduces the quality of sealing. We strongly advise that cable drums and sleeve sets be kept in a warm room for at least 24 hours before installation if they have been stored in the cold.

Necessary tools and materials for high-quality installation

A successful high voltage splice operation is impossible without the right set of tools. The use of improvised means, such as ordinary knives instead of strict insulation strippers, is a gross violation of technology. Damage to the semiconducting screen or the appearance of scoring on the main insulation creates points of concentration of electric field strength, which will inevitably lead to breakdown. Below is a list of mandatory equipment that the installation team must have.

• Strict stripping kit:includes knives with adjustable cutting depth, semiconductive layer strippers and core stripping tools. It is important that the blades are perfectly sharp and do not leave deep scratches on copper or aluminum.
• Measuring instruments:caliper for checking diameters, megohmmeter (at least 2500 V for medium voltage cables), ohmmeter for checking contact connections.
• Heating equipment (for heat-shrinkable sleeves):a gas burner with a soft flame or an industrial hair dryer with temperature control. The open flame should be blue, without soot, so as not to deposit carbon on the surface of the insulation.
• Cleaning and lubricating agents:lint-free wipes, a special cleaner based on isopropyl alcohol, silicone grease to facilitate the sliding of the coupling elements.
• Protective equipment:dielectric gloves, safety glasses, fire-resistant clothing (when working with fire).

Particular attention should be paid to the quality of consumables. Pastes for equalizing the electric field (conductive and semiconducting) must be fresh and stored in airtight packaging. Dried paste loses its properties and does not provide the necessary contact between the insulation layers and the screen. At the laboratory of Shijiazhuang Hist Electric Co., Ltd., we conduct adhesion and dielectric strength tests on each batch of pastes to ensure that they comply with international standards IEC 60502-4.

Another critical point is the fasteners. Bolts and nuts of connecting elements must be made of corrosion-resistant alloys and have the correct strength class. The use of conventional uncoated steel bolts in conditions of high humidity leads to oxidation of the contacts, an increase in contact resistance and overheating of the connection. We recommend using only the original fastening kits supplied with the couplings, as they are designed for specific tightening forces and materials of live parts.

Step-by-step instructions for installing the coupling

The installation process can be divided into several key stages. Each of them requires extreme concentration and accuracy. A deviation from the instructions even by a millimeter can be fatal for the entire cable line. Below is a detailed methodology developed based on many years of experience of our engineers and adapted to various types of high-voltage cables.

1. Cutting cable ends.

   The first step is to accurately mark and remove the outer sheath, metal screen and core insulation. The cutting length must strictly correspond to the dimensions specified in the passport of the specific coupling model. Use special stripping tools to avoid damaging live wires. The depth of the cut of the semiconducting screen must be controlled with an accuracy of 0.1 mm. After removing the insulation, the surface must be thoroughly cleaned of semiconductor particles and treated with a solvent. Any remaining dirt impairs contact and creates air voids.

2. Installation of core connectors.

   Current-carrying conductors are connected using crimping or bolted connections. When crimping, it is important to use the correct size dies and follow the crimping sequence specified by the manufacturer. When bolting, it is necessary to ensure uniform tightening force of all bolts using a torque wrench. Insufficient tightening leads to sparking and overheating, and excessive tightening leads to metal deformation and poor contact. After connection, the joint must be insulated with a layer of semiconducting tape or heat-shrinkable tube to equalize the electric field.

3. Restoration of insulation and screen.

   At this stage the main coupling insulation is applied. For heat-shrinkable sleeves, tubes and sleeves are put on and then heat-shrink. Heating should be done evenly, starting from the center and moving towards the edges to avoid the formation of air bubbles. For cold-shrinkable couplings, the elements are pulled onto the cable and fixed in the desired position. The key is the correct positioning of the field leveling cones. They must exactly coincide with the cut boundaries of the semiconducting cable screen. A displacement of the cone even by 5 mm leads to a local excess of the field strength and breakdown.

