Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Installation of Overhead Wires
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Installation of Overhead Wires
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Subject: Electrical Installation And Maintenance Work
Class: Senior Secondary 2
Term: 3rd Term
Week: 2
Theme: Workshop Practices
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Identity cable/wires, to ols and equipment used in over head distribution/transmission and state the ir uses. Draw line with appropriate tension and state the uses of different types of cross-arms used in transmission.
forces exerted by the conductors (e.g., tension at corners, ends, or due to wind). They prevent the pole from bending or collapsing.
Components of a Stay Wire Assembly: Stay Rod: A galvanized steel rod anchored into the ground.
Stay Plate: A plate attached to the stay rod in the ground for greater anchorage.
Stay Wire/Guy Wire: Stranded galvanized steel wire connected from the pole to the stay rod.
Guy Grip/Thimble: Used for secure attachment of the stay wire to the pole and the stay rod.
Stay Insulator (Egg/Strain Insulator): An insulator inserted in the stay wire to break its continuity and prevent ground faults in case the stay wire comes into contact with a live conductor. It also prevents the transfer of potential to ground.
6. Fittings and Accessories: Line Taps/Connectors: Devices used to make electrical connections between a main conductor and a service drop or another feeder. E.g., parallel groove clamps, wedge connectors, bolted connectors.
Tension Clamps/Dead-end Clamps: Used at terminal poles or strain points to hold the conductor securely and transmit its tension to the pole structure via insulators. E.g., wedge-type tension clamps, compression-type dead-end clamps.
Earth Wires/Ground Wires: Wires running along the top of poles/towers, connected to the earth, to protect against lightning strikes.
Danger Notices/Warning Signs: Posted on poles to warn the public of electrical hazards.
C. Tools for Overhead Installation:
1. Personal Protective Equipment (PPE): Essential for safety.
Safety Helmets: Protect against head injuries.
Safety Boots: Steel-toed boots protect feet.
Insulating Gloves: Protect against electrical shock.
Safety Harnesses and Lanyards: Used when working at heights to prevent falls.
Safety Goggles: Protect eyes from dust and debris.
2. Hand Tools: Pliers: Various types (combination, long nose, side cutting) for gripping, bending, and cutting wires.
Spanners/Wrenches: For tightening nuts and bolts on cross-arms, clamps, etc.
Screwdrivers: For various fasteners.
Hacksaw/Cable Cutters: For cutting conductors and metal components.
Measuring Tape/Ruler: For accurate measurements of spans, heights, etc.
Spirit Level: To ensure poles are vertical and cross-arms are horizontal.
3. Specialized Tools: Winches/Pulling Machines: Motorized or manual devices used to pull and tension conductors during stringing.
Come-alongs/Wire Grips: Temporary gripping devices used to hold conductors under tension, allowing linemen to work on them. They grip the wire without damaging it.
Cable Rollers/Pulleys: Used to support conductors and allow them to run smoothly during stringing operations, reducing friction and preventing damage.
Sagging Charts/Sagging Boards: Instruments or tables used to determine the correct sag (and thus tension) for conductors based on span length, temperature, and conductor type.
Hydraulic Crimpers/Compressors: Used with appropriate dies to create strong, permanent compression joints and terminations on conductors.
Pole Climbers (Gaffs) and Safety Belts: For linemen to ascend and work on wooden poles (less common now with bucket trucks, but still used in remote areas). Ladder/Scaffolding/Bucket Trucks (Cherry Pickers): Provide safe access for working at height.
D. Tensioning of Wires (Sagging): Concept of Sag: When a conductor is strung between two poles, it does not form a perfectly straight line but hangs down in a curve due to its weight and gravity. This curve is called "sag." Importance of Correct Tension/Sag: Preventing Over-stress: Too much tension can over-stress the conductor and supporting structures, leading to breakage, especially under adverse conditions (e.g., high winds).
Maintaining Electrical Clearance: Too much sag reduces the clearance between the conductor and the ground, buildings, or other conductors, posing a safety hazard (risk of electrocution).
Economic Factors: Proper tensioning balances material stress with required clearances, optimising costs.
Factors Affecting Sag: Span Length: Longer spans result in greater sag.
