25 practice questions on Fuses and Electrical Safety from the chapter Electricity: Magnetic and Heating Effects (NCERT Class 8 Science). Every question includes a written explanation. Try the sample set below - the answer stays hidden until you ask for it.
8 Easy12 Medium5 Hard
Sample questions
Q1
medium
Fuse wire is usually made of a metal or alloy that has
Aa low melting point and a higher resistance than the circuit wiring
Ba strong magnetic property
Cno resistance to the current at all, like a perfect conductor
Dthe highest melting point of all the metals, so that it never breaks during use
Show answer and explanation
Correct answer: A - a low melting point and a higher resistance than the circuit wiring
A fuse must heat up faster than the rest of the circuit and melt before anything else is damaged, so a low melting point together with a somewhat higher resistance is needed. A very high melting point would stop it ever melting, no resistance would mean no heating, and magnetism plays no part.
Q2
hard
Why is earthing provided for metal-bodied appliances such as a refrigerator?
AIt gives a leaking current a safe path to the earth instead of through a person who touches the body
BIt makes the appliance run faster than it otherwise would, by giving the current a shorter path to follow
CIt reduces the cost of the electricity the appliance uses
DIt keeps the inside of the appliance cool
Show answer and explanation
Correct answer: A - It gives a leaking current a safe path to the earth instead of through a person who touches the body
If a live wire touches the metal body, the earth wire carries the current safely away and the fuse or MCB cuts off the supply, so a person touching it is protected. Earthing has no effect on how fast an appliance runs, on its running cost, or on its cooling.
Q3
medium
The plastic covering of a flex has worn away, and the bare wires inside touch each other while the appliance is switched on. What is most likely to happen first?
ANothing at all, because the covering is only for appearance
BA very large current flows and the fuse blows or the circuit breaker trips
CThe appliance works at half its usual power
DThe two wires slowly turn into permanent magnets
Show answer and explanation
Correct answer: B - A very large current flows and the fuse blows or the circuit breaker trips
Bare wires touching create a short circuit, so a very large current flows and the protective device cuts it off. The appliance is bypassed rather than run at half power, insulation is a genuine safety part and not decoration, and the wires do not become magnets.
Q4
easy
Which of these is a safe practice at home?
ASwitching off the supply at the mains before changing a bulb
BTouching a switch with wet hands
CJoining broken wires with sticky tape and using them
DPulling a plug out of its socket by the wire
Show answer and explanation
Correct answer: A - Switching off the supply at the mains before changing a bulb
Switching off at the mains removes the current before anyone touches the fitting. Wet hands conduct electricity, pulling a plug by its wire damages the connections inside it, and taped joints are not proper insulation.
Q5
easy
A fuse in a circuit is a
Athin piece of wire that melts and breaks the circuit if the current becomes too large
Bswitch that has to be pressed by hand every hour
Csmall cell that supplies extra current to the circuit
Dthick metal bar that can safely carry any current at all, however large it becomes in a fault
Show answer and explanation
Correct answer: A - thin piece of wire that melts and breaks the circuit if the current becomes too large
A fuse is a deliberately weak link, a thin wire that melts when the current rises past a safe value and so breaks the circuit. It is not a thick bar built to carry any current, not a hand-operated switch, and it supplies no current of its own.
Q6
medium
A house circuit is protected by a fuse rated $5\ \text{A}$. A fault sends a current of about $30\ \text{A}$ through it. What should happen?
AThe fuse increases the current still further
BThe fuse turns the extra current into heat inside the appliance
CThe fuse lets the large current pass through unchanged
DThe fuse wire melts quickly and breaks the circuit
Show answer and explanation
Correct answer: D - The fuse wire melts quickly and breaks the circuit
A fuse rated $5\ \text{A}$ is made to melt once the current rises well past that value, so a fault current of $30\ \text{A}$ melts it almost at once. A fuse cannot pass such a current unchanged, cannot increase it, and its job is to break the circuit rather than push heat into the appliance.
Q7
easy
A short circuit occurs when
Aa fuse of the correct rating is fitted in the circuit, so that the current takes a shorter route
Bthe live and neutral wires touch each other directly, giving the current a very easy path
Can appliance is switched off at its socket
Da bulb is replaced with a new one of the same kind
Show answer and explanation
Correct answer: B - the live and neutral wires touch each other directly, giving the current a very easy path
When the two supply wires touch, the current bypasses the appliance along a path with almost no resistance, so a very large current flows. Switching off, fitting the correct fuse and changing a bulb are all normal and safe actions.
Q8
hard
Assertion (A): A fuse of a higher rating should be fitted whenever a fuse blows again and again. Reason (R): A fuse that blows repeatedly is a signal that the circuit is being overloaded or has a fault.
ABoth A and R are true, and R is the correct explanation of A
BBoth A and R are true, but R is not the correct explanation of A
CA is true, but R is false
DA is false, but R is true
Show answer and explanation
Correct answer: D - A is false, but R is true
A is false, because fitting a larger fuse removes the protection instead of removing the cause. R is true and says what a repeatedly blowing fuse actually means, so the fault must be found and put right. The assertion therefore fails while the reason stands.
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