Description

Elsewedy unarmored 3-core aluminum cable joint, 12/20 kV

Introduction: Economical and reliable connection for medium-voltage networks

Unarmored three-phase cable joints are vital components in medium-voltage networks where cables are routed through conduits, protected tunnels, or indoor substation connections. The joint reconstructs the cable at the connection point for each phase separately, restoring the insulation and electrical properties of the aluminum conductor and XLPE insulation. It is designed to withstand the high thermal and electrical stresses of 20 kV service while addressing aluminum-specific physical challenges such as rapid oxidation and thermal expansion.

Chapter One: Engineering anatomy and functional layer system

The Elsewedy unarmored 3-core joint uses an advanced polymer structure in which each layer performs a specialized protective function to maintain safe power flow across all three phases:

1. Advanced phase stress-control system

When aluminum-cable insulation is cut and the copper screen is removed from each phase, strong electric-field concentrations can form at the cut edges and may cause rapid insulation breakdown when voltage is applied.

• Stress-control tubes:Black polymer tubes with high-permittivity (High-K) characteristics are installed independently over each phase to absorb concentrated charge and redistribute the electric field evenly along the joint area. This helps prevent partial discharge and extend joint service life.
• Stress-relief mastic:A chemical mastic applied carefully over connectors and at copper-screen ends to fill microscopic air gaps and help keep the region free from electrical ionization.

2. Main insulation and reconstruction of the polymer structure

• Triple-layer insulation tube:A large tube combining a red insulating layer with a black semiconductive layer. Its main function is to rebuild the XLPE insulation removed from each phase, keeping the electric field contained around the aluminum conductor under 20 kV service conditions.
• Thermal stability:The materials are designed to withstand aluminum-conductor temperatures up to 90°C under full load and up to 250°C during short-circuit conditions.

3. Restoring the copper screen and earthing

Because the cable is unarmored, engineering focus is placed on restoring the electrical screen around the three phases:

• Copper mesh:Surrounds the three-phase joint area to maintain continuity of the cable’s copper screen and provide a safe path for earth-fault currents, while helping shield the system from electromagnetic interference.
• Field distribution:This layer helps distribute the electric field symmetrically around the aluminum conductors, reducing localized hot spots that could damage the outer sheath.

4. Environmental sealing and outer protection

• Heavy-duty outer jacket:A thick heat-shrink tube covers all three phases and is internally lined with active hot-melt adhesive.
• Function:When heated, the adhesive melts and bonds to the original cable sheath, creating a hermetic seal that prevents moisture and chemical vapors from entering the joint. This is especially important for preventing white oxidation of aluminum.

Chapter Two: Technical performance and strategic advantages

• Shear-bolt connector technology:The joint includes connectors designed for aluminum that mechanically penetrate the insulating oxide layer to provide stable electrical contact without hydraulic crimping tools, while accommodating aluminum’s thermal expansion characteristics.
• Chemical and environmental resistance:The polymer materials resist oils, grease, and acids and are also UV-resistant, allowing use in outdoor substations and demanding industrial environments.
• Fast, dry installation:Heat-shrink technology reduces installation time and allows the circuit to be returned to service once the joint has cooled and passed testing, reducing service downtime.
• High flexibility:The joint is flexible enough for installation along curved routes, inside narrow conduits, and on vertical risers in high-rise buildings.

Chapter Three: Joint applications in major projects

1. Administrative and commercial buildings:For joining rising unarmored aluminum feeder cables inside electrical ducts supplying medium-voltage loads in residential and commercial towers.
2. Conduit and tunnel installations:A suitable solution for joining cables routed through PVC conduits or protected concrete tunnels in new and smart cities.
3. Indoor substations and switchboards:For connecting cables from medium-voltage switchgear to transformers inside enclosed electrical rooms in factories and hospitals.
4. Oil and gas sector:For securing joints on protected routes requiring complete sealing against combustible vapors, gases, and high humidity in coastal areas.

Chapter Four: Engineering installation and testing guide

• Accurate preparation of all three phases:Clean the aluminum conductors thoroughly with a wire brush and immediately apply conductive contact grease before joining to prevent a new insulating oxide layer from forming.
• Tube sequence and spacing:Place all inner and outer tubes in their correct positions before connecting the conductors, maintaining the required spacing between phases to prevent contact.
• Professional heat distribution:Use a gas torch with a soft flame and shrink from the center toward the ends to expel air completely and prevent air bubbles that could cause dielectric failure.
• Mandatory tests before energization:After installation, perform a high-voltage VLF test and an insulation-resistance Megger test to verify the insulation integrity of all three phases before closing the route and energizing the circuit.