Conductor Weight: Heavier conductors have more sag.
Tension: Higher tension results in less sag (and vice-versa).
Temperature: Conductors expand in heat (increasing sag) and contract in cold (decreasing sag). This is a critical factor in Nigeria's varying weather.
Wind Load: Lateral forces from wind can increase sag temporarily or cause sway. Practical Methods for Sagging (Simplified for SS2): * Visual Method: For short spans, experienced linemen balances material stress with required clearances, optimising costs.
Factors Affecting Sag: Span Length: Longer spans result in greater sag.
Conductor Weight: Heavier conductors have more sag.
Tension: Higher tension results in less sag (and vice-versa).
Temperature: Conductors expand in heat (increasing sag) and contract in cold (decreasing sag). This is a critical factor in Nigeria's varying weather.
Wind Load: Lateral forces from wind can increase sag temporarily or cause sway. Practical Methods for Sagging (Simplified for SS2): Visual Method: For short spans, experienced linemen can visually estimate correct sag against a pre-marked point on an adjacent pole.
Timing Method (Wave Method): A lineman shakes the wire at one end, and another observes the time it takes for the wave to travel to the next pole and return. This time corresponds to a specific tension/sag.
Use of Sagging Board/Chart: Pre-calculated tables or boards provide the required sag for specific span lengths, conductor types, and temperatures. The linemen then adjust tension until the actual sag matches the chart. (This is the most accurate for distribution lines).
Dynamometer/Tension Meter: For critical installations, a device is inserted in the line to directly measure the tension.
Drawing a Line with Appropriate Tension: When drawing, the line should depict a curve (catenary curve), not a straight line, representing the sag. The sag should be realistic, not excessively deep or too shallow (which would indicate excessive tension).
Illustration: ``` P1 ------------------------------ P2 | | | | | ____V____ | | / \ | <-- Sag | / \ | | / \ | | / \ | |______|_________________|__________| Ground Level ``` (Where P1 and P2 are poles, and 'V' indicates the lowest point of the sag).
E. Jointing and Termination of Overhead Cables:
1. Jointing (Splicing): The process of connecting two conductor ends together to form a continuous electrical path.
Purpose: To extend the length of a conductor, repair a broken conductor, or connect a tap-off. Method (Compression Jointing - most common for overhead lines): Materials: Compression jointing sleeve (barrel), conductor.
Tools: Hydraulic crimper with appropriate dies, wire brush, measuring tape, hacksaw/cable cutters.
Steps:
1. Prepare Conductor: Cut the conductor ends squarely. Measure and mark the correct stripping length as per the sleeve specifications.
2. Clean Conductor: Thoroughly clean the conductor strands with a wire brush to remove oxidation and ensure good electrical contact.
3. Insert Conductor: Insert one conductor end into one side of the compression sleeve until it reaches the central stop.
4. Crimping: Using the hydraulic crimper and correct die, make a series of compressions (crimps) along the sleeve as specified by the manufacturer (e.g., starting from the center outwards). Ensure each crimp is firm and complete.
5. Repeat: Insert the second conductor end into the other side of the sleeve and repeat the crimping process.
6. Inspection: Visually inspect the joint for proper crimps and absence of sharp edges.
2. Termination: The process of connecting a conductor end to a piece of equipment (e.g., transformer bushing, disconnect switch, insulator).
Purpose: To create a secure and electrically sound connection between the overhead line and other electrical apparatus.
Method (Compression Termination): Materials: Compression lug (terminal connector), conductor.
Tools: Hydraulic crimper with appropriate dies, wire brush, measuring tape, cable cutters.
Steps:
1. Prepare Conductor: Cut the conductor end squarely. Measure and mark the correct stripping length as per the lug specifications.
2. Clean Conductor: Clean the exposed conductor strands thoroughly.
3. Insert Conductor: Insert the clean conductor into the barrel of the compression lug until it bottoms out.
4. Crimping: Using the hydraulic crimper and correct die, make a series of compressions along the barrel of the lug as specified.
