The Night the Fuse Blew
An apartment has two circuits at 220 V: a 5 A lighting circuit and a 15 A power circuit. The 5 A circuit already serves four 60 W bulbs, a 75 W ceiling fan and a 750 W refrigerator. The family buys a 1500 W immersion rod for hot water and plugs it into a socket on the 5 A circuit. Within minutes the 10 A fuse in that circuit melts and the lights go out. The father replaces the fuse with a loop of thick copper wire "just for tonight", saying the fuse was too weak for the load. Next morning the refrigerator's metal body gives a mild shock when touched with wet hands. The landlord says the wiring is fine and the family simply used the wrong socket; the father says the fuse rating was too low and the landlord should upgrade it. A neighbour suggests fitting a 20 A fuse wire in the same holder so that it will never blow again.
1. Calculate the currents on the 5 A circuit before and after the immersion rod was plugged in, and decide whether the fuse blew because it was faulty or because it did its job.
Read the worked answer
Before: 4 × 60 = 240 W for bulbs, 75 W fan, 750 W refrigerator → 1065 W. I = P/V = 1065/220 ≈ 4.8 A. That is already close to the 5 A rating of the wiring, so there was almost no margin.
After: 1065 + 1500 = 2565 W. I = 2565/220 ≈ 11.7 A.
A 10 A fuse in a circuit that is meant to carry 5 A continuously will melt at 11.7 A, and it should. The fuse is a deliberate weak point placed in series so that it melts before the cable and sockets overheat. So the fuse was not weak or faulty; it did precisely its job, and the fault was loading a 5 A lighting circuit with a 1500 W heating appliance.
Correct action: run the immersion rod, and any other heavy appliance, from the 15 A power circuit through its own 15 A socket, and leave the lighting circuit for lamps and the fan. The father's conclusion ("raise the fuse rating") is dangerous: the fuse rating must match the cable, not the appliance.
2. Evaluate the father's copper-wire repair and the neighbour's suggestion of a 20 A fuse wire, explaining what each would actually do to the circuit.
Read the worked answer
Both defeat the only protection the circuit has. A fuse wire is chosen to melt at a current slightly above the safe rating of the cable. Thick copper wire will carry far more than 11.7 A without melting - perhaps 30 A or more - so the next overload heats the cable, the socket and the insulation instead. The insulation carbonises, the wall wiring can smoulder and catch fire, and the fault may occur where nobody can see it. A 20 A fuse wire is the same mistake in a neater form: the fuse holder and cable are rated for 5 A, so 20 A will pass long after the cable is dangerously hot.
The judgement to teach is that a fuse protects the wiring, not the appliance, and a fuse that blows repeatedly is a message about the load. Correct responses are: reduce the load, move the appliance to the correctly rated circuit, and fit a fuse (or an MCB) of the correct rating. Oversizing a fuse or replacing it with wire is illegal in most installations and is a leading cause of domestic fires.
3. Explain why the refrigerator's body gave a shock, why wet hands made it worse, and what should be checked and done before the fridge is used again.
Read the worked answer
The metal body of a refrigerator is bonded to the earth pin of the plug. If the live wire inside chafes against the body, current flows to earth through the earth wire and the body stays at earth potential, so nobody gets a shock. A shock on touching the body therefore means the leakage path exists but the earth path does not: the earth wire in the plug or the wall is broken, the socket has no earth connection, or the earth pit is dried out.
Wet hands make it worse because moist skin has much lower resistance than dry skin; the same leakage current that went unnoticed now passes through the body, which is why the shock appeared only with wet hands. It is a warning, not a minor annoyance.
Action before reuse: stop using the refrigerator, have the earth continuity checked from the plug pin through the socket to the earth pit, get the appliance checked for insulation failure, and use the 15 A circuit with proper earthing. Adding a 30 mA RCCB/ELCB on the circuit would trip on such leakage and is strongly recommended.