4. Sealing and protection.

   The outer shell of the coupling must ensure complete tightness from moisture and mechanical damage. External protective covers, reinforcing tapes and metal screens are installed. All joints are carefully sealed with mastics and tapes. Particular attention is paid to the places where the cable enters the coupling. Here, current leaks often occur due to incomplete seals. We recommend additional use of heat-shrinkable gloves or cuffs to enhance the seal in these areas.

5. Final inspection and testing.

   After installation is completed, it is necessary to conduct a visual inspection of all elements. Electrical tests are then performed: measuring the DC resistance of the contact connections and testing the insulation with increased DC or AC voltage. Resistance values ​​must be symmetrical for all three phases and not exceed standard values. The test voltage is selected in accordance with the PUE and cable type. Only after successfully passing the tests can the cable line be put into operation.

It is important to note that every step must be documented. Photographic recording of installation stages, especially hidden work (for example, the state of cores after stripping), helps in the investigation of possible future accidents and serves as evidence of compliance with the technology. At Shijiazhuang Hist Electric LLC, we require our partners to provide such reports, which allows us to provide more effective technical support and analyze the causes of rare failures.

Typical errors during high-voltage splicing and ways to prevent them

Even experienced installers make mistakes that can be very costly. Analysis of hundreds of emergency cases allows us to identify several of the most common problems. Understanding these risks will help you avoid them in your practice.

Mistake #1: Damage to the main insulation during stripping.
This is the most common reason for early failures. Using dull knives or excessive force when removing the semiconductive layer will result in micro-cuts in the XLPE insulation. These cuts are invisible to the eye, but under the influence of high voltage, partial discharges begin in them, which gradually burn out the channels in the insulation.
Solution:Use only certified cable cutting tools with cutting depth gauges. After stripping, be sure to inspect the insulation surface under a bright light and, if necessary, sand small defects with a special abrasive tape.

Mistake #2: Incorrect positioning of field alignment elements.
Cones or tapes for leveling the electric field must accurately cover the cut edge of the semiconducting screen. If the cone is shifted towards the core, the field strength at the edge of the screen becomes critical. If biased towards insulation, the opposite problem occurs.
Solution:Measure distances carefully before installing elements. Use the marking tape supplied with the coupling as a guide. Do not rely on the “eye gauge”.

Mistake #3: Moisture or dirt getting inside the coupling.
Installation in rain, snow or dusty weather without proper cover will result in moisture getting between the layers of insulation. Water is a conductor and initiates the development of wood discharges.
Solution:Never install high voltage couplings outdoors in adverse weather conditions. Use mobile tents or awnings. Clean all surfaces thoroughly with solvent before assembly. Wear clean gloves to avoid leaving grease marks.

Mistake #4: Insufficient shrinkage of heat-shrinkable elements.
If the tube is not seated tightly along its entire length, air cavities remain underneath it. Air has a much lower dielectric strength than solid insulation, which leads to breakdowns.
Solution:When heating, observe the recommended temperature and holding time. Move the burner flame evenly, without staying in one place for too long, so as not to overheat the material. Check the tightness visually and by touch (after cooling).

Comparative analysis: heat-shrinkable vs cold-shrinkable couplings

The choice between heat-shrinkable and cold-shrinkable technologies is often controversial. Both types have their advantages and disadvantages, and the choice should depend on the specific project conditions. Below is a comparison table based on our supply experience and customer feedback from different climate zones.

For projects in northern regions or for urgent emergency restoration work, we definitely recommend cold-shrinkable couplings. Their ability to maintain elasticity at low temperatures and the lack of need for a heat source make them indispensable. For permanent installations in controlled areas, heat shrink sleeves remain a cost-effective solution, provided that the heating technology is strictly followed.

Shijiazhuang Hist Electric LLC produces both types of couplings using its own developments in the field of silicone compounds and thermopolymers. Our cold shrink couplings are temperature cycle tested from -60°C to +90°C to ensure they are suitable for the harshest operating conditions. We provide detailed technical advice on selecting the optimal coupling type for your specific project, taking into account load, climate and budget.

