PhysicsICSE Class 10 · Household Circuits

Electric Fuse and MCB: Principle, Rating, Overloading and Short Circuit (ICSE Class 10)

Q: What is an electric fuse?

An electric fuse is a safety device: a short, thin wire of low melting point (an alloy of tin and lead) connected in series in the live wire. It melts and breaks the circuit when the current exceeds its rating.

Q: Why is a fuse connected in the live wire?

So that when it melts, the appliance is disconnected from the high-potential live wire. A fuse in the neutral would stop the current but leave the appliance live, and touching it could cause a shock.

Q: How is an MCB better than a fuse?

An MCB trips automatically when the current is too large and can simply be switched on again after the fault is cleared. It acts quickly and reliably, and its rating cannot be altered by fitting the wrong wire.

Updated 16 October 2026 · Fact-checked

An electric fuse is a short, thin wire of low melting point connected in series in the live wire. When the current exceeds its rating, the wire heats up, melts and breaks the circuit. An MCB is an automatic switch that trips instead and can be reset. The rating should be slightly above the normal current, I = P/V.

The fuse sits in series on the live wire; if current exceeds its rating the thin wire melts, breaking the circuit and cutting the appliance off.

Illustration drawn for QuestionBuddy.

Understand Electric Fuse and MCB

Why a safety device is needed. The heat produced in a wire is $H = I^{2} R t$ . If the current in a household circuit becomes too large, the wires heat up, their insulation can melt and a fire can start. Two faults cause such large currents:

Overloading: too many appliances (or too high a total power) are connected to one circuit at the same time, so the total current $I = \frac{P _{t o t a l}}{V}$ exceeds what the wiring is meant to carry.
Short circuit: the live and neutral wires touch directly, for example where the insulation is damaged. The resistance of the path becomes almost zero, so by $I = \frac{V}{R}$ the current becomes enormous.

The electric fuse.

Principle: the heating effect of current. The fuse wire is chosen so that it melts when the current exceeds a certain value (its rating).
Material: a short, thin wire of a material with a low melting point, commonly an alloy of tin and lead.
Connection: in series with the circuit it protects, and always in the live wire.
Action: when the current exceeds the rating, the fuse wire melts and breaks the circuit before the house wiring or the appliance can be damaged. The fault must then be found and corrected before the fuse is replaced.

Why in the live wire? When the fuse in the live wire melts, the appliance is cut off from the high potential. If the fuse were in the neutral wire, it would still break the current, but the appliance would remain connected to the live wire, and a person touching it could get a shock.

Choosing the rating. Find the normal current of the appliance (or the total for a circuit):

$I = \frac{P}{V}, I_{t o t a l} = I_{1} + I_{2} + I_{3} + \dots$

The fuse rating should be slightly more than this current:

rated below the normal current, it melts as soon as the appliance is switched on;
rated far above, a dangerous current can flow without melting it, so it gives no protection.

A circuit with a fuse rating $I_{f}$ on a supply $V$ can supply a maximum total power $P_{ma x} = V I_{f}$ . In Indian homes, lighting circuits (bulbs, fans) commonly have a 5 A rating and power circuits (geysers, coolers) a 15 A rating.

Where fuses are found. A sealed company (service) fuse on the incoming live wire, before the meter; a fuse or MCB for each circuit at the distribution board; and in the ring system, a fuse in each appliance plug.

The MCB (miniature circuit breaker). An MCB is an automatic switch connected in the live wire of a circuit. When the current exceeds its rating, it trips (switches off) and breaks the circuit. Once the fault is removed, it is simply switched on again.

Fuse	MCB
Wire melts; must be replaced	Trips; reset by switching on again
Slower; replacing it can be unsafe	Acts quickly and reliably
Wrong wire may be used as a replacement	Rating is fixed; cannot be tampered with easily
Cheap	Costlier, but reusable

Never replace a blown fuse with a thick wire. A thick copper wire has a low resistance and a high melting point, so it will not melt even when a dangerous current flows. The wiring may then overheat and catch fire.

Electric Fuse and MCB: formulas and key facts

Normal current: $I = \frac{P}{V}$; Fuse rating slightly above this.
Total current of a circuit: $I_{t o t a l} = I_{1} + I_{2} + I_{3} + \dots = \frac{P _{1} + P _{2} + P _{3} + \dots}{V}$; Appliances are in parallel.
Maximum power on a fuse: $P_{ma x} = V I_{f}$; I_f is the fuse or MCB rating.
Short-circuit current: $I = \frac{V}{R}, R \to 0 \Rightarrow I very large$; Why a short circuit is dangerous.

