Beatrice Lang
Laying electrical cable underground is a popular method for transmitting telecommunications or electric power. It is simple and inexpensive, and it avoids the safety issues associated with aerial cables. However, it is important to carefully plan and execute the process, adhering to rigorous installation procedures and local electrical codes. Before laying underground electrical cables, it is necessary to obtain the required permissions and approvals from relevant authorities and to understand the unique advantages and challenges of underground solutions. The type of soil, presence of obstacles, and local depth regulations will impact the process. This article will provide a comprehensive guide to safely and successfully installing underground electrical cables in the UK.
| Characteristics | Values |
|---|---|
| Advantages | Accident prevention, continuous service, zero obstruction, minimum space requirement, uninterrupted power supply and telecommunication |
| Soil type | Granular, rocky, heavy clay |
| Installation cost | High |
| Permissions and approvals | Required from relevant authorities |
| Cable type | Armoured or unarmored |
| Cable material | Copper, aluminium, steel wire, steel tape, polyethylene, polyvinyl chloride |
| Cable protection | Tar paper, burlap tape, concrete protective plate |
| Cable laying procedure | Dig trench, lay cable on sand, cover with sand, bricks, and other materials |
| Trench depth | Minimum 0.9m for 3.3kV to 11kV cables, 1.05m for 22kV to 33kV cables, 1.5m recommended |
| Trench width | 0.45m |
| Cable spacing | 30 cm horizontal or vertical inter-axial spacing for multiple cables |
| Conduit material | Metal, plastic |
| Conduit depth | 6 inches for galvanized rigid metal electrical conduit, 12 inches for GFCI-protected direct-buried Type UF cable, 18 inches for PVC conduit |
| Safety | Avoid sites with severe acid, alkali, or stray current corrosion, prevent moisture, comply with local electrical codes |
| Installation method | Traditional trenching, trenchless technology |
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What You'll Learn
Planning and permissions
Firstly, you should contact your local building inspection department to determine whether electrical permits and inspections are required for your project. An electrical inspector will be able to verify the depth of your trench and inspect the wiring before it is covered, ensuring your work is safe and compliant.
Secondly, careful planning is required to determine the type of wiring and method of installation. Underground cable and wire can be installed at different depths, depending on the type of conduit, wire, or cable used. For example, rigid metal electrical conduit with individual insulated wires should be installed at a depth of six inches, whereas PVC conduit with individual insulated wires should be installed at a depth of 18 inches. The type of soil will also impact your planning; if the soil is rocky or heavy clay, you may need to protect the wiring in metal or plastic conduit, whereas granular soil may allow you to dig deeper and avoid the need for conduit.
Additionally, when planning the route of your underground electrical cable, it is important to consider any obstacles that may be present. Avoid areas with severe acid or alkali corrosion, termite damage zones, heat sources, and sections that are easily damaged by external forces. Ensure you comply with local depth regulations and, if your cable is to be laid continuously through multiple sections, do not hang the cables in the middle of hand holes or manholes.
Finally, it is important to note that the installation process for underground electrical cables can be complex, and professional assistance may be required for certain installations.
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Digging trenches
The depth and width of the trench will depend on local regulations, the type of soil, and the chosen cable. In the UK, the minimum depth for cable laying below the ground varies according to the voltage capacity of the cable. For example, the depth should be at least 0.9 metres for 3.3kV to 11kV cables and 1.05 metres for 22kV to 33kV cables. The trench should be wide enough to accommodate the cable comfortably, typically ranging from 0.45 metres to 0.5 metres.
When digging the trench, it is important to avoid areas with severe acid or alkali corrosion, stray current electrochemical corrosion, termite damage zones, heat sources, and sections vulnerable to external forces. If the soil is rocky or composed of heavy clay, or if there are numerous tree roots, it is advisable to minimise digging and protect the wiring with metal or plastic conduit. Alternatively, trenchless technology, such as NODIG systems, can be employed to install underground cables without digging any trenches, making it suitable for environmentally sensitive areas.
Once the trench is dug, a protective layer of soft soil or sand, at least 100 mm thick, should be placed along the full length of the trench's upper and lower adjacent sides. A protective plate made of concrete, with a width of at least 50 mm on both sides of the cable, should be placed along the entire length for added protection.
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Cable laying
Laying underground electrical cables is a complex process that requires careful planning and execution. Before beginning any work, it is important to adhere to electrical standards and local requirements, such as obtaining the necessary permissions and approvals from relevant authorities.
The first step is to determine the type of soil and decide on the depth of the trench. If the soil is granular and easy to dig, you can dig deeper and may not need to install metal or plastic conduit to protect the wiring. Conversely, if the soil is rocky or clay-heavy, you can minimise digging and protect the wiring with conduit. The trench should be at least 0.9 metres deep for 3.3kV to 11kV cables and 1.05 metres deep for 22kV to 33kV cables.
Once the trench is dug, the cables are laid inside, ensuring they are suitable for underground use and are rated for use in a wet location. The cables should be placed on a 10-centimetre layer of fine sand, which protects them from moisture. If multiple cables are laid, a horizontal or vertical spacing of about 30 centimetres is recommended to prevent mutual heating.
