An electric iron requires a ________ fuse to prevent short circuiting.

Correct — D. Size a fuse from the current the appliance actually draws, then take the next standard rating above it.

I = P/V
= 1000 W / 220 V = 4.54 A for a 1 kW iron on a 220 V supply.

NCERT gives the domestic fuse ratings as 1 A, 2 A, 3 A, 5 A, 10 A. The smallest of those above 4.54 A is 5 A, so a 5 A fuse carries the iron's working current and still melts on a fault.

Why the others are wrong

• (a)
  4 A — 4 A is below the 4.54 A the iron draws, so a fuse of that size would melt in ordinary use — and 4 A is not one of the ratings the NCERT series lists.
• (b)
  3 A — 3 A is well under the working current — the fuse goes the moment the iron is switched on, which is nuisance blowing, not protection.
• (c)
  2 A — 2 A is less than half the iron's demand, so this fuse cannot carry the appliance at all, fault or no fault.

A fuse is a short piece of wire of low melting point, connected in series with the appliance. Current above its rating heats it until it melts, breaking the circuit before the wiring or the device is damaged.

So the rating is picked by a rule with two sides. It must sit above the normal working current, or the fuse blows in ordinary use, and as close above it as possible, or a fault current passes unchecked.

Key facts

• A fuse rating is chosen just above the appliance's normal working current.
• A 1 kW electric iron on a 220 V supply draws 1000/220 = 4.54 A, so it takes a 5 A fuse.
• NCERT gives the fuse ratings used for domestic purposes as 1 A, 2 A, 3 A, 5 A, 10 A — a list with no 4 A in it.
• The fuse is connected in series with the device and works by melting.

Common traps

• Taking the rating nearest the working current from below instead of above
• Computing with 240 V when the standard example assumes 220 V
• Treating the fuse as something that stops a fault rather than something that breaks the circuit

No directly related past PYQ was found.