How to solve Electric Fuse and MCB questions

Every fuse question is a current comparison: work out the current, then compare it with the rating.

1Write the power of each appliance in watts and the supply voltage.
2Find each current with $I = \frac{P}{V}$ , or add the powers first and divide once.
3Compare the current with the fuse or MCB rating: if it is larger, the fuse melts (MCB trips).
4To choose a fuse, pick the nearest rating just above the normal current.
5To find how much can be connected, use $P_{ma x} = V I_{f}$ .
6State the answer in words ('Yes, because…' or 'No, because…') with the current in amperes.

Electric Fuse and MCB in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Overloading a circuit

Suppose in a hostel room you switch on a 1.5 kW room heater and then a 1 kW electric kettle on the same 230 V socket circuit, which is protected by a 10 A MCB.

Explain what happens.

Try it first, then show the solutionSolution

Total current $= \frac{1500 + 1000}{230} \approx 10.9$ A.
This is more than 10 A, so the MCB trips and both appliances go off.
Switch one off, then reset the MCB.

Answer: The MCB trips because about 10.9 A exceeds its 10 A rating.

Memory hook: Add the amps; over the rating, off it goes.

The thick-wire 'repair'

Imagine a fuse keeps blowing, and someone replaces the fuse wire with a piece of thick copper wire so that 'it won't happen again'.

Why is this dangerous?

Try it first, then show the solutionSolution

The fuse kept blowing because of a fault or an overload; that cause is still there.
Thick copper has low resistance and a high melting point, so it will not melt even with a very large current.
The large current now heats the house wiring instead, which can melt the insulation and start a fire.

Answer: The circuit is left unprotected; the wiring can overheat and catch fire.

Memory hook: A fuse that can't melt is no fuse.

Tricks to remember Electric Fuse and MCB

Short memory aids. Say each one aloud once, then use it on a question the same day.

Fuse: S-L-L
Series, Live wire, Low melting point.
Use it when: Recalling how a fuse is connected and made.
Just above
Fuse rating = just above the normal current: below, it blows; far above, it sleeps.
Use it when: Choosing a fuse.
Overload adds, short shorts
Overload: too many amps added together. Short circuit: R falls to almost zero.
Use it when: Telling the two faults apart.
MCB = Melt? Certainly not; Breaks and resets
A fuse melts; an MCB trips and resets.
Use it when: Fuse versus MCB.

Electric Fuse and MCB: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Fuse wire material · 1 mark · MCQBoard level

A fuse wire is usually made of:
(a) copper
(b) tungsten
(c) an alloy of tin and lead
(d) nichrome

Try it first, then show the solutionSolution

A fuse must melt at a low temperature; an alloy of tin and lead has a low melting point. Copper, tungsten and nichrome do not melt easily.

Answer: (c) an alloy of tin and lead

Example 2: How a fuse protects · 3 marks · Short answerBoard level

(a) State the principle of an electric fuse. (b) How is it connected in a circuit? (c) What happens when the current exceeds its rating?

Try it first, then show the solutionSolution

(a) The heating effect of electric current. (1)
(b) In series with the appliance or circuit, in the live wire. (1)
(c) The fuse wire heats up, melts and breaks the circuit, so the current stops. (1)

Answer: (a) Heating effect (b) In series, in the live wire (c) It melts and breaks the circuit

Example 3: Fuse for a toaster · 3 marks · Short answerHarder board level

A toaster is rated 1210 W, 220 V. Fuses of 2 A, 6 A and 13 A are available. Which should be used? Give a reason for rejecting each of the others.

Try it first, then show the solutionSolution

Normal current $I = \frac{1210}{220} = 5.5$ A. (1)
The 2 A fuse is below 5.5 A, so it would melt every time the toaster is switched on. (1)
The 13 A fuse is far above 5.5 A and would let a dangerous current pass. The 6 A fuse is just above the normal current, so it is the right choice. (1)

Answer: 6 A fuse

Example 4: Is the lighting circuit overloaded? · 3 marks · Short answerHarder board level

A lighting circuit on a 220 V supply has a 5 A fuse. (a) Find the maximum power it can supply. (b) Eight 75 W fans and six 20 W lamps are on together. Find the current. (c) A 1000 W heater is also plugged into this circuit. What happens?

