Power Cables: Classification and Basic Structure

What is a power cable?

A power cable is a flexible conductive assembly that transmits and distributes electrical energy. Composed of a conductor, insulation layer, and protective sheath, it reliably transmits alternating or direct current in complex environments such as underground or underwater. Compared to ordinary wires, it features higher voltage ratings, greater transmission capacity, and enhanced protection, serving as the core transmission medium in power systems.

The Importance of Power Cables in Transmission and Distribution Systems

Power cables serve as the “connecting hub” of power systems, responsible for conveying electricity from power plants via high-voltage cables to substations. After voltage reduction, medium- and low-voltage cables distribute power to end users. Approximately 70% of urban distribution networks rely on cable transmission, particularly in densely populated areas where their reliability directly impacts power supply stability.

Why Understanding Classification and Structure is Critical for Selection, Performance, and Safety

Cable classification and structure determine suitability, operational performance, and safety: Mixing cables of different voltage ratings risks insulation breakdown, triggering short-circuit fires; selecting the wrong structure (e.g., using non-armored cables underground) accelerates damage. Additionally, substandard cables often reduce insulation thickness or use impure conductors, posing leakage and overheating hazards. Mastering this knowledge is key to ensuring accurate selection and safe operation.

What is a power cable?

Definition and Core Function

Power cables are composite conductors comprising insulated conductors, insulation layers, and optional shielding, armor, and outer sheaths. They enable long-distance, low-loss transmission and distribution of electrical energy within power systems while providing protection and leakage prevention. Their core functions include: conducting electrical energy, insulating against leakage, and adapting to diverse environments through protective structures.

Basic Structure of Power Cables

Conductor: Materials and Types
As the core conductive element, the conductor's material and structure impact transmission efficiency and mechanical properties. Copper and aluminum are mainstream materials: copper offers low resistivity, superior conductivity, and oxidation resistance, suitable for medium- and high-voltage applications; aluminum is cost-effective and lightweight but has poorer conductivity, now modified for low-voltage distribution. Structures include solid and stranded conductors.
Conductor Shield Layer (for Medium/High-Voltage Cables)
Conductors in medium/high-voltage cables (1kV+) exhibit surface irregularities prone to partial discharge. The shield layer, made of semiconductive material, envelops the conductor to equalize electric fields. With a thickness of 0.5-1mm and resistivity of 100-1000Ω・m, it prevents insulation breakdown.
Insulation Layer: Materials and Characteristics
The insulation layer prevents current leakage. Different materials determine the cable's application scenarios:
PVC: Low cost, flexible, corrosion-resistant. Used for low-voltage residential applications. Not heat-resistant; emits toxic fumes when burning.
XLPE: Heat-resistant, excellent insulation. Used for medium- and high-voltage applications. Poor low-temperature flexibility.
EPR: Excellent weather resistance and flexibility, suitable for outdoor low-temperature environments, but not oil-resistant.
Insulation Shield Layer
The insulation shield layer works with the conductor shield layer to equalize electric fields, provide a grounding path, and protect the insulation layer. Essential for medium- and high-voltage applications, optional for low-voltage based on requirements. Made of semiconductive material, thickness 0.3-0.8mm.
Metal Shielding / Armor Layer: Mechanical Protection and Grounding
Metal Shielding Layer: Copper-based, used for grounding and electromagnetic shielding, suitable for scenarios with high electromagnetic compatibility requirements.
Armor Layer: Steel tape armor provides crush resistance, suitable for underground applications; steel wire armor offers high tensile strength, used for underwater and aerial applications.
Padding Layer and Filler
Padding layer cushions armor compression and provides waterproofing; filler maintains compact multi-core cable structure, with some materials enhancing flame retardancy.
Outer Sheath: Outer Protective Barrier
PVC Sheath: Low cost, for indoor use, not weather-resistant.
PE Sheath: Weather-resistant and waterproof, mainstream for outdoor applications.
LSZH Sheath: Low smoke, zero halogen, used in densely populated areas, higher cost.
Optional Auxiliary Structures
Water-Swelling Tape: Absorbs water and expands, used in humid environments.
Tear-Out Cord: Facilitates installation and prevents internal damage.

