The Extension Lead That Keeps Tripping
A student’s study room is supplied by a 240 V circuit protected by a 13 A fuse. The room contains a 1.20 kW fan heater, a 60 W desk lamp, a 150 W laptop charger, a 900 W printer and a 2.00 kW kettle. All are connected through a single 4-way extension lead rated at 13 A, which is plugged into one wall socket. The student switches on the heater, lamp, laptop charger and printer, then boils the kettle. The fuse in the plug of the extension lead melts. The student replaces it with another 13 A fuse, but the new fuse also melts when the kettle is switched on. A friend suggests wrapping the fuse in aluminium foil “just to finish the printing”. The room also has a metal desk bolted to a concrete floor and an old radio whose plug has a missing earth pin.
1. Calculate the current drawn when all five appliances are on. Show whether the 13 A fuse should melt, and identify which appliance combination is already close to the limit.
Read the worked answer
Use I = P / V for each appliance at 240 V:
- Fan heater: 1200 / 240 = 5.00 A
- Desk lamp: 60 / 240 = 0.25 A
- Laptop charger: 150 / 240 = 0.625 A
- Printer: 900 / 240 = 3.75 A
- Kettle: 2000 / 240 = 8.33 A
Total current = 5.00 + 0.25 + 0.625 + 3.75 + 8.33 = 17.96 A ≈ 18.0 A.
This is greater than 13 A, so a 13 A fuse should melt. It is doing its job, not failing accidentally. Without the kettle, the total is 5.00 + 0.25 + 0.625 + 3.75 = 9.63 A, which is within 13 A. The heater and kettle alone draw 5.00 + 8.33 = 13.33 A, already above 13 A, so they must not be run together on one 13 A extension lead.
2. Explain why the aluminium-foil “repair” is dangerous. Explain also why the fuse may fail to protect someone who touches the metal desk if a live wire inside the printer touches the desk.
Read the worked answer
Aluminium foil has a very low resistance and a very high melting point compared with the fuse wire. It will carry 18 A without melting, so the extension lead, plug and socket overheat. The insulation can melt, exposing live conductors, and a fire can start. The foil defeats the purpose of the fuse.
A fuse is an overcurrent device, not an earth-leakage device. The metal desk is bolted to a concrete floor and may provide a path to earth, but the current that flows when a live wire touches the desk depends on the resistance of that path. If the current is below 13 A, the fuse will not melt quickly, so the desk can remain live. A person touching the desk then provides a parallel path to earth. The current through the person can be lethal even though the fuse has not blown. A residual-current device (RCD) rated at 30 mA is needed to detect the imbalance and trip.
3. Recommend a safe way to run the appliances. Include a calculation to justify your arrangement, and state one fixed protective measure for the room.
Read the worked answer
Do not run the kettle and heater on the same 13 A extension lead. Put the kettle on a separate wall socket, preferably on a different circuit, or use it only when the heater is off. On the 13 A extension lead, keep the heater, lamp, charger and printer: 5.00 + 0.25 + 0.625 + 3.75 = 9.63 A, which is below 13 A. The kettle alone draws 8.33 A, which is also below 13 A on its own socket. Do not daisy-chain extension leads, and check that the extension lead is rated for 13 A.
Replace the old radio plug or have the radio checked; never remove or bypass the earth pin. As a fixed protective measure, install a 30 mA RCD on the room circuit. It trips when the current to earth differs from the current returning, protecting people even when the fuse does not blow. Keep the metal desk’s earth connection intact.