The Heater, the Extension Reel and the Warm Plug
A family uses a 3 kW electric fan heater in a garage workshop. The heater is plugged into a 4-way extension lead with 1.0 mm² cable, rated at 10 A and fitted with a 13 A fuse. The extension lead is plugged into a socket protected by a 30 mA RCD, and the garage circuit is protected by a 32 A MCB. The supply voltage is 230 V. The extension lead is left coiled on a plastic reel. After 20 minutes of use, the reel feels warm, the cable is soft, and the plug connecting the heater to the extension lead is slightly discoloured and smells hot. The heater is still working normally.
1. Calculate the current drawn by the heater and explain why the extension lead is unsuitable for it.
Read the worked answer
Power P = VI, so I = P/V = 3000 W ÷ 230 V = 13.0 A (13.04 A).
The extension lead is rated at 10 A, so the heater draws about 30% more than the cable is designed to carry continuously. The 13 A fuse protects the lead only up to its rating; the fuse is rated above the cable's 10 A capacity, so the cable can overheat without the fuse blowing.
Overheating happens because the cable has resistance. Power dissipated as heat