Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Joints and Termination of Cables
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Joints and Termination of Cables
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Subject: Electrical Installation And Maintenance Work
Class: Senior Secondary 2
Term: 2nd Term
Week: 4
Theme: Workshop Practices
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Identify and use all to ols and materials related to cable joining and termination Recognize various types of conductors and in sulating materials Make different types of joints.
material for low-voltage cables in Nigeria. Good dielectric strength, flame-retardant, resistant to oils, acids, and alkalis. Relatively inexpensive. Limited temperature range.
Cross-Linked Polyethylene (XLPE): Offers superior temperature resistance, higher dielectric strength, and better chemical resistance than PVC. Used for medium and high-voltage cables, and in applications requiring higher performance (e.g., industrial wiring, direct burial).
Rubber: Good flexibility and moisture resistance. Used in flexible cords and some industrial cables. Less common for fixed installations due to ageing properties compared to plastics.
Paper (Impregnated): Historically used for high-voltage power cables, often impregnated with oil. Offers good dielectric strength but is susceptible to moisture and requires a protective sheath. Less common now due to XLPE advancements. 2.6 Making Different Types of Joints (Step-by-Step Procedures): General Principles for all Joints:
1. Safety First: Ensure power is OFF before working on any cable. Wear appropriate PPE (gloves, safety glasses).
2. Strip Insulation: Carefully remove insulation to the required length without nicking or damaging the conductor strands.
3. Clean Conductors: Scrape or clean conductors to remove dirt, grease, or oxidation, ensuring good electrical contact.
4. Mechanical Strength: Twist/wrap conductors to achieve a strong mechanical connection before soldering (if applicable).
5. Solder (if required): Apply flux, then heat the joint (not the solder) until hot enough to melt the solder, allowing it to flow into the strands. Allow to cool naturally.
6. Insulate: Re-insulate the joint using insulating tape, heat shrink, or other suitable materials to match or exceed the original insulation level. 2.6.1 Rattail (Pig-tail)
Joint: Application: Connecting flexible cables, inside junction boxes for fixture connections.
Procedure:
1. Strip about 3-4 cm of insulation from the ends of the two (or more) conductors to be joined.
2. Hold the stripped ends together, parallel to each other.
3. Twist the conductors tightly and evenly together using combination pliers, forming a neat spiral, resembling a rat's tail.
4. Cut off any excess wire to leave about 1.5 - 2 cm of twisted wire.
5. Solder the joint for enhanced conductivity and strength (optional but recommended).
6. Insulate the entire joint with insulating tape or heat shrink. 2.6.2 Married Joint: Application: Extending multi-strand flexible cables where flexibility is important.
Procedure:
1. Strip about 5-7 cm of insulation from the ends of the two conductors to be joined.
2. Divide the strands of each conductor into two equal halves.
3. Interlace the two sets of strands from each conductor (like fingers locking).
4. Carefully twist the strands from one side of the first conductor around the main body of the second conductor, wrapping neatly.
5. Repeat the process for the remaining strands.
6. Solder the joint.
7. Insulate the joint. 2.6.3 Tee Joint (Standard Tee Splice): Application: Branching off a connection from a continuous main conductor without cutting the main conductor. Common in distribution systems and domestic wiring.
Procedure:
1. Prepare Branch Wire: Strip about 5-7 cm of insulation from the end of the branch wire.
2. Prepare Main Wire: Carefully remove about 5-7 cm of insulation from the middle of the main conductor without cutting the conductor itself.
3. Twist: Wrap the stripped end of the branch wire tightly around the exposed main conductor, starting from one end and moving towards the other, ensuring even and close turns.
4. Cut Excess: Cut off any excess branch wire.
5. Solder (Optional but recommended): Solder the entire joint.
6. Insulate: Thoroughly insulate the joint, ensuring full coverage and protection, extending beyond the original insulation of both wires. 2.6.4 Western Union Joint (Long Form): Application: For extending overhead bare conductors where high mechanical strength and good conductivity are required. Less common in insulated domestic wiring. * Procedure:
1. Strip about 8-10 cm of insulation from each conductor end.
2. Clean the conductors.
3. Bring the two conductors together, overlapping by about 2-3 cm at the center of the stripped sections, and twist them to form a slight bend or 'hook'.
