The Kettle That Blew the 5 A Fuse
A small flat is wired on 220 V mains with a single 5 A fuse on one kitchen circuit; the cable used for that circuit is rated for a maximum of 10 A. The circuit already feeds three 60 W lamps and a 375 W mixer-grinder. The family adds a 1500 W electric kettle to the same circuit. The instant the kettle is switched on, the fuse wire melts.
An electrician replaces the burnt 5 A fuse wire with a piece of 15 A fuse wire and assures the family that the circuit will now "never blow again".
Separately, the family reports a mild tingling sensation when touching the metal body of the mixer while standing on a wet floor. The mixer runs normally. On inspection, the earth wire of that socket is found to be disconnected at the plug, and the insulation inside the mixer near the live terminal is frayed.
1. Calculate the current drawn by the lamps-and-mixer group and by the kettle alone. Using these numbers, explain why the fuse melted, and state whether the fuse was faulty.
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Lamps + mixer: P = 3 × 60 + 375 = 180 + 375 = 555 W. I = P/V = 555/220 = 2.52 A.
Kettle alone: I = 1500/220 = 6.82 A.
Everything together: I = 2055/220 = 9.34 A.
Interpretation: the kettle by itself asks for 6.82 A, which already exceeds the 5 A rating of the fuse. So the fuse melted the moment the kettle was switched on, even before the lamps and mixer are counted - the blowing is not a symptom of a short circuit or a fault in the kettle. It is the fuse doing exactly its job on an undersized circuit.
The kettle's element resistance is R = V²/P = 220²/1500 ≈ 32.3 Ω, so at 220 V the current is fixed at about 6.8 A; no arrangement of plugs can reduce it. The correct reading is "circuit capacity too small for this appliance", not "appliance faulty". A 10 A fuse would carry the full 9.34 A load, since the cable is rated 10 A.
2. Evaluate the electrician's 15 A replacement, and state why a fuse must be fitted in the live wire rather than the neutral wire.
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The 15 A wire is dangerous. A fuse protects the cable, not the appliance. The cable is rated 10 A; with a 15 A fuse in place, a current of, say, 12 A could flow indefinitely through insulation designed for 10 A. The cable would heat up, the PVC insulation would soften and eventually char, and a fire could start inside the wall - while the fuse sat there intact. The fuse rating must be the lowest value that carries the normal load but below the cable's safe limit: here 10 A, because the full load of 9.34 A is just under the cable rating.
Why the live wire: the fuse must disconnect the appliance from the high-potential line. If the fuse is in the neutral wire and melts, the appliance is still connected to the live 220 V line, so its internal wiring and metal body remain at a dangerous potential; anyone touching a faulty part and earth completes a circuit through their body. With the fuse in the live wire, a blown fuse leaves the appliance dead and safe to handle. This is why switches and fuses are always placed in the live conductor.
3. Explain the tingling sensation using the fault described, and explain why no fuse rating - 5 A, 10 A or 15 A - would have prevented the shock.
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What is happening: the frayed insulation lets the live conductor touch the metal body of the mixer. Standing on a wet floor, a person touching the body completes a path: live wire → body of mixer → person → wet floor → earth. Wet skin has low resistance, so a current of a few milliamperes flows, which is felt as tingling; a slightly better contact could push tens of milliamperes through the chest, which is lethal.
Why the fuse is irrelevant: a fuse melts only when the current exceeds its rating, typically many amperes. The shock current is a few milliamperes - thousands of times below 5 A - so the fuse element is not heated at all and the supply stays on. The appliance also continues to run normally, because the motor current is unaffected.
What would have prevented it: a properly connected earth wire. It gives the leakage current a low-resistance metal path back to earth, so a large current flows, the fuse or MCB trips almost instantly, and the body never stays live. In addition, an RCCB (Earth Leakage Circuit Breaker) sensing a 30 mA imbalance would trip even without a large fault current. The tingling is therefore a warning that the earthing has failed, not that the fuse rating is wrong.