Kettle and toaster on a 13 A extension lead
A householder plugs a 2.0 kW electric kettle and a 1.5 kW toaster into a four-way extension lead. The lead is rated at 13 A maximum and is fitted with a 3 A fuse in its plug. It is plugged into a normal 230 V socket on a circuit protected by a 32 A miniature circuit breaker. When both appliances are switched on, the 3 A fuse in the extension lead blows almost immediately. The householder replaces it with a 13 A fuse and the appliances then work. A week later the extension lead feels warm and the plug socket is slightly discoloured. The cable is marked 1.25 mm², which has a current-carrying capacity of about 13 A. The householder asks whether the 13 A fuse has 'fixed' the problem.
1. Calculate the current drawn by each appliance at 230 V and the total current. Show your working.
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Use I = P/V. Kettle: 2000 W ÷ 230 V = 8.70 A. Toaster: 1500 W ÷ 230 V = 6.52 A. Total = 15.2 A (or 3500 W ÷ 230 V = 15.2 A). This assumes both are on the same 230 V supply and are resistive heating appliances with a power factor close to 1. The total exceeds the 13 A rating of the lead.
2. Explain why the 3 A fuse blew and why fitting a 13 A fuse does not make the arrangement safe. Include the idea of fuse rating, cable rating and heating.
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The 3 A fuse blew correctly because 15.2 A is about five times its rating; a fuse is designed to melt and break the circuit on overload. Fitting a 13 A fuse raises the threshold. The current is still above the lead's 13 A maximum, but only by about 17%, and a 13 A fuse may carry that overload for a long time without melting. Meanwhile the cable, rated about 13 A, heats up, which explains the warm lead and discoloured socket. The 32 A circuit breaker will not help because it protects the fixed wiring, not the extension lead. The fuse has hidden the symptom, not fixed the cause.
3. Recommend a safe solution and explain the principle behind it.
Read the worked answer
Take the kettle off the extension lead and plug it directly into a separate wall socket, or use the toaster and kettle one at a time. A four-way lead is not suitable for two high-power appliances whose combined current exceeds its rating. Do not replace the cable or fuse with a higher-rated one unless the whole lead, plug and socket are rated for the current; a domestic 13 A plug is limited to 13 A. Check for heat damage and replace the lead if the socket or cable is discoloured. The safe principle is that the fuse protects the cable, so the fuse rating must not exceed the cable's current-carrying capacity.