Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: 3rd Term
Week: 2
Age: 15 years
Duration: 45 minutes
Subject: Electrical Installation and Maintenance Work
Curriculum Theme: Technology
Previous Lesson: Introduction to Protective Devices: Meaning and Types.
Topic: FUSES
Subject Matter: Definition of fuse, types of fuses, IEE regulation on fuses, terminologies such as current rating and fusing factor, calculating current rating for fuse selection.

Specific Objectives

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

Cognitive Domain:

Define a fuse.
State at least three types of fuses.
Explain the importance of IEE regulation regarding fuses.
Define key terminologies such as current rating and fusing factor.
Calculate the appropriate current rating for basic fuse selection.

Affective Domain:

Appreciate the role of fuses in ensuring electrical safety.
Show willingness to follow safety guidelines when working with electrical installations.

Psychomotor Domain:

Identify different types of fuses when presented with samples.
Select the correct fuse rating for a given electrical appliance based on calculation.

Discuss with peers the importance of using correctly rated fuses.

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.
Federal Ministry of Education. (2007). Electrical Installation and Maintenance Work for Senior Secondary Schools. NERDC.

Instructional Materials

The teacher will teach this lesson with the aid of:

Samples of fuses (e.g., rewirable fuse, cartridge fuse, HRC fuse).
A chart illustrating different fuse types and their applications.
Whiteboard and markers.
Calculator.

Rationale for the Lesson

This lesson helps pupils understand how fuses protect electrical circuits and appliances from damage due to overcurrent. Knowing about fuses enables pupils to make safe choices when installing or maintaining electrical systems, which is important for preventing electrical hazards in homes and workplaces.

Prerequisite/Previous Knowledge

Pupils should have basic knowledge of electrical circuits, current, voltage, and power from previous lessons.

Lesson Content/Board Summary

FUSES

Definition of a Fuse

A fuse is a safety device used in electrical circuits to protect appliances and wiring from damage caused by excessive current. It contains a thin wire or metal strip that melts and breaks the circuit when the current exceeds a safe level, thus preventing overheating and potential fire.

Types of Fuses

The following are common types of fuses:

Rewirable Fuses (Open Wire/Kit-Kat Fuses): These are older types where the fuse wire can be replaced after it blows. They are typically used in older installations.
Cartridge Fuses: These fuses have a sealed body, usually ceramic or glass, containing the fuse element. Once they blow, the entire cartridge is replaced. They are common in plugs and some appliances.
High Rupturing Capacity (HRC) Fuses: These are cartridge fuses designed to safely interrupt very large fault currents without exploding. They are used in industrial and commercial installations.
Miniature Fuses: Small fuses often used in electronic equipment and automotive applications.

Key Terminologies Related to Fuses

Current Rating (Rated Current): This is the maximum current that a fuse can carry continuously without melting. It is usually marked on the fuse (e.g., 5A, 13A).
Fusing Current: This is the minimum current at which the fuse element will melt and break the circuit. It is always higher than the current rating.
Fusing Factor: This is the ratio of the fusing current to the current rating. A low fusing factor means the fuse blows quickly after exceeding its rated current.
Breaking Capacity (Rupturing Capacity): This is the maximum prospective current that a fuse can safely interrupt without damage to itself or the surrounding equipment.

IEE Regulation on Fuses

The Institution of Electrical Engineers (IEE) Regulations (now BS 7671 – Requirements for Electrical Installations) provide guidelines for the safe design, installation, and maintenance of electrical systems, including the use of fuses. Key points related to fuses include:

Fuses must be correctly rated for the circuit and connected load to provide adequate protection against overcurrent and short circuits.
It is important to ensure that the breaking capacity of a fuse is sufficient for the maximum fault current that could occur at its point of installation.
Fuses should be easily accessible for inspection and replacement.
Only fuses of the correct type and rating should be used as replacements.

Calculating Current Rating for Fuse Selection

To select the correct fuse for an appliance, it is important to calculate the normal operating current of the appliance.

The power (P) of an appliance is given by the formula:

P = V x I

Where:

P = Power in Watts (W)
V = Voltage in Volts (V) (e.g., 230V for mains supply in Nigeria)
I = Current in Amperes (A)

Therefore, to find the current (I):

I = P / V

Once the operating current is calculated, a fuse with a current rating slightly higher than the operating current should be selected to allow for normal startup surges but still provide protection against overloads. Common standard fuse ratings are 3A, 5A, 10A, 13A, etc.

Example: An appliance has a power rating of 2300W and operates on a 230V supply. Calculate the operating current and suggest a suitable fuse rating.

I = P / V = 2300W / 230V = 10A

A suitable fuse rating would be 13A, as it is the next standard rating above 10A, allowing for slight variations without immediately blowing.

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 and asks them what they think protects their home electrical appliances from damage when there is a sudden power surge or fault. The teacher displays various fuse samples.
Pupils’ Activity: Pupils respond to the questions and observe the displayed samples.
Learning Point: Pupils are introduced to the concept of electrical protection and fuses.

Step 2: Definition of a Fuse

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines a fuse as a safety device that protects electrical circuits and appliances by melting and breaking the circuit when current exceeds a safe level. The teacher explains its basic working principle.
Pupils’ Activity: Pupils listen attentively and take notes.
Learning Point: Pupils understand what a fuse is and its primary function.

Step 3: Types of Fuses

Time: 8 minutes
Teaching Skill: Demonstration/Classification
Teacher’s Activity: The teacher displays and explains the different types of fuses such as rewirable, cartridge, and HRC fuses, highlighting their characteristics and common applications. The teacher uses the chart for illustration.
Pupils’ Activity: Pupils observe the samples, ask questions, and take notes.
Learning Point: Pupils can identify and differentiate between various types of fuses.

Time: 8 minutes
Teaching Skill: Explanation/Clarification
Teacher’s Activity: The teacher explains and defines key terminologies such as current rating, fusing current, fusing factor, and breaking capacity, giving practical examples where necessary.
Pupils’ Activity: Pupils listen, ask for clarification, and record the definitions.
Learning Point: Pupils understand the technical terms associated with fuses.

Step 5: IEE Regulation on Fuses

Time: 5 minutes
Teaching Skill: Explanation/Safety Awareness
Teacher’s Activity: The teacher briefly explains the importance of IEE regulations concerning the correct selection and installation of fuses for electrical safety and compliance.
Pupils’ Activity: Pupils listen and understand the need for regulation in electrical work.
Learning Point: Pupils recognise the importance of safety standards in fuse application.

Step 6: Calculating Current Rating for Fuse Selection

Time: 7 minutes
Teaching Skill: Problem Solving/Demonstration
Teacher’s Activity: The teacher demonstrates how to calculate the operating current of an appliance using the formula I = P/V and then how to select an appropriate fuse rating. An example is worked on the board.
Pupils’ Activity: Pupils follow the calculation, ask questions, and attempt to solve simple problems.
Learning Point: Pupils learn how to calculate and select the correct fuse rating.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

What is a fuse and what is its main purpose?
Mention three types of fuses you have learned today.
Explain the meaning of “current rating” and “fusing factor.”
Why is it important to follow IEE regulations when installing fuses?
An electric kettle has a power rating of 2000W and operates on a 230V supply. Calculate its operating current and suggest a suitable fuse rating.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summary/Assignment
Teacher’s Activity: The teacher summarises the key points of the lesson on fuses and assigns homework: “Find out the fuse ratings of three different electrical appliances in your home.”
Pupils’ Activity: Pupils listen to the summary and copy the homework.
Learning Point: The lesson is reinforced, and pupils are encouraged to extend their learning.

Lesson Keywords

Fuse – A safety device that breaks an electrical circuit when current exceeds a safe level.
Current Rating – The maximum current a fuse can carry continuously without melting.
Fusing Factor – The ratio of fusing current to current rating.
Rewirable Fuse – A fuse whose wire can be replaced after it blows.
Cartridge Fuse – A sealed fuse that is replaced entirely after it blows.
HRC Fuse – High Rupturing Capacity fuse, designed to interrupt large fault currents.
IEE Regulation – Guidelines for safe electrical installations.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide more complex calculations or ask them to research different fuse characteristics like time-current curves. For those who need more support, the teacher can provide simpler examples and additional visual aids, focusing on identification and basic function before calculations.

Note for teachers using this lesson plan

Ensure to bring actual fuse samples to class for demonstration, as hands-on observation greatly aids understanding. Emphasise the safety aspects of correctly rated fuses and the dangers of using incorrect replacements or bypassing fuses.