4. Using pliers, wrap the free end of one conductor tightly around the main body of the other conductor for 5-7 turns.
5. Repeat for the other free end, wrapping it tightly around the mechanical strength and good conductivity are required. Less common in insulated domestic wiring.
Procedure:
1. Strip about 8-10 cm of insulation from each conductor end.
2. Clean the conductors.
3. Bring the two conductors together, overlapping by about 2-3 cm at the center of the stripped sections, and twist them to form a slight bend or 'hook'.
4. Using pliers, wrap the free end of one conductor tightly around the main body of the other conductor for 5-7 turns.
5. Repeat for the other free end, wrapping it tightly around the first conductor.
6. Ensure all turns are tight and neat.
7. Solder the entire joint.
8. Insulate thoroughly if it's an insulated cable. 2.6.5 Britannia Straight Joint: Application: For joining large single-strand conductors (e.g., overhead line conductors) where high mechanical strength and good current carrying capacity are essential. Requires a separate 'binding' wire.
Procedure:
1. Strip about 10-12 cm of insulation from each conductor end.
2. Taper the ends of the conductors (file them to a wedge shape) for about 2-3 cm.
3. Clean the conductors.
4. Overlap the tapered ends of the two main conductors, ensuring good contact.
5. Use a smaller gauge bare copper 'binding wire' (e.g., SWG 18 or 20). Starting from one side, wrap the binding wire tightly and closely around the overlapped main conductors, covering the tapered section and extending a short distance onto the un-tapered part.
6. Repeat the wrapping from the other side.
7. Solder the entire joint liberally to ensure good electrical conductivity.
8. Insulate the joint. 2.7 Cable Termination to Accessories (e.g., 13A Socket Outlet or Switch): Application: Connecting cables to switches, sockets, lamp holders, circuit breakers, fan regulators, etc. Procedure (
Example: Connecting to a 13A Socket Outlet):
1. Safety: Ensure the circuit is completely de-energized at the consumer unit/distribution board.
2. Prepare Cable: Measure the required length of cable to reach the terminal cleanly, with some slack.
3. Remove Outer Sheath: Carefully remove the outer sheath of the cable (e.g., twin and earth) using an electrician's knife or appropriate stripper, exposing the insulated core conductors (live, neutral, earth). Be careful not to damage the inner insulation.
4. Strip Core Insulation: For each core conductor, strip about 10-15 mm of insulation from the end using a wire stripper. For stranded wires, gently twist the strands together to keep them tidy.
5. Form Loop (if required): If the terminal is a screw type that requires wrapping, bend the stripped end into a small loop that matches the direction of the screw tightening (usually clockwise for right-hand threads). For clamp-type terminals (common in modern accessories), a straight end is sufficient; sometimes ferrules are used for stranded wires.
6. Connect to Terminals: Live (Brown or Red): Connect to the "L" terminal.
Neutral (Blue or Black): Connect to the "N" terminal.
Earth (Green/Yellow): Connect to the "E" terminal.
7. Secure Connection: Insert each prepared conductor into its correct terminal. Tighten the terminal screw firmly but without overtightening, which can damage the conductor or terminal. Gently tug on the wire to ensure it is secure.
8. Dressing: Ensure the cables are neatly dressed within the accessory box, avoiding stress on the terminals or sharp bends.
9. Cover: Replace the accessory cover or faceplate.
1
0. Test: After completing all connections, carry out relevant electrical tests (e.g., continuity, insulation resistance) before re-energizing the circuit. --- 2.1 Cable Joining: Cable joining refers to the process of mechanically and electrically connecting two or more cable ends to extend a circuit, create a branch, or repair a damaged section. The primary goals are to ensure low resistance (good electrical conductivity), high mechanical strength, and proper insulation comparable to the original cable. 2.2 Cable Termination: Cable termination is the process of connecting a cable end to an electrical device, accessory, terminal block, or other equipment. This involves preparing the cable end (stripping insulation, twisting strands if necessary) and securely fastening it to ensure a stable electrical connection and mechanical support. 2.3 Tools and Materials Related to Cable Joining and Termination: Tools: Combination Pliers: Used for gripping, twisting, cutting, and sometimes stripping wires. Essential for general electrical work.