5. Inspection: Inspect the termination for proper crimps. The lug's tongue is then bolted onto the equipment terminal. ---
A. Definition of Overhead Wires: Overhead wires, also known as overhead lines, are electrical conductors suspended above the ground by poles, towers, or other supporting structures. They are used for the transmission and distribution of electrical power over long distances or within local networks.
B. Components of Overhead Line Installation:
1. Cables/Wires (Conductors): These are the materials that carry electrical current. Aluminium Conductors Steel-Reinforced (ACSR): Very common in Nigeria for primary transmission and distribution lines due to its high tensile strength (from the steel core) and good conductivity (from the aluminium layers), combined with its relatively low weight and cost.
Construction: Consists of a central core of steel wires, surrounded by one or more layers of high-purity aluminium wires.
Use: Medium to long span transmission lines and primary distribution feeders.
All Aluminium Conductors (AAC): Made entirely of aluminium.
Use: Short-span distribution lines, urban areas, or where line sag is not a major concern. Less strength than ACS
R. All Aluminium Alloy Conductors (AAAC): Made from heat-treatable aluminium-magnesium-silicon alloy.
Use: Provides better strength-to-weight ratio and improved sag characteristics compared to AAC, suitable for longer spans than AAC but generally not as strong as ACS
R. Service Drop Cables: Insulated conductors used to connect the overhead distribution line from the pole to the consumer's meter point.
Types (common in Nigeria): Duplex: Two insulated conductors twisted together (one phase, one neutral).
Triplex: Three insulated conductors (two phases, one neutral or one phase, one neutral, one street light).
Quadruplex: Four insulated conductors (three phases, one neutral).
Use: Final connection to individual buildings.
2. Poles/Towers: Support structures for overhead conductors.
Wooden Poles: Treated timber poles (e.g., creosoted) are commonly used for distribution lines (up to 33kV) in many parts of Nigeria due to their cost-effectiveness and ease of installation.
Concrete Poles: Prestressed concrete poles offer greater durability, resistance to rot and termites, and longer lifespan than wooden poles. Used for distribution and sometimes sub-transmission lines.
Steel Lattice Towers: Used for high-voltage (e.g., 132kV, 330kV) transmission lines over long distances, providing high strength and stability for very long spans.
3. Insulators: Devices that provide electrical isolation between the conductor and the pole/tower, while also providing mechanical support for the conductors. They are typically made of porcelain or glass.
Pin Type Insulators: Used for lower voltage distribution lines (up to 33kV). The conductor is tied to the top groove of the insulator, which is mounted on a pin on the cross-arm.
Shackle Type Insulators: Used for low voltage (415V) distribution lines, especially at strain points or sharp bends. Disc Type Insulators (Suspension or Strain Insulators): Suspension Type: Used for high voltage lines, where insulators are connected in a string and the conductor hangs freely below the cross-arm. The number of discs depends on the voltage level.
Strain Type: Used at terminal poles, corner poles, or where there is a change in the direction of the line, to withstand high tension. The insulator string is placed horizontally.
4. Cross-Arms: Horizontal members attached to poles or towers, providing support for insulators and maintaining the required spacing between conductors. They ensure safe electrical clearance.
Wooden Cross-Arms: Common for wooden poles in distribution systems.
Steel Cross-Arms: Made from angle iron or channel steel, offering greater strength and durability. Used for concrete poles and higher voltage distribution lines.
Types/Configurations: Single cross-arm, double cross-arm (for strain poles), vertical configuration, horizontal configuration.
5. Stay Wires/Guy Wires: Supporting wires connected from the pole to the ground (or another pole) to counteract the lateral forces exerted by the conductors (e.g., tension at corners, ends, or due to wind). They prevent the pole from bending or collapsing.
Components of a Stay Wire Assembly: Stay Rod: A galvanized steel rod anchored into the ground.
Stay Plate: A plate attached to the stay rod in the ground for greater anchorage.
Stay Wire/Guy Wire: Stranded galvanized steel wire connected from the pole to the stay rod.
Guy Grip/Thimble: Used for secure attachment of the stay wire to the pole and the stay rod. *Stay
A. Teacher Activities: Introduction and Motivation (10 minutes): Engage students by showing pictures or short video clips of overhead power lines in Nigerian towns and villages. Ask students about their observations regarding how electricity reaches their homes.
