Summary of Cable Installation Knowledge in Power Engineering

The operational quality, safety, and reliability of a power supply system depend not only on the quality of the wires and cables themselves but also on the construction quality of cable accessories and lines.

1. Cable Installation Methods

Cable installation methods include the following: direct burial, duct installation, shallow trench installation, cable trench installation, cable tunnel installation, and overhead installation. Each method has its advantages and disadvantages. Generally, considerations must include urban development planning, the density of existing buildings, cable line length, the number of installation routes, and the impact of the surrounding environment. Technically, cable tunnel and cable trench installation methods are more convenient for cable construction, maintenance, and inspection. In some developed countries, public tunnels have been planned during urban development. Practice has shown that public tunnels operate effectively, significantly reducing repeated investments and road excavation. However, the initial investment is substantial, and material costs are high. In China, due to various constraints, this installation method is rarely used. In comparison, direct burial and shallow trench installation are more economical methods. Direct burial is the most economical and widely used method, suitable for suburban areas and locations with infrequent vehicle traffic. However, it is not conducive to cable maintenance and inspection. Once a cable fault occurs, even if testing equipment identifies the fault point, the cable trench must be excavated again, which is highly inconvenient. Therefore, the choice of cable installation method should consider actual conditions, project requirements, environmental characteristics, cable types, and quantities. A forward-looking approach should be taken, balancing operational reliability, ease of maintenance, and technical-economic rationality.

2. Cable Selection

Common power cables include oil-impregnated cables, PVC-insulated cables, and cross-linked polyethylene cables. Depending on the application, these extend to various types of special cables. Currently, with advancements in production technology and processes, cross-linked polyethylene cables have become the most widely used. When selecting cables, factors such as the operating environment and conditions should be considered, and choices should be made based on specific circumstances. Efforts should be made to minimize crossings with pipelines, railways, roads, and communication cables. For direct burial or shallow trench installation, steel-armored cables should be considered.

3. Selection of Cable Cross-Sectional Area

The choice of cable cross-sectional area relates to investment costs, line losses, voltage quality, and cable service life. If the cross-sectional area is too small, voltage quality may decline, and line losses may increase. Conversely, an oversized cross-sectional area leads to higher initial investment. Therefore, based on load forecasting results and development planning, an appropriate cross-sectional area should be selected to ensure the cable meets the core temperature requirements under maximum operating current, voltage drop requirements, and thermal stability requirements under maximum short-circuit current. Given the challenges and inaccuracies in load forecasting, cable cross-sectional area selection must also comply with the Technical Guidelines for Urban Medium and Low Voltage Distribution Network Renovationand the Guidelines for Urban Power Grid Planning.

In three-phase four-wire low-voltage power grids, the selection of the neutral conductor cross-sectional area must also be considered. In public low-voltage networks, due to significant user influence, it is difficult to control three-phase load balance. To improve voltage quality and reduce line losses, the cross-sectional area of the neutral conductor should match that of the phase conductors.

4. Cable Networks and Cable Network Automation

With the increasing adoption of power cables in distribution networks, distribution networks can be divided into cable networks and overhead networks (including hybrid overhead-cable networks). The Implementation Status and Supplementary Opinions on the Technical Guidelines for Urban Medium and Low Voltage Distribution Network Renovationalso specifies requirements for cable distribution network automation. Therefore, when implementing cable networks in distribution network areas, new technologies and equipment should be adopted in line with the requirements of distribution automation. Where conditions permit, automation pilot projects should be considered. Even where conditions are not yet mature, equipment selection should allow sufficient flexibility to lay the foundation for future automation.

5. Issues to Note in Power Cable Construction

1. Eddy Current Issues Caused by High-Current Power Cables

   During power cable construction, steel supports, steel protective pipes, or cable clamps for overhead installation may be used. Any formation of a closed steel (iron) loop around the power cable may generate eddy currents, particularly in high-current power cable systems where eddy currents are more significant. During power cable installation, measures must be taken to prevent the formation of closed steel (iron) loops around the cable to avoid eddy currents.

2. Mechanical Damage Caused by Cable Bending

   Due to the large outer diameter of power cables, transportation and installation can be challenging, and power cables have strict requirements for bending radius. During installation, excessive bending angles may cause internal mechanical damage to the conductors. Such damage reduces the cable’s insulation strength, potentially leading to faults. In one instance, a cable head failure occurred during installation because the three cable heads were of equal length during fabrication. When connecting to equipment, the middle-phase cable head was longer due to terrain constraints, forming an arch shape and causing discharge damage at the root. Measures were taken to shorten the middle-phase cable head connection appropriately, ensuring all three cable heads were free from external force. Practice has shown that this approach works well. Therefore, during cable installation, torsional forces on the cable should be minimized. When bending or leaving slack, the cable should be allowed to follow a natural curve to prevent internal mechanical damage.

3. Moisture Prevention for Power Cables

   Operational experience shows that most faults in medium and low-voltage power cables occur at intermediate joints and terminations, often due to poor sealing and moisture ingress, which reduces insulation strength. Medium and low-voltage power cable networks typically use a tree-like power supply structure, resulting in a large number of cable terminations. Therefore, ensuring proper sealing of cable terminations and intermediate joints is one of the key measures to guarantee the safe and reliable operation of cables.

4. Grounding Issues for Medium and Low-Voltage Power Cables

   In public medium and low-voltage power cable networks, due to unequal three-phase loads, if cables with metal sheaths are used, the grounding of the metal sheath must be considered. The normal induced voltage at any non-grounded point of the metal sheath should not exceed 100V. It is recommended that in medium and low-voltage cable networks, grounding electrodes (grids) be installed at all cable joints, and the metal sheath should be reliably grounded.

6. Quality Inspection

• Cables arranged neatly with clear labeling.

• Joint fabrication complies with standards.

• Fire sealing is tight and secure.

• Test records are complete.

7. Common Issues and Solutions

8. Relevant Standards and Specifications

• GB 50168: Code for Construction and Acceptance of Cable Lines in Electrical Installation Engineering

• GB 50217: Code for Design of Power Cable Engineering

• DL/T 5161: Specification for Quality Inspection and Assessment of Electrical Installation Engineering

Reminder: Cable installation is a specialized task and must be performed by certified professionals in strict compliance with safety operating procedures!