Long-nose Pliers: Useful for gripping small components, bending wires in tight spaces, and making loops.
Side-cutting Pliers (Diagonal Cutters): Specifically designed for cutting electrical wires cleanly.
Wire Strippers: Used for removing insulation from conductors without damaging the metallic core. Available in manual and automatic types. Electrician's Knife (Pocket Knife/Utility Knife): Used for removing tough outer sheathing, scraping oxidation from conductors, or general cutting tasks. Requires careful handling. Screwdrivers (Flat-head and Phillips-head): Used for tightening and loosening terminal screws on accessories and equipment.
Crimping Tool: Used to mechanically press cable lugs or ferrules onto cable ends, creating a gas-tight, highly conductive connection.
Soldering Iron/Gun: Used for applying solder to electrical joints to ensure a highly conductive and mechanically strong connection. Requires flux and solder.
Multimeter: Used for testing continuity, voltage, and resistance to ensure correct connections and identify faults.
Materials: Insulating Tape (PVC Tape): A flexible, adhesive tape used for insulating exposed conductors, protecting against short circuits, and providing a degree of mechanical protection. Available in various colours for phase identification (e.g., red, yellow, blue, black, green/yellow).
Self-Amalgamating Tape: A rubber-based tape that fuses into a solid mass when stretched and wrapped, providing excellent waterproofing and insulation, often used for outdoor joints or high-moisture environments.
Heat Shrink Tubing: Polymer tubing that shrinks when heated, providing robust insulation, mechanical protection, and strain relief for joints and terminations.
Solder: An alloy (e.g., tin-lead or lead-free alternatives) with a low melting point used to form a metallurgical bond between conductors, enhancing electrical conductivity and mechanical stability.
Flux: A chemical agent (e.g., rosin-based) used during soldering to clean the metal surfaces, remove oxides, and facilitate the flow of solder.
Cable Lugs/Ferrules: Metal terminals (usually copper or brass) crimped onto the ends of larger cables to provide a secure and conductive connection to terminal blocks or busbars. Ferrules are used for stranded wires to prevent fraying and ensure good contact in screw terminals.
Connector Blocks (Terminal Blocks): Insulated blocks with metal inserts used to join multiple conductors or terminate cables securely within enclosures.
Junction Boxes: Enclosures used to house and protect cable joints and terminations, providing safety and organisation. 2.4 Types of Conductors: Copper: Excellent electrical conductivity, high ductility (can be drawn into thin wires), good corrosion resistance, and high tensile strength. Widely used for domestic, commercial, and industrial wiring due to its superior performance. More expensive than aluminum.
Aluminium: Good electrical conductivity (about 60% of copper), lighter than copper, and more affordable. Used extensively in overhead transmission lines and larger distribution cables due to its cost-effectiveness and weight. Requires special consideration for termination due to its tendency to creep under pressure and form oxides. 2.5 Types of Insulating Materials: Polyvinyl Chloride (PVC): Most common insulating material for low-voltage cables in Nigeria. Good dielectric strength, flame-retardant, resistant to oils, acids, and alkalis. Relatively inexpensive. Limited temperature range.
Cross-Linked Polyethylene (XLPE): Offers superior temperature resistance, higher dielectric strength, and better chemical resistance than PVC. Used for medium and high-voltage cables, and in applications requiring higher performance (e.g., industrial wiring, direct burial).