After the cables are in place, cover them with another 10-centimetre layer of sand. Then, to protect against mechanical injury, cover the trench with bricks or other materials. Finally, integrate the cables into the electrical system, connecting them to the power supply and distribution points.
It is important to note that cable network alterations are difficult, so careful planning is essential. Additionally, pay attention to moisture prevention and take appropriate measures, such as keeping manholes or hand holes dry.
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System integration
Before initiating the system integration, it is essential to have a comprehensive understanding of the properties of underground electrical cables. This includes considerations such as the type and application of the cables, as well as the cost of underground wiring. Selecting the appropriate cable for the specific project is of utmost importance.
During system integration, the cables are carefully connected to the power supply and distribution points, ensuring compliance with local electrical codes. This process involves establishing secure connections and implementing safety measures to protect against moisture. Moisture protection is a crucial aspect of underground cable installations.
Additionally, the system integration phase encompasses verifying that the cable joints and branch connections are of superior quality. This verification process contributes to the overall reliability of the underground cable network. Proper cable joints and branch connections are essential for the efficient transmission of electrical signals.
Furthermore, system integration may involve the utilisation of innovative technologies such as trenchless installation methods. These methods, including the NODIG system, offer an environmentally friendly and minimally invasive approach to installing underground cables. Trenchless technology eliminates the need for disruptive trenching, making it suitable even for nature conservation areas and locations where traditional trenching methods are impractical.
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Inspection and testing
Underground electrical cables require regular maintenance to prevent problems and ensure the reliability and safety of power distribution systems. Before laying underground electrical cables, it is important to understand the testing and inspection requirements to ensure compliance with local regulations and to safeguard against future issues.
Firstly, it is crucial to determine the required depth for the trench in your specific area. Each region has regulatory organisations that oversee electrical installations, and non-compliance can lead to safety hazards and legal issues. After digging the trench to the specified depth, the cable should be installed inside a PVC conduit, which offers better protection than simply placing the cable directly into the trench.
Once the cable is laid, a visual inspection should be conducted to identify any potential issues. This includes checking for corrosion on copper components, cracks in insulation, moisture on cables, and other indicators of damage. It is also essential to compare cable data with drawings and specifications, paying attention to details such as the number of sets, cable size, routing, and insulation ratings.
Before energising low-voltage cables rated 600V or below, several tests must be performed. These tests include checking for material damage on uncovered cable parts, inspecting the condition of the cable jacket and insulation of exposed sections, and verifying that connection points match the project's single-line diagram. Bolted electrical connections should be checked for high resistance using specialised tools such as a calibrated torque wrench, low-resistance ohmmeter, or thermographic survey.
Additionally, the continuity test, also known as low-resistance measurement, is used to measure cable resistance from 1 mΩ to 250 Ω. This test can be performed in two or four wires, depending on the resistance being measured. The high-voltage test, also called the dielectric strength test or hipot test, can be conducted using alternating current (AC) or direct current (DC). This test detects any sudden rise in current and helps ensure the safety and integrity of the cable system.
After the initial inspections and tests, it is standard to have the installation inspected by local authorities or inspectors. They will verify that the conduit is buried to the proper depth and that the cable is laid correctly. In some cases, they may require the installation of warning tape in the trench to alert future diggers to the presence of underground cables. This safety measure can help prevent accidental damage to the cables and protect individuals from potential hazards.
Finally, after the inspector has approved the installation, you can proceed to fill in the trench and complete the wiring. However, it is recommended to have the entire installation inspected a third time after completion. This final inspection provides added assurance that everything was done correctly and protects you from potential issues or liabilities in the future.
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Frequently asked questions
Underground electrical cables are safer than aerial cables, which can collapse on buildings and cars and cause electric shocks. They also provide uninterrupted power, are out of sight, and require less space.
You will need to contact your local building inspection department to find out if an electrical permit and inspections are required. You will also need to adhere to electrical standards, such as IEC or local requirements.
The depth of the trench depends on the type of conduit, wire, or cable. For example, galvanised rigid metal electrical conduit with individual insulated wires should be installed 6 inches deep, while PVC conduit with individual insulated wires should be installed 18 inches deep. The minimum depth for cable laying below the ground also varies according to the voltage capacity of the cable. The depth should be at least 0.9m for 3.3kV to 11kV cables and 1.05m for 22kV to 33kV cables.
Armoured cables are designed with a metal protective layer to provide mechanical protection against physical damage and rodent attacks. They are suitable for direct burial in the ground. The wires must be rated for use in a wet location, such as Type THWN-2 (thermoplastic/wet location/nylon jacket).
First, plan the route of the cables, taking into account any obstacles and avoiding sites with severe acid or alkali corrosion. Then, dig the trench to the required depth, following local regulations. Lay the cables in the trench, ensuring proper cable joins and branch connections. Cover the cables with a layer of sand and then bricks or other materials for protection. Finally, integrate the cables into the electrical system and connect them to the power supply and distribution points. Perform a thorough system inspection to ensure everything is functioning properly and safely.
