Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: Second Term
Week: 7
Age: 15 years
Duration: 45 minutes
Subject: Electrical Installation and Maintenance Work
Curriculum Theme: Technology
Previous Lesson: Two Points of Light Controlled by a Switch.
Topic: CURRENT RATING OF A CABLE
Subject Matter: Definition of current rating of a cable, factors affecting current rating of a cable, types of conductors, copper conductor, aluminium conductor, observing cable sizes and clips.

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define the term “current rating of a cable.”
State at least three factors that affect the current rating of a cable.
Identify the two main types of conductors used in electrical cables.
Describe two characteristics of copper conductors and two of aluminium conductors.

Affective Domain:

Appreciate the need for selecting the correct cable size for electrical safety.
Show willingness to learn more about safe electrical practices.

Psychomotor Domain:

Observe and identify different sizes and types of electrical cables.
Point out examples of cable clips and their uses.

Contribute to class discussions on the importance of proper cable selection.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum for Electrical Installation and Maintenance Work.
State Unified Scheme of Work for Senior Secondary Schools.
Electrical Installation and Maintenance Work for Senior Secondary Schools, Book 1.

Instructional Materials

The teacher will teach this lesson with the aid of:

Various sizes and types of electrical cables (e.g., PVC insulated copper, aluminium conductor).
Different types of cable clips.
Charts showing cable current ratings.

Rationale for the Lesson

This lesson helps pupils understand why different electrical cables are used for different purposes. It enables them to identify the right cable for a specific electrical load, which is important for preventing electrical hazards and ensuring safety in homes and workplaces.

Prerequisite/Previous Knowledge

Pupils should have basic knowledge of electric current, conductors, and insulators from previous lessons.

Lesson Content/Board Summary

CURRENT RATING OF A CABLE

Definition of Current Rating of a Cable

The current rating of a cable is the maximum electric current (measured in Amperes) that a cable can safely carry continuously without exceeding its temperature limits and causing damage to its insulation or the cable itself.

Factors Affecting Current Rating of a Cable

The following are factors that affect the current rating of a cable:

Conductor Material: Copper conductors generally have a higher current rating than aluminium conductors of the same size due to better conductivity.
Conductor Size (Cross-sectional Area): Larger cross-sectional areas allow higher current to flow without excessive heating.
Type of Insulation: Different insulation materials (e.g., PVC, XLPE) have different temperature withstand capabilities, affecting the maximum current.
Ambient Temperature: Higher surrounding temperatures reduce the cable’s ability to dissipate heat, thus lowering its current rating.
Method of Installation: How the cable is installed (e.g., in air, in conduit, buried in ground, bunched with other cables) affects heat dissipation.
Number of Cables in a Bunch/Conduit: When multiple cables are run together, their heat dissipation is reduced, lowering individual current ratings.

Types of Conductors

Conductors are materials that allow electric current to flow through them easily. In electrical installations, the most common conductor materials used for cables are copper and aluminium.

The following are the main types of conductors used in cables:

Copper Conductor
Aluminium Conductor

Copper Conductor

Copper is a widely used conductor due to its excellent electrical conductivity, high tensile strength, and good corrosion resistance. It is often preferred for its reliability and efficiency in carrying current.

Characteristics of copper conductors include:

High electrical conductivity.
Good mechanical strength.
Resistant to corrosion.
More expensive than aluminium.
Ductile and malleable, making it easy to work with.

Aluminium Conductor

Aluminium is a lighter and less expensive conductor compared to copper. While it has lower conductivity than copper for the same cross-sectional area, it is often used for larger cables, especially in overhead power lines, due to its weight advantage and cost-effectiveness.

Characteristics of aluminium conductors include:

Lower electrical conductivity than copper (for the same size).
Lighter in weight than copper.
Less expensive than copper.
More prone to corrosion and oxidation, especially at connections.
Lower mechanical strength compared to copper.