Rubber: Good flexibility and moisture resistance. Used in flexible cords and some industrial cables. Less common for fixed installations due to ageing properties compared to plastics. * Paper (Impregnated): Historically used for high-voltage power cables, often 3.1 Teacher Activities: Introduction (10 minutes): Initiate a discussion on the importance of electricity in daily life in Nigeria and the potential dangers of faulty electrical connections (e.g., fire incidents, electric shocks from exposed wires). Briefly introduce the topic "Joints and Termination of Cables" as essential skills for safe and reliable electrical installations. Present the learning objectives clearly to the students. Tools and Materials Identification & Demonstration (20 minutes): Display a collection of actual tools and materials relevant to cable joining and termination. For each tool, demonstrate its correct use (e.g., how to strip a wire using wire strippers, how to cut with side cutters, how to hold with pliers). For materials, explain their function (e.g., how insulating tape provides insulation, the purpose of solder). Engage students by asking them to identify tools and state their uses. Conductors and Insulators Explanation (15 minutes): Explain the properties of common conductors (copper, aluminum) and insulators (PVC, XLPE) using visual aids (samples of cables). Discuss the advantages and disadvantages of each, relating them to practical applications and cost implications in the Nigerian market. Demonstration of Cable Joints (30 minutes): Using actual cables (e.g., 2.5mm2 single core or flexible cable), perform a step-by-step demonstration of making at least three different types of joints: Rattail Joint Tee Joint Married Joint Emphasize safety precautions and the importance of neatness, mechanical strength, and proper insulation at each step. Demonstrate how to solder a joint correctly and then insulate it. Demonstration of Cable Termination (20 minutes): Using a section of appropriate cable (e.g., 2.5mm2 twin and earth) and a common accessory (e.g., a 13A surface socket outlet or a one-gang switch), demonstrate the correct procedure for terminating the cable into the accessory's terminals. Highlight the importance of correct core identification (L, N, E) and ensuring no stray strands cause short circuits. Practical Work Introduction & Supervision (5 minutes): Organise students into small groups (if resources permit) and distribute tools, cables, and accessories. Clearly state the practical tasks students are to perform. Circulate around the classroom/workshop, observing, guiding, and correcting students' practical work.
Question and Answer Session (10 minutes): Address any questions or difficulties students encountered during the demonstrations or initial practical work. Reinforce key safety messages. 3.2 Student Activities: Active Observation and Participation (Throughout): Pay close attention during demonstrations. Participate in discussions, identify tools, and answer questions posed by the teacher. Tool and Material Identification (15 minutes): Working individually or in pairs, students will handle the provided tools and materials, correctly naming them and stating their basic uses.
Cable Preparation Practice (20 minutes): Practice using wire strippers to remove insulation from various cable types without damaging the conductor. Practice using side cutters to cut cables to specific lengths.
Joint Making Practice (45 minutes): Under teacher supervision, students will practice making: Rattail Joint Tee Joint Married Joint Students should strive for neatness, mechanical strength, and proper insulation. Soldering practice can be included if time and resources allow.
Cable Termination Practice (30 minutes): Students will practice terminating a cable (e.g., a two-core or three-core flexible cable or a twin-and-earth cable) into an accessory such as a switch or a socket outlet. They should focus on correct core connection, secure terminal tightening, and prevention of stray strands.
Peer Review (10 minutes): Students can review and constructively critique each other's practical work, identifying areas for improvement, under teacher guidance.
Documentation (5 minutes): Students may be required to sketch the joints they have made and label the tools used. ---
Home Electrical Maintenance and Upgrades (Community & Economy): Students will understand how to safely repair damaged appliance cords (e.g., for pressing irons, blenders, fans), saving household expenses and preventing safety hazards. They can competently extend power points, install new light fittings, or replace faulty switches/sockets in their homes or for neighbours, contributing to safer living environments and potentially earning income as apprentices or skilled labour in their communities. This directly supports local electricians and small businesses. Solar Power and Rural Electrification (Environment & Economy): In Nigeria, with increasing adoption of solar energy, especially in rural areas lacking grid access, proper cable jointing and termination are critical for installing solar panels, connecting charge controllers, inverters, and batteries. Students will gain skills directly applicable to these emerging 'green' technologies, contributing to sustainable energy solutions and opening up employment opportunities in this growing sector. Learning these skills ensures that such installations are efficient and reliable. Industrial Wiring and Fault Rectification (Economy & Safety): Many small and medium-scale industries in Nigeria rely on electrical machinery. Proper cable joints and terminations are essential for connecting motors, control panels, and other equipment. Students skilled in these techniques can assist in the installation and maintenance of industrial electrical systems, preventing costly downtime due to electrical faults and ensuring the safety of workers by eliminating potential fire or shock hazards caused by substandard connections. This reinforces the importance of professional standards in the workplace. ---