fuses-and-electrical-safety MCQs for UPSC Prelims

25 practice questions on fuses-and-electrical-safety from the Electricity: Magnetic and Heating Effects section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

8 Easy 12 Medium 5 Hard

Sample questions

Q1
medium
Fuse wire is usually made of a metal or alloy that has
  1. A a low melting point and a higher resistance than the circuit wiring
  2. B a strong magnetic property
  3. C no resistance to the current at all
  4. D the highest melting point of all the metals
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?
  1. A It gives a leaking current a safe path to the earth instead of through a person who touches the body
  2. B It makes the appliance run faster than it otherwise would
  3. C It reduces the cost of the electricity the appliance uses
  4. D It 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?
  1. A Nothing at all, because the covering is only for appearance
  2. B A very large current flows and the fuse blows or the circuit breaker trips
  3. C The appliance works at half its usual power
  4. D The 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?
  1. A Switching off the supply at the mains before changing a bulb
  2. B Touching a switch with wet hands
  3. C Joining broken wires with sticky tape and using them
  4. D Pulling 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
  1. A thin piece of wire that melts and breaks the circuit if the current becomes too large
  2. B switch that has to be pressed by hand every hour
  3. C small cell that supplies extra current to the circuit
  4. D thick metal bar that can safely carry any current at all
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?
  1. A The fuse increases the current still further
  2. B The fuse turns the extra current into heat inside the appliance
  3. C The fuse lets the large current pass through unchanged
  4. D The 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
  1. A a fuse of the correct rating is fitted in the circuit
  2. B the live and neutral wires touch each other directly, giving the current a very easy path
  3. C an appliance is switched off at its socket
  4. D a 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.
  1. A Both A and R are true, and R is the correct explanation of A
  2. B Both A and R are true, but R is not the correct explanation of A
  3. C A is true, but R is false
  4. D A 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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