At the time of short circuit, the current in an electric circuit
- (a)becomes zero
- (b)remains same
- (c)increases sharply
- (d)decreases sharply
Correct — C, increases sharply. A short circuit is an unintended low-resistance path that lets the live wire reach the neutral wire without going through the appliance. Once the appliance is bypassed, almost the only resistance left in the loop is that of the copper wiring itself, which is a fraction of an ohm. The supply voltage has not changed, so I = V/R forces the current up by two or three orders of magnitude — hundreds of amperes where the appliance drew a few. That surge is exactly what the fuse or the miniature circuit breaker is there to interrupt.
- (a)becomes zero — Zero current belongs to the opposite fault — an open circuit, where a wire is broken and the loop is incomplete. A short leaves the loop complete and makes it far too easy.
- (b)remains same — Current would hold steady only if the loop resistance held steady. Bypassing the appliance removes nearly all of it.
- (d)decreases sharply — For current to fall, resistance has to rise. Nothing in a short raises resistance; the fault is defined by resistance collapsing.
Ohm's law read as I = V/R settles this in one line. Domestic supply holds the voltage fixed at about 230 V, so the current in any loop is decided entirely by the resistance in that loop. An appliance is the resistance; a short circuit removes it from the path.
Many candidates mix up the two household faults. An open circuit breaks the path and the current stops. A short circuit shortens the path and the current explodes. The word 'short' describes the route, not the size of the current — which is why the wrong intuition that something is being cut down is so easy to fall for.
- A 1 kW heater on a 230 V supply draws about 4.3 A, so its resistance is roughly 53 ohms.
- If the live and neutral conductors touch directly, the loop resistance can fall below one ohm, taking the current into the hundreds of amperes.
- A fuse is a short length of tin-lead alloy wire of low melting point, connected in series with the live wire, that melts and breaks the circuit during such a surge.
- Overloading is a separate fault: too many appliances on one socket raise the current without any collapse of resistance.
- Earthing is a third, different protection — it carries leakage current from a metal body to the ground and guards against shock, not against the surge itself.
Only one of the four takes the current up by orders of magnitude, and the question asks about that one.
- Reading 'short' as 'less current' instead of 'shorter path'.
- Confusing a short circuit with an open circuit, which is where zero current comes from.
- Assuming the fuse protects against electric shock; that is the earth wire's job.
Asked as a one-line statement about what happens to current during a short circuit, or as a matching item pairing fuse, earth wire and MCB with the fault each one answers.
When the short circuit condition occurs, the current in the circuit
- (a) becomes zero
- (b) remains constant
- (c) increases substantially
- (d) keeps on changing randomly
Answer(c) increases substantially
The same question in almost the same words, set two years earlier. Both papers reward the one idea that a short removes resistance from the loop while the voltage stays put.
Which of the following items is used in the household wirings to prevent accidental fire in case of short circuit ?
- (a) Insulated wire
- (b) Plastic switches
- (c) Non-metallic coatings on conducting wires
- (d) Electric fuse
Answer(d) Electric fuse
The other half of the topic — this asks what stops the surge rather than what the surge does. Read the two together and the fuse's low melting point stops being an arbitrary fact.
- practice — not a real PYQ
A fuse wire used in a domestic circuit should have
- (a)high melting point and high resistance
- (b)low melting point and high resistance
- (c)low melting point and very low resistance
- (d)high melting point and very low resistance
Answer(b) low melting point and high resistance — the fuse must heat up and melt before the rest of the wiring does, so it is made of a tin-lead alloy of low melting point and higher resistance than the line conductors.
- practice — not a real PYQ
In a household circuit, the fuse is always connected in
- (a)the neutral wire
- (b)the earth wire
- (c)the live wire
- (d)parallel with the appliance
Answer(c) the live wire — breaking the live wire isolates the appliance from the supply voltage; a fuse in the neutral would stop the current but leave the appliance live.