Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Underground Cable Installation
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Underground Cable Installation
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Electrical Installation And Maintenance Work
Class: Senior Secondary 2
Term: 3rd Term
Week: 1
Theme: Workshop Practices
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
Select appropriate types of cable for under ground cable to site and prepare to appropriate depth for cable laying. Make cable joints/termination, solder under ground cable joint. Identify various materials and to ols used for joints and termination in under ground cable.
This section provides the core content necessary for the teacher to deliver the lesson effectively. A. Introduction to Underground Cables Underground cables are electrical cables designed to be buried directly in the ground or laid in conduits beneath the surface. They offer advantages such as improved aesthetics (no overhead lines), reduced vulnerability to weather conditions (storms, lightning), less risk of accidental contact, and a lower electromagnetic field impact.
However, they are more expensive to install, and fault location and repair can be more challenging. B. Types of Cables for Underground Installation The selection of an appropriate cable type depends on factors such as voltage level, current carrying capacity, environmental conditions (soil type, moisture), mechanical stress, and cost. Common types suitable for underground installation in Nigeria include:
1. PVC (Polyvinyl Chloride) Insulated, Armoured Cable: Construction: Consists of copper or aluminium conductors, PVC insulation, an inner bedding layer, steel wire or tape armouring (for mechanical protection against impact and rodents), and an outer PVC sheath.
Application: Widely used for low voltage (LV) distribution (e.g., 230V/400V) in residential, commercial, and industrial settings. Suitable for direct burial where mechanical protection is required.
Advantage: Relatively cost-effective, good mechanical protection due to armouring.
2. XLPE (Cross-linked Polyethylene) Insulated, Armoured Cable: Construction: Similar to PVC armoured but uses XLPE for insulation. XLPE is a thermosetting material with superior thermal, electrical, and chemical properties compared to PV
C. It can withstand higher temperatures and is less prone to thermal deformation.
Application: Preferred for medium voltage (MV) (e.g., 11kV, 33kV) and some low voltage applications where higher current carrying capacity and better thermal performance are needed. Often used by DisCos for feeders.
Advantage: High current carrying capacity, excellent heat resistance, good insulation properties, longer lifespan.
3. PILC (Paper Insulated Lead Covered)
Cable: Construction: Copper or aluminium conductors insulated with layers of oil-impregnated paper, a lead sheath (to prevent moisture ingress), bedding, armouring, and an outer serving.
Application: Historically used for high voltage (HV) and MV distribution. Less common in new installations due to environmental concerns with lead and complex jointing requirements, but still found in older installations across Nigeria.
Advantage: High reliability and long life when properly installed.
4. SWA (Steel Wire Armoured)
Cable: Construction: A general term referring to cables with steel wire armouring for mechanical protection. Can have PVC or XLPE insulation. Often used in industrial settings.
Application: Versatile for various voltage levels where robust mechanical protection is paramount. C. Site Preparation and Trenching for Cable Laying Proper site preparation and trenching are critical for the safety, longevity, and reliability of underground cable installations.
1. Route Planning and Survey: Identify the exact cable route, considering existing underground utilities (water pipes, gas lines, other electrical cables, telecommunication lines). Consultation with relevant authorities (e.g., Water Board, Gas Company, other DisCos) is mandatory in Nigeria to prevent damage during excavation. Mark the route clearly on the ground using paint or pegs.
2. Excavation (Trenching): Depth: The depth of the trench is critical for safety and protection.
Typical recommended depths in Nigeria are: Low Voltage (LV) (up to 1kV): Minimum 0.75 meters (2.5 feet) in areas not subject to heavy vehicular traffic. 1.0 meter (3.3 feet) under roads or pavements.
Medium Voltage (MV) (1kV to 33kV): Minimum 1.0 - 1.2 meters (3.3 - 4 feet).
Width: The trench width should be sufficient to accommodate the cable(s) with adequate spacing and allow for proper backfilling and compaction, typically 300mm to 450mm (12 to 18 inches) for a single cable.
Safety: Use appropriate personal protective equipment (PPE) like hard hats, safety boots, and gloves. Ensure trench stability, especially in loose soil, by shoring if necessary. Provide warning signs and barriers around the excavation site.
3. Trench Bedding: Once the trench is excavated to the required depth, the bottom should be leveled and cleared of sharp stones or debris. A layer of fine sand (minimum 75mm to 100mm, 3-4 inches) should be laid as a "bedding" to protect the cable from sharp objects and to provide a uniform, stable base.
4. Cable Laying: * The cable is (PPE) like hard hats, safety boots, and gloves. Ensure trench stability, especially in loose soil, by shoring if necessary. Provide warning signs and barriers around the excavation site.
3. Trench Bedding: Once the trench is excavated to the required depth, the bottom should be leveled and cleared of sharp stones or debris. A layer of fine sand (minimum 75mm to 100mm, 3-4 inches) should be laid as a "bedding" to protect the cable from sharp objects and to provide a uniform, stable base.