Try it first, then show the solutionSolution

(a) $P_{ma x} = V I_{f} = 220 \times 5 = 1100$ W. (1)
(b) $P = 8 \times 75 + 6 \times 20 = 720$ W; $I = \frac{720}{220} \approx 3.27$ A, below 5 A. (1)
(c) Total $= 1720$ W; $I = \frac{1720}{220} \approx 7.82$ A, more than 5 A, so the fuse melts (overloading). (1)

Answer: (a) 1100 W (b) ≈ 3.27 A, safe (c) ≈ 7.82 A; the fuse blows

Example 5: Short circuit · 3 marks · Short answerHarder board level

An iron of resistance 44 Ω works on 220 V. Its flex is damaged and the live and neutral wires touch, making a path of resistance 0.5 Ω. (a) Find the normal current. (b) Find the current in the short circuit. (c) Explain the role of the fuse.

Try it first, then show the solutionSolution

(a) $I = \frac{220}{44} = 5$ A. (1)
(b) $I = \frac{220}{0.5} = 440$ A, nearly 90 times the normal current. (1)
(c) Such a current would overheat the wiring at once; the fuse in the live wire melts immediately and breaks the circuit. (1)

Answer: (a) 5 A (b) 440 A (c) The fuse melts and cuts off the supply

Example 6: Thick and thin fuse wires · 2 marks · Short answerHOTS / competency

A 5 A fuse and a 15 A fuse use wire of the same alloy and length. Which wire is thicker? Explain using the heating effect.

Try it first, then show the solutionSolution

The 15 A fuse wire is thicker. (1)
A thicker wire has a lower resistance and more material to heat, so a larger current is needed before $I^{2} R t$ raises it to its melting point. (1)

Answer: The 15 A wire is thicker

Example 7: Case study: the power circuit trips · 4 marks · Case-basedHOTS / competency

A 220 V power circuit is protected by a 15 A MCB. A 2 kW geyser, a 500 W washing machine and a 1 kW iron are switched on together.
(a) Find the current drawn by each appliance.
(b) Find the total current and state what happens.
(c) Which one appliance should be switched off so that the other two can run? Show the new current.
(d) Give two advantages of using an MCB here instead of a fuse.

Try it first, then show the solutionSolution

(a) Geyser $\frac{2000}{220} \approx 9.09$ A; washing machine $\frac{500}{220} \approx 2.27$ A; iron $\frac{1000}{220} \approx 4.55$ A. (1)
(b) Total $\approx 15.91$ A, more than 15 A, so the MCB trips and the circuit is cut off. (1)
(c) Switching off the iron leaves $\frac{2500}{220} \approx 11.36$ A, below 15 A. (Switching off only the washing machine leaves about 13.64 A, which also works, but the iron leaves a bigger margin.) (1)
(d) It can be reset by switching it on again, with no wire to replace; it acts quickly and its rating cannot be changed by using the wrong wire. (1)

Answer: (a) ≈ 9.09 A, 2.27 A, 4.55 A (b) ≈ 15.91 A; MCB trips (c) Iron: ≈ 11.36 A (d) Resettable; quick and tamper-proof

Where students lose marks in Electric Fuse and MCB

The slips examiners see most often, and what to write instead.

Mistake: Fuse rated below the normal current
A fuse must carry the normal current; it melts only when the current goes above its rating.
Wrong working
'The fuse rating should be less than the appliance's current, to protect it.'
Right working
'The fuse rating should be slightly more than the normal current.'
Do this: Picture switching the appliance on: a lower-rated fuse would blow at once.
Usually costs: Reported by CISCE examiners for 2024 Q3(ii).
Mistake: No supporting calculation
'Yes' or 'No' alone earns nothing; the current must be calculated and compared.
Wrong working
'No, the fuse cannot be used.' (no working)
Right working
'I = 1320/220 = 6 A, which is more than 5 A, so No.'
Do this: Calculate I = P/V first, with the unit A, then answer.
Usually costs: Reported by CISCE examiners for 2024 Q3(ii).
Mistake: Finding R instead of I
The fuse rating is a current, so the comparison needs I = P/V.
Wrong working
R = V²/P ≈ 36.7 Ω for a 1320 W, 220 V appliance, 'so the fuse cannot be used'.
Right working
I = P/V, compared with the fuse rating in amperes.
Do this: Match the unit of what you calculate to the unit of the rating.
Usually costs: Reported by CISCE examiners for 2024 Q3(ii).
Mistake: Wrong fuse named in the meter room
The company (service) fuse protects the whole supply and has a much larger rating than the fuses of individual circuits.
Wrong working
Giving the rating of a circuit fuse in the ring system, or a random value, for the company fuse.
Right working
Identify which fuse the question shows before giving a rating.
Do this: Learn the order: company fuse, meter, main switch, distribution board fuses or MCBs.
Usually costs: Reported by CISCE examiners for 2025 Q8(i).

