At the time of short-circuit, the current in the circuit:
- (a)reduces substantially.
- (b)does not change.
- (c)increases heavily.
- (d)keeps on fluctuating.
Correct — C, increases heavily. A short circuit is what happens when the live and neutral wires come into direct contact — through damaged insulation, a loose strand, or water bridging two terminals — so the current has a path back to the supply that misses the appliance altogether. Everything that used to resist the flow has been bypassed, and the only resistance left in the loop is the small resistance of the wiring itself, a fraction of an ohm. Ohm's law then does the rest: the mains voltage is unchanged at about 220 volts, the resistance has collapsed towards zero, and I = V/R therefore rises to hundreds of amperes where the appliance drew a few. Because the heat produced in the wiring goes as I²R, that surge can melt insulation and start a fire within seconds, which is precisely the event a fuse or a miniature circuit breaker is fitted to interrupt.
- (a)reduces substantially. — This is what an added resistance would do. A short does the opposite — it removes resistance from the loop rather than adding it.
- (b)does not change. — The current in a circuit is fixed by the supply voltage and the resistance. When the resistance collapses and the voltage does not, the current cannot stay where it was.
- (d)keeps on fluctuating. — Describes a loose or arcing contact, where the connection makes and breaks repeatedly. A genuine short is a solid low-resistance path, and its signature is a single large surge, not an unsteady reading.
In a household circuit the appliance is the resistance that limits the current; the wiring is designed to have as little resistance as possible so that it wastes no power. A short circuit joins live to neutral directly, leaving only the wiring in the loop. Since the mains voltage is held constant by the supply, Ohm's law forces the current up by whatever factor the resistance has fallen. The protective devices work off exactly that signature: a fuse is a thin wire chosen to melt at a stated current, and a miniature circuit breaker trips a switch magnetically or thermally when the rated current is exceeded.
The reasoning that settles this in one line is to hold the voltage fixed and watch the resistance. A short is defined by near-zero resistance, so the current must go up, and the option that says so is the answer. It is worth separating the short circuit from the other domestic fault, the earth leakage, in which current escapes to earth through a person or a wet casing without live and neutral ever touching; that fault may not raise the total current enough to blow a fuse, which is why residual-current devices exist alongside circuit breakers. In Indian homes today the rewirable fuse of the older textbooks has largely given way to miniature circuit breakers on the distribution board, though the physics they respond to is unchanged.
- A short circuit is a direct low-resistance connection between the live and neutral wires that bypasses the appliance.
- With the supply voltage fixed, Ohm's law I = V/R sends the current up as the resistance falls towards zero.
- Heat produced in a conductor goes as I²R, so the surge heats the wiring very quickly.
- A fuse is a wire that melts above its rated current; a miniature circuit breaker trips a switch instead and can be reset.
- Earth leakage is a separate fault that a fuse may not catch, which is why residual-current devices are fitted as well.
- Assuming a fault must reduce the current because something has gone wrong.
- Treating a short circuit and an overload as the same event — an overload is too many appliances on one line, a short is live meeting neutral.
- Expecting a fuse to protect against electric shock; that is what earthing and residual-current devices are for.
As a direct one-line question on what the current does, or through the protective device — which component guards against a short, and what its rating means.
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 asked of NDA candidates, with the same four ideas offered and the same one keyed. Anyone who can say why the resistance collapses answers both without hesitating.
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 practical consequence of this answer. Because a short sends the current up rather than down, the protective device has to be one that responds to excess current — a fuse or a breaker, not better insulation.
- practice — not a real PYQ
The fuse wire in a domestic circuit is connected
- (a)in parallel with the live wire
- (b)in series with the live wire
- (c)in series with the earth wire
- (d)in parallel with the appliance
Answer(b) in series with the live wire — the whole current must pass through the fuse for it to melt and break the circuit when the current becomes excessive.
- practice — not a real PYQ
An electric heater rated for a 220 V supply draws 5 A in normal use. If its terminals are accidentally joined by a wire of negligible resistance, the current in the supply line will
- (a)stay at 5 A
- (b)fall below 5 A
- (c)rise far above 5 A
- (d)become zero
Answer(c) rise far above 5 A — the heating element has been bypassed, so almost no resistance is left in the loop and I = V/R becomes very large.