4. Cable Laying: The cable is carefully laid on the sand bedding. Avoid sharp bends or kinks. Cables should be laid with sufficient slack to accommodate ground movement. For multiple cables, ensure proper spacing (e.g., 150mm for LV, more for MV) to prevent overheating.
5. Initial Backfilling (First Cover): After laying the cable, it is covered with another layer of fine sand (minimum 75mm to 100mm, 3-4 inches) to further protect it.
6. Warning Tape: A brightly colored plastic warning tape (often yellow with "ELECTRIC CABLE BELOW" or "POWER CABLE" written on it) is laid approximately 300mm (12 inches) above the cable, within the backfill. This serves as a visible warning to future excavators.
7. Final Backfilling and Compaction: The remaining trench is backfilled with excavated soil, free from large stones or construction debris. The soil is compacted in layers to prevent subsidence, ensuring the ground level is restored to its original state.
D. Cable Jointing and Termination
1. Cable Jointing: Jointing is the process of connecting two or more lengths of cable to extend the run or to create a branch. The primary goals are electrical continuity, effective insulation, and mechanical protection.
Types of Joints: Straight-through Joint: Connects two cable ends in a straight line to extend the cable length.
Branch Joint (T-Joint or Y-Joint): Connects a feeder cable to a main cable. General Steps for Making a Straight-through Joint:
1. Preparation: Clean the cable ends. Measure and cut back the outer sheath, armouring, bedding, and insulation layers according to the jointing kit instructions, exposing the conductors.
2. Conductor Connection: Crimping: For larger conductors, use a crimping tool and appropriate lugs/connectors. Ensure a secure, low-resistance connection.
Soldering: For smaller conductors, clean conductors, apply flux, heat the joint with a soldering iron or blow torch (carefully), and apply solder until it flows freely into the strands, forming a strong bond. Allow to cool.
3. Insulation: Re-insulate each conductor individually using self-amalgamating tape, PVC tape, or heat/cold shrink tubes provided in the jointing kit. Build up layers to match the original insulation thickness.
4. Overall Insulation and Sealing: Apply outer insulation layers (e.g., another layer of self-amalgamating tape, followed by PVC tape, or a heat/cold shrink tube) to seal the entire joint against moisture and provide electrical isolation.
5. Mechanical Protection: For armoured cables, restore the armouring continuity (e.g., using armour clamps or bonding braids) and provide overall mechanical protection (e.g., resin-filled joint box, outer heat shrink sleeve with mastic sealant).
2. Cable Termination: Termination is the process of connecting the end of a cable to an electrical apparatus, such as switchgear, transformers, motors, or distribution boards. The aims are secure electrical connection, insulation, and environmental sealing.
General Steps for Termination:
1. Preparation: Measure and cut back the outer sheath, armouring (if present), and insulation layers according to the equipment's terminal block and termination kit requirements.
2. Lugging: Crimp or solder a suitable cable lug onto each conductor. Ensure the lug size matches the conductor and the equipment terminal.
3. Insulation and Sealing: Insulate each lug and the exposed conductor-lug connection area. This is typically done using heat shrink tubes, cold shrink tubes, or specialized insulating shrouds. The aim is to provide proper electrical isolation and environmental protection.
4. Connection: Secure the terminated cable lugs to the equipment's terminals, ensuring tight and proper connections.
E. Materials and Tools Used for Joints and Terminations
1. Materials: Cable Jointing/Termination Kits: Specific kits (e.g., heat shrink, cold shrink, resin-filled) designed for different cable types and voltage levels. These often include all necessary tapes, tubes, resin, and connectors. Insulate each lug and the exposed conductor-lug connection area. This is typically done using heat shrink tubes, cold shrink tubes, or specialized insulating shrouds. The aim is to provide proper electrical isolation and environmental protection.
4. Connection: Secure the terminated cable lugs to the equipment's terminals, ensuring tight and proper connections.
E. Materials and Tools Used for Joints and Terminations
1. Materials: Cable Jointing/Termination Kits: Specific kits (e.g., heat shrink, cold shrink, resin-filled) designed for different cable types and voltage levels. These often include all necessary tapes, tubes, resin, and connectors.
Cable Lugs/Connectors: Copper or aluminium lugs for terminating conductors to equipment terminals or joining conductors.
Insulation Tapes: PVC Electrical Tape: General-purpose insulation, color-coding.
Self-Amalgamating Tape: Rubber-based tape that fuses into a solid, waterproof mass, excellent for primary insulation and moisture sealing.
Mastic Tape: Used for filling voids and sealing.
Solder and Flux: For soldered connections. Solder (lead-free preferred for environmental reasons) and appropriate flux to ensure good solder flow and prevent oxidation.
Cable Grease/Petroleum Jelly: To ease installation of shrinkable components and provide additional moisture protection.
Warning Tape: Yellow/black "ELECTRIC CABLE" tape for backfilling.
Fine Sand: For trench bedding and cable covering.
2. Tools: Cable Cutters: Heavy-duty cutters for severing cables.
Cable Strippers/Knives: For carefully removing cable sheaths and insulation without damaging conductors.