Quality control and acceptance of work

Completion of installation does not mean the end of the job. High-quality high-voltage splicing must be confirmed by test results. Acceptance of work includes several stages of control that guarantee the safety and reliability of the connection.

An initial visual inspection allows you to identify obvious defects: lack of heating, displacement of elements, damage to the outer shell. Electrical measurements are then carried out. The DC resistance of contact connections should not differ by more than 20% between phases. This value indicates the uniformity of tightening and the quality of contact.

High voltage insulation testing is the final and most important step. For 6-10 kV cables, the DC test voltage is usually 6U0 (where U0 is the phase voltage) for 15 minutes. The absence of breakdown and stable leakage current indicate the quality of the installation. If the leakage current increases over time, this is a sign of defective insulation or contamination.

All test results must be recorded in a protocol, which is signed by the responsible persons. This document is the basis for allowing the cable line into operation. It is recommended that records and photographs of installation be retained for the life of the line as this will assist in the analysis of any future incidents.

We at Shijiazhuang Hist Electric LLC emphasize the importance of not only the installation process itself, but also the subsequent monitoring of the condition of the connections. Modern diagnostic systems make it possible to identify developing defects at an early stage, preventing accidents. Our technical support includes consultations on methods of such diagnostics and interpretation of the data obtained.

Frequently Asked Questions

What is the service life of high voltage couplings?

With proper installation and compliance with operating conditions, the service life of high-quality couplings is at least 30 years. However, this indicator strongly depends on external factors: temperature cycles, mechanical loads and the presence of aggressive environments. Cold-shrinkable silicone rubber couplings, such as those from Shijiazhuang Hist Electric Co., Ltd., exhibit better aging resistance due to silicone's UV and ozone resistance.

Is it possible to use one coupling to connect cables of different sections?

Yes, there are special adapter couplings designed to connect cables with different core cross-sections. However, the difference in sections should not exceed established standards (usually no more than two adjacent standard sections). In this case, it is necessary to use special adapter connectors or adapt the field leveling area. The use of a standard straight coupling for cables with large differences in cross-section is prohibited, as this will lead to a violation of the geometry of the electric field.

What to do if a breakdown occurs during testing?

A breakdown during testing indicates a serious installation defect or cable damage. Repeated testing of the same area without eliminating the cause is unacceptable. It is necessary to localize the breakdown site, disassemble the coupling and carefully inspect all layers. Most often, the reason lies in moisture, damage to the insulation during cutting, or displacement of the field leveling cones. After eliminating the defect, the coupling must be replaced with a new one, since the restored insulation does not guarantee the same reliability.

Does cable length affect the choice of coupling type?

The length of the cable line affects the amount of capacitive current and charges accumulating in the insulation. Long lines may require couplings with improved shielding and grounding systems. It is also important to consider the mechanical stress on the connection due to thermal expansion of a long cable. In such cases, it is recommended to use couplings with enhanced mechanical protection and flexible compensation elements.

Conclusion: Investing in Grid Reliability

High-voltage splicing is not just a technical operation, but a critically important process that determines the stability of the entire energy system of an enterprise or city. Neglect of installation details, the use of low-quality materials or lack of personnel qualifications inevitably lead to accidents, the cost of eliminating which is many times greater than the savings at the procurement stage. Selecting trusted suppliers such as Shijiazhuang Hist Electric Co.,Ltd., which has its own laboratory facilities and many years of manufacturing experience, is the first step to ensuring reliability.

We encourage you to treat each connection with the utmost responsibility. Follow the instructions, use the right tool, control every step and don't skimp on quality. Investments in quality cable accessories and professional installation will pay off with years of trouble-free operation and no unexpected repair costs. If you have any questions about the selection of equipment or installation technology, our engineers are ready to provide qualified advice.

For detailed technical documentation, product catalogs or a customized quote, contact us today. We will help you choose the optimal solution for your needs, ensuring a balance between cost and reliability.High voltage couplings from the manufactureris your choice in favor of safety and effectiveness.