Exam tips

In any diagram, draw the fuse in the live wire and in series with the circuit.
Write 'fuse rating slightly more than the normal current' in words in addition to the calculation.
For 'can it be used' questions, finish with a clear Yes or No.
Name both faults (overloading and short circuit) when asked why a fuse is needed.

Practice questions on Electric Fuse and MCB

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
When the current in a circuit exceeds its rating, an MCB:
(a) melts
(b) trips and can be reset
(c) earths the appliance
(d) raises the voltage
Hint
It is an automatic switch.
Answer
(b) trips and can be reset
Q2 · 1 mark · Very short answerBoard level
On which effect of electric current does a fuse work?
Hint
What makes the wire melt?
Answer
The heating effect of current.
Q3 · 1 mark · Very short answerBoard level
What is meant by overloading of a household circuit?
Hint
Too much total power.
Answer
Connecting so many appliances (so much total power) to one circuit that the total current exceeds the safe limit of the circuit.
Q4 · 2 marks · Short answerHarder board level
An iron is rated 750 W, 220 V. Which fuse should be used: 3 A, 5 A or 10 A? Justify.
Hint
Find P/V.
Answer
$I = \frac{750}{220} \approx 3.4$ A. The 3 A fuse would melt; 10 A gives poor protection; 5 A, just above the normal current, should be used.
Q5 · 2 marks · Short answerHarder board level
A 230 V circuit has a 10 A fuse. (a) Find the maximum power it can supply. (b) Can a 1.2 kW microwave oven and a 1 kW kettle run on it together? Justify.
Hint
$P_{ma x} = V I_{f}$ .
Answer
(a) $230 \times 10 = 2300$ W. (b) Yes: together they need 2200 W, less than 2300 W (current $= \frac{2200}{230} \approx 9.57$ A < 10 A), so the fuse does not blow, though there is little margin left for anything else.
Q6 · 2 marks · Short answerHarder board level
Why should the rating of a fuse be slightly more than the normal current of the appliance, and not much more?
Hint
Two sides of the choice.
Answer
Slightly more, so that the normal current does not melt it; not much more, because then a dangerously large current could flow without the fuse melting, and the appliance and wiring would not be protected.
Q7 · 2 marks · Short answerHarder board level
Give two differences between a fuse and an MCB.
Hint
What happens after each acts?
Answer
(i) A fuse wire melts and must be replaced; an MCB trips and is simply reset. (ii) An MCB acts more quickly and reliably, and its rating cannot be changed by fitting the wrong wire.
Q8 · 2 marks · Short answerHarder board level
Distinguish between overloading and a short circuit, saying how each produces a large current.
Hint
$I = \frac{P}{V}$ and $I = \frac{V}{R}$ .
Answer
Overloading: too many appliances in parallel, so the currents add to more than the safe value ( $I = \frac{P _{t o t a l}}{V}$ ). Short circuit: live and neutral touch directly, so the resistance is almost zero and $I = \frac{V}{R}$ becomes very large.
Q9 · 2 marks · Short answerHOTS / competency
A kettle rated 2.4 kW, 240 V has a 5 A fuse in its plug. Explain what happens when it is switched on, and what fuse should be fitted.
Hint
Normal current.
Answer
$I = \frac{2400}{240} = 10$ A, twice the fuse rating, so the fuse melts every time the kettle is switched on. A fuse slightly above 10 A (for example 13 A) should be fitted.
Q10 · 2 marks · Short answerHOTS / competency
A blown fuse is replaced with a thick copper wire. Give two reasons why this is dangerous.
Hint
Melting point and resistance.
Answer
(i) Copper has a high melting point and the thick wire has low resistance, so it will not melt even when a very large current flows. (ii) The fault that blew the fuse is still there, so the wiring may overheat and start a fire.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Principle of a fuse?Show answer
Heating effect of current.
2.Fuse wire material?Show answer
Alloy of tin and lead (low melting point).
3.Fuse connected in which wire?Show answer
Live wire, in series.
4.Full form of MCB?Show answer
Miniature circuit breaker.
5.Maximum power on a 5 A fuse at 220 V?Show answer
1100 W.

Electric Fuse and MCB: frequently asked questions

What is an electric fuse?

An electric fuse is a safety device: a short, thin wire of low melting point (an alloy of tin and lead) connected in series in the live wire. It melts and breaks the circuit when the current exceeds its rating.

Why is a fuse connected in the live wire?

So that when it melts, the appliance is disconnected from the high-potential live wire. A fuse in the neutral would stop the current but leave the appliance live, and touching it could cause a shock.

How is an MCB better than a fuse?

An MCB trips automatically when the current is too large and can simply be switched on again after the fault is cleared. It acts quickly and reliably, and its rating cannot be altered by fitting the wrong wire.