Crimping Tool (Hydraulic/Manual): For securely attaching cable lugs/connectors to conductors.
Soldering Iron/Blow Torch: For heating conductors and melting solder.
Hacksaw: For cutting armouring (if not using specialised armour cutters).
Measuring Tape/Rule: For accurate measurements. Shovel, Pickaxe, Hoe: For excavation and backfilling of trenches.
Spirit Level/Plumb Bob: For ensuring trench levels and straightness. Gloves (Leather, Rubber): For safety and protection.
Safety Boots/Hard Hat: Essential PPE for site work.
Cleaning Agents: Solvent wipes for cleaning cable surfaces before jointing.
Heat Gun (for Heat Shrink Kits): To uniformly shrink heat shrink tubes.
A. Teacher Activities: Introduction (10 minutes): Initiate a discussion by asking students to identify common methods of electrical power distribution (overhead vs. underground). Present real-life scenarios in Nigeria where underground cables are used (e.g., Victoria Island/Ikoyi estates, major industrial layouts, airport facilities, new shopping malls).
Introduce the topic: Underground Cable Installation, and briefly outline the lesson's objectives. Concept Presentation and Explanation (30 minutes): Utilize diagrams, charts, and actual cable samples (if available) to explain the construction of various underground cables (PVC armoured, XLPE armoured, SWA). Explain the properties and applications of each cable type, relating them to Nigerian contexts (e.g., why XLPE is preferred for DisCo feeders). Detail the steps for site preparation, trenching (depths, width, bedding), cable laying, initial backfilling, warning tape placement, and final compaction. Use visual aids like photos of trenches or a simulated trench layout in the school compound. Explain the principles and procedures for making cable joints (straight-through, branch) and terminations. Emphasize the importance of clean connections, proper insulation, and mechanical protection. Clearly explain the soldering process, including flux application and heating techniques, stressing safety. Present and describe the function of all key materials and tools used in underground cable installation, jointing, and termination. Show actual tools and materials if possible.
Demonstration (30 minutes): Trench Preparation Simulation: If an outdoor space is available, guide students to mark out a small trench area (e.g., 2m long, 0.4m wide, 0.75m deep) and demonstrate the initial steps of excavation, leveling, and laying a sand bed. Alternatively, use a sand tray or diagram to simulate.* Cable Preparation and Jointing/Termination (using scrap cable): Demonstrate safe and accurate stripping of cable sheath, armouring, and insulation using appropriate tools. Demonstrate crimping a lug onto a conductor. Demonstrate a simple soldered connection on a smaller conductor (e.g., 2.5mm2 or 4mm2 multi-strand wire), explaining flux and solder application. Demonstrate how to apply insulation tapes (PVC, self-amalgamating) for re-insulation. If a heat-shrink kit is available for demonstration, show how to use it with a heat gun. Facilitate Q&A and Discussion (10 minutes): Encourage students to ask questions regarding the concepts, procedures, and safety measures. Prompt discussions on challenges encountered in underground installations in Nigeria (e.g., vandalism, poor soil conditions, existing utility conflicts).
B. Student Activities: Active Listening and Note-Taking: Students will listen attentively to explanations and take comprehensive notes.
Observation: Students will observe all teacher demonstrations carefully, asking clarifying questions.
Identification: Students will identify various types of cables, materials, and tools displayed.
Group Practical Exercise (30 minutes): In small groups, students will practice measuring and marking a trench area. Using scrap cables and appropriate tools, students will practice: Carefully stripping cable sheaths and insulation. Crimping lugs onto conductors. Applying insulation tapes (PVC and self-amalgamating) to create a mock joint insulation.
Note: Soldering practice can be done under strict supervision with proper safety measures due to heat.* Discussion and Reporting: Groups will discuss their observations and practical challenges, reporting back to the class.
Urban Development and Aesthetics: In modern Nigerian cities like Lagos, Abuja, and Port Harcourt, underground cable installation is crucial for enhancing the urban landscape by eliminating unsightly overhead power lines. This supports master plans for smart cities, new residential estates, and commercial hubs, leading to cleaner visuals and increased property value.
Power Reliability and Safety: Underground cables significantly improve the reliability of power supply by being less susceptible to weather-related damages (storms, high winds), vehicular accidents, and vandalism compared to overhead lines. This translates to fewer power outages for homes and businesses in Nigeria, enhancing economic productivity and daily life convenience.
Furthermore, they drastically reduce the risk of electrocution from fallen lines or accidental contact, a significant safety benefit in densely populated areas.
Industrial and Commercial Zones: Industrial estates (e.g., Agbara Industrial Estate, Ikeja Industrial Estate) and large commercial complexes often rely heavily on underground cable networks for their internal power distribution. This ensures a robust and uninterrupted power supply for machinery, offices, and retail operations, critical for sustained economic activity and minimizing operational downtime. Knowledge of underground cable maintenance is thus vital for technicians working in these sectors.