Observing Cable Sizes and Clips

Electrical cables are manufactured in various sizes, indicated by their cross-sectional area (e.g., 1.5mm², 2.5mm², 4mm²). The size determines the maximum current the cable can safely carry. Cable clips are used to secure cables neatly and safely to surfaces during installation, preventing damage and ensuring proper routing.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils, reviews the previous lesson briefly, and then displays various sizes and types of electrical cables and clips. The teacher asks pupils to identify what the items are and what they are used for.
Pupils’ Activity: Pupils respond to greetings, recall previous knowledge, and identify the displayed items.
Learning Point: Pupils are introduced to the lesson’s topic and connect it to familiar objects.

Step 2: Definition of Current Rating of a Cable

Time: 8 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines “current rating of a cable” using simple language and explains its importance in electrical safety. The teacher writes the definition on the board.
Pupils’ Activity: Pupils listen attentively, ask questions for clarity, and copy the definition into their notes.
Learning Point: Pupils understand the meaning of current rating.

Step 3: Factors Affecting Current Rating of a Cable

Time: 8 minutes
Teaching Skill: Explanation/Listing
Teacher’s Activity: The teacher explains the various factors that affect the current rating of a cable, such as conductor material, size, insulation type, and ambient temperature. The teacher uses examples and charts to illustrate these points.
Pupils’ Activity: Pupils listen, observe the charts, and note down the factors explained by the teacher.
Learning Point: Pupils learn the different elements that influence how much current a cable can carry.

Step 4: Types of Conductors

Time: 7 minutes
Teaching Skill: Classification/Demonstration
Teacher’s Activity: The teacher explains that copper and aluminium are the main types of conductors used in electrical cables. The teacher shows samples of copper and aluminium conductors.
Pupils’ Activity: Pupils identify the different types of conductors and observe their physical differences.
Learning Point: Pupils identify the common materials used as electrical conductors in cables.

Step 5: Characteristics of Copper and Aluminium Conductors

Time: 7 minutes
Teaching Skill: Comparison/Description
Teacher’s Activity: The teacher describes the specific characteristics of copper conductors (e.g., high conductivity, strength, cost) and aluminium conductors (e.g., lighter, less costly, lower conductivity for the same size). The teacher highlights their respective advantages and disadvantages.
Pupils’ Activity: Pupils listen, compare the characteristics, and ask questions about their applications.
Learning Point: Pupils understand the properties of copper and aluminium conductors.

Step 6: Observing Cable Sizes and Clips

Time: 5 minutes
Teaching Skill: Practical Observation
Teacher’s Activity: The teacher guides pupils to closely observe the displayed cables, pointing out how different sizes are marked (e.g., 1.5mm², 2.5mm²). The teacher also demonstrates the use of different types of cable clips.
Pupils’ Activity: Pupils examine the cables and clips, noting the size markings and how clips are used.
Learning Point: Pupils gain practical insight into identifying cable sizes and the function of clips.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define the term “current rating of a cable.”
Mention three factors that affect the current rating of a cable.
List the two main types of conductors used in electrical cables.
State two characteristics of copper conductors and two of aluminium conductors.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization/Assignment
Teacher’s Activity: The teacher summarizes the key points of the lesson, emphasizing the importance of selecting the correct cable size for safety. The teacher assigns homework for pupils to research the current ratings for common domestic cable sizes.
Pupils’ Activity: Pupils listen to the summary and note down the homework.
Learning Point: Pupils consolidate their learning and prepare for further study.

Lesson Keywords

Current Rating – The maximum current a cable can safely carry.
Conductor – A material that allows electricity to flow through it easily.
Insulation – A material that prevents the flow of electricity, covering conductors.
Copper – A highly conductive metal commonly used for electrical cables.
Aluminium – A lighter, less expensive metal also used as a conductor, especially for larger cables.
Ambient Temperature – The temperature of the surrounding environment.

Differentiation

For pupils who grasp concepts quickly, the teacher can encourage them to research the specific current ratings for different cable sizes and installation methods. For pupils needing more support, the teacher can provide simplified charts or visual aids and offer one-on-one explanations of the main definitions and factors.

Note for teachers using this lesson plan

Ensure that all cable samples and clips are handled safely and that pupils understand the practical implications of incorrect cable selection. Emphasize safety throughout the lesson. Encourage pupils to ask questions and share any relevant experiences they may have.