A 5 A Fuse and Four Appliances in One Room
A household in a Haryana town is connected to a 220 V supply through a 5 A fuse. At one time the following appliances are switched on: a 1000 W electric iron, a 60 W bulb, an 80 W fan, and a 150 W television. The owner wants to add a 750 W microwave oven to the same circuit. The wiring in the room is old, with a single copper wire of 1.0 mm² cross-section for the whole circuit. The fuse is rated 5 A. The owner argues that because the supply is 220 V, a 5 A fuse can safely carry up to 1100 W, so the total should be kept below 1100 W. A shopkeeper suggests replacing the fuse wire with a thicker piece of the same fuse. The appliances are resistive or nearly so for this estimate.
1. Calculate the total power and current with and without the microwave. Is the existing circuit already safe?
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Without the microwave: P = 1000 + 60 + 80 + 150 = 1290 W. Current I = P/V = 1290/220 ≈ 5.86 A. This is already greater than the 5 A fuse rating, so the circuit is overloaded even before the microwave is added. With the microwave: P = 1290 + 750 = 2040 W, so I = 2040/220 ≈ 9.27 A. The owner's statement that 5 A × 220 V = 1100 W is arithmetically correct as a power limit, but the existing appliances already exceed that limit. The fuse is a current-operated safety device, and the actual current is what matters. Adding the microwave would almost double the current above the fuse rating and could overheat the old 1.0 mm² wiring.
2. Explain what a fuse does and why replacing the fuse wire with a thicker piece is dangerous.
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A fuse is a thin wire placed in series with the circuit. It has a defined current rating and melts when the current exceeds that rating for a sufficient time, breaking the circuit before the wiring overheats. A thicker fuse wire has lower resistance and a higher melting current, so it will not melt at the intended 5 A. The wiring and appliances would then carry excessive current, causing overheating, insulation damage, or fire. The fuse would no longer protect the circuit. A fuse must never be uprated by using a thicker wire or a coin; the correct replacement is a fuse of the same rating, or better, a properly rated miniature circuit breaker installed by a qualified electrician.
3. Suggest a safe plan for using the microwave and the other appliances in this room.
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The high-power appliances should not all run on one old 5 A lighting circuit. A safe plan is to have a qualified electrician install a separate 15 A circuit with properly sized copper wiring for the iron and microwave, keep the lighting and fan on their own circuit, and use a correctly rated MCB or fuse. The microwave should be plugged into its own socket, not into an extension board shared with the iron. Earthing should be checked, and damaged old wiring should be replaced. In the short term, the owner should avoid running the iron and microwave together and should not replace the fuse with a thicker wire. The fact that the 5 A fuse may not have blown yet is not evidence of safety; it may indicate a wrongly rated fuse or a faulty connection that needs checking.