Power Cables Classified by Voltage Rating

Low-Voltage Power Cables (≤1kV)

Low-voltage power cables are widely used, accounting for over 60% of total power cable consumption. Key specifications:
Voltage: Rated 0.6/1kV, compatible with 220V/380V systems; some specialized types reach 1kV.
Structure: Typically 2-5 cores, conductor cross-section 10-240mm², PVC or XLPE insulation, PVC or PE outer sheath. Outdoor/underground types feature steel tape armor. Simple structure with low cost.

Medium-voltage power cables (1-35kV)

Medium-voltage cables form the backbone of urban and industrial power distribution, with characteristics as follows:
Voltage: 1kV < rated voltage ≤ 35kV, with 10kV specifications accounting for over 70% of applications.
Structure: Includes conductor and insulation shielding layer, featuring XLPE insulation, copper tape for metallic shielding, steel tape armoring for underground applications, and steel wire armoring for outdoor/submarine use. Conductor cross-section ranges from 35-400mm².

Power Cables Classified by Insulation Type

PVC-Insulated Cables

PVC-insulated cables utilize polyvinyl chloride as insulation material, serving as the “cost-effective choice” for low-voltage distribution. Core characteristics include:
Performance Characteristics: Costs 60%-70% of XLPE-insulated cables, offers excellent flexibility (bending radius 4-6 times outer diameter), and resists acids/alkalis (pH 4-9). However, it has a maximum continuous operating temperature of 60-70°C, emits toxic gases when burning, and exhibits poor aging resistance.
Suitable Applications: Indoor low-voltage distribution in dry environments and industrial non-high-temperature, non-oil-contaminated auxiliary circuits.

XLPE Insulated Cable

XLPE (cross-linked polyethylene) insulated cable is the mainstream choice for medium- and high-voltage applications, with core advantages as follows:
Performance Characteristics: Cross-linking process creates a three-dimensional network structure. Long-term operating temperature: 90–125°C. Insulation resistance ≥10¹⁴Ω・cm. Resistant to aging and cracking. Low smoke emission during combustion (halogen-free type).
Applications: Medium- and high-voltage transmission, power collection lines for new energy power stations, power supply for industrial high-temperature equipment, and distribution in high-rise buildings.

Power Cables Classified by Application Scenario

Underground Power Cables

Underground power cables are specifically designed for subterranean installation, featuring the following core characteristics:
Performance Requirements: Capable of withstanding soil pressure, waterproofing, and corrosion resistance. Typically armored with steel tape or steel wire, featuring a PE outer sheath, with some incorporating waterproof tape.
Installation Methods: Direct burial (minimum depth ≥0.7 m, ≥1 m under vehicular traffic with warning tape) or conduit installation (PVC or galvanized steel pipes, cable cross-section ≤40% of conduit volume).​

Industrial Power Cables​

Industrial power cables are customized for industrial environments, featuring:​
Performance Requirements: Withstands high temperatures, oil contamination, vibration, corrosion, and similar environments. Insulation typically consists of XLPE or fluoroplastic, with an outer sheath made of oil-resistant rubber or LSZH material. Some feature steel tape armor, and conductors are copper.
Typical Types: Includes high-temperature resistant, oil-resistant, explosion-proof, and flame-retardant cables.

Conclusion

Summary of Power Cable Classification and Structure

Power cables exhibit diverse classifications, with performance closely tied to application scenarios: - Voltage classification: Low-voltage (≤1kV), medium-high voltage (1~35kV and above), affecting transmission capacity. - Insulation material classification: PVC, XLPE, etc., determining temperature resistance and insulation properties. - Application classification: Underground, submarine, etc., influencing protective design. - Armor classification: Steel tape, steel wire, and non-armored, relating to mechanical protection. By core count: single-core and multi-core, affecting current-carrying capacity and installation.

The Importance of Selecting Appropriate Cables for Safety and Efficiency

Cable selection impacts the safety and efficiency of power systems:
Safety: Cables suited to the environment mitigate risks like leakage and fire.
Efficiency: Cables matched to voltage and current reduce losses and conserve energy.