Introduce the topic: "Installation of Overhead Wires" and briefly explain its importance for electrical technicians in Nigeria.
Key Components and Materials (20 minutes): Present actual samples (if available) or large, clear diagrams/posters of different types of overhead cables (ACSR, service drop), insulators (pin, disc, shackle), cross-arms (wood, steel), and components of a stay wire (wire, rod, insulator). Explain the construction, function, and typical application of each component in the Nigerian context. Facilitate a short Q&A session to check for understanding.
Tools and Equipment (15 minutes): Display various tools (PPE, come-alongs, wire grips, hydraulic crimper, cable rollers, safety harnesses). Demonstrate or explain the proper use and safety precautions for each tool, emphasizing their role in overhead line installation. Discuss the importance of PP
E. Conductor Tensioning (Sagging) (15 minutes): Explain the concept of sag using a simple physical model (e.g., a piece of string or thin wire held between two points with varying tension). Illustrate how sag changes with tension and span length. Discuss the dangers of too much or too little sag. Explain (verbally/with diagrams) how a sagging chart is used, without going into complex calculations. Draw on the board a simple diagram of an overhead line showing appropriate sag, and have students replicate it. Jointing and Termination Demonstration (25 minutes): If practicals are possible: Prepare scrap pieces of conductor (e.g., service drop cable, small ACSR) and a compression sleeve/lug, along with a hydraulic crimper.
Demonstrate step-by-step: Preparation of conductor (stripping, cleaning). Performing a compression joint. Performing a compression termination. Emphasize neatness, secure connections, and safety during the demonstration.
If practicals are not possible: Use clear, sequential diagrams or a detailed video illustration of the processes. Safety and Real-life Applications (10 minutes): Discuss critical safety measures when working on overhead lines (e.g., lockout/tagout, earthing, weather considerations). Reiterate the connection of this topic to career opportunities and Nigeria's power sector.
Conclusion and Assignment (5 minutes): Summarize key learning points. Assign independent practice questions.
B. Student Activities: Observation and Identification: Students carefully observe the displayed components, tools, and the teacher's demonstrations. They identify and name the different cables, tools, and equipment.
Discussion and Q&A: Actively participate in discussions, asking questions about the functions and applications of various components and tools.
Drawing Practice: Students draw a diagram of an overhead line, correctly depicting the sag of the conductor and identifying key components like poles, insulators, and cross-arms.
Practical Engagement (if feasible): Simulation: In groups, students might simulate the use of tools (e.g., handling pliers, mimicking crimping motions) with non-live components.
Observation of Jointing/Termination: Closely observe the teacher's demonstration of jointing and termination, noting each step. If materials are available, students could try preparing conductor ends under supervision.
Note-Taking: Take comprehensive notes on key definitions, uses of tools and materials, and safety procedures.
Group Work: In small groups, students might discuss and list common hazards associated with overhead line work in Nigeria and suggest preventive measures. ---
Rural Electrification in Nigeria: The installation of overhead wires is fundamental to extending electricity access to remote and rural communities across Nigeria. Projects by the Rural Electrification Agency (REA) heavily rely on this technology. Students learning this topic will understand how their skills can directly contribute to improving livelihoods, powering schools, health centers, and small businesses in underserved areas, aligning with national development goals. Maintenance and Repair of Existing Power Infrastructure: Nigeria's extensive power distribution network, largely comprising overhead lines, requires continuous maintenance, fault location, and repair. Knowledge of overhead wire installation principles is crucial for identifying defects, performing repairs (like re-stringing, jointing broken conductors, replacing damaged insulators or poles) for Electricity Distribution Companies (DisCos), ensuring reliable power supply to homes and industries.
Safety and Public Awareness: The topic inherently integrates safety considerations. Understanding how overhead lines are installed (e.g., proper sag, use of stay insulators, maintaining clearances) helps students grasp the importance of electrical safety. They can then become advocates for safe practices in their communities, warning others about the dangers of illegal connections, unauthorized climbing of poles, or building too close to power lines, which are significant issues in Nigeria. This directly impacts community well-being and reduces electrical accidents. ---