Electric Fuse: Working, Material and Rating | Class 10

Quick answer: An electric fuse protects a circuit from overloading and short-circuiting. It works on the heating effect of electric current ($H = I^2Rt$): when too much current flows, the fuse wire heats up, melts and breaks the circuit. It is always connected in series in the live wire. For a $1\text{ kW}$ electric iron on $220\text{ V}$, the current is about $4.5\text{ A}$, so a 5 A fuse is used.

When several high-power appliances such as heaters, geysers and air conditioners run at the same time, the current in the household wiring can rise sharply. If damaged insulation lets the live wire touch the neutral wire, a short circuit causes an even larger current. Too much current can overheat the wiring, damage appliances and even start a fire.

An electric fuse is a simple safety device that prevents this.

In CBSE Class 10 Science, Chapter 11 (Electricity), the working of a fuse, its material, why it is connected in the live wire and how to choose the correct fuse rating are frequently asked board exam topics.

What You Will Learn

• What an electric fuse is and what it protects against
• How a fuse works using the heating effect of current, $H = I^2Rt$
• What fuse wire is made of, and why
• Why a fuse is connected in series in the live wire
• How to choose the correct fuse rating, with a solved example

1. What Is an Electric Fuse?

Definition: An electric fuse is a safety device connected in series in a circuit. It protects the circuit and appliances from damage due to overloading or short-circuiting by melting and breaking the circuit when the current becomes too large.

2. How a Fuse Works: The Heating Effect of Current

The heat produced in a wire is given by Joule's law of heating:

$H = I^2Rt$

Step 1: Under normal conditions, the current is within the safe limit and the fuse wire stays below its melting point.

Step 2: During overloading or a short circuit, the current rises sharply.

Step 3: Heat produced depends on the square of the current ($H \propto I^2$), so the fuse wire heats up very quickly.

Step 4: The fuse wire melts, breaking the circuit and stopping the current. This protects the wiring and the appliances.

3. What Is Fuse Wire Made Of?

A fuse wire must:

1. Have a suitable (fairly low) melting point, so that it melts before the rest of the wiring gets dangerously hot.
2. Be chosen so that it melts at the rated current. The material and thickness of the wire decide this.

Fuse wires are made of metals or alloys with suitable melting points, such as aluminium, copper, iron or lead, or alloys of tin and lead. A fuse wire is thin and chosen for a specific current rating.

Remember: Never replace a blown fuse with a thick piece of ordinary copper wire. It will not melt at the safe current, so it gives no protection.

4. Why Is a Fuse Connected in the Live Wire?

Common board question: Why is a fuse always connected in the live wire and not in the neutral wire?

Answer:

1. The live wire is at a high potential ($220\text{ V}$), and the neutral wire is at about zero potential.
2. If the fuse is in the live wire and it blows, the appliance is cut off from the high voltage, so it is safe.
3. If the fuse were in the neutral wire, the appliance would stop working when the fuse blew, but parts of it would still be at high voltage. Touching it could cause a dangerous electric shock.

5. Solved Example: Choosing the Fuse Rating

Question

An electric iron consumes $1\text{ kW}$ of power when operated at $220\text{ V}$. Which fuse rating should be used in its circuit: $1\text{ A}$, $2\text{ A}$, $3\text{ A}$, $5\text{ A}$ or $10\text{ A}$?

Solution

Step 1: Write the given values.

• Power: $P = 1\text{ kW} = 1000\text{ W}$
• Voltage: $V = 220\text{ V}$

Step 2: Find the current.

$I = \frac{P}{V} = \frac{1000}{220} \approx 4.5\text{ A}$

Step 3: Choose the fuse. A $1\text{ A}$, $2\text{ A}$ or $3\text{ A}$ fuse would blow during normal use. The $10\text{ A}$ fuse would allow too much current before blowing. The best choice is the next rating above the normal current.

Answer: The iron draws about $4.5\text{ A}$, so a 5 A fuse should be used.

6. Summary Table

7. Exam Tips and Common Mistakes

Exam Tip: When asked how a fuse works, mention the heating effect of current and that the wire melts and breaks the circuit.

Exam Tip: For rating questions, first find $I = \frac{P}{V}$, then choose the next standard rating above it.

Common Mistake: Saying a fuse is connected in parallel. It is always connected in series, so that the full current passes through it.

Common Mistake: Saying the fuse can go in the neutral wire. It must be in the live wire.

Frequently Asked Questions

What is an electric fuse?

An electric fuse is a safety device connected in series in a circuit. It melts and breaks the circuit when the current becomes too large, protecting the wiring and appliances.

On which principle does a fuse work?

A fuse works on the heating effect of electric current (H = I²Rt). Excess current heats the fuse wire until it melts.

Why is a fuse connected in the live wire?

So that when it blows, the appliance is disconnected from the high-voltage live wire and is safe to touch.

What fuse rating should be used for a 1 kW iron on 220 V?

The current is about 4.5 A, so a 5 A fuse should be used.

Why should thick copper wire not be used as a fuse?

It will not melt at the safe current, so it will not break the circuit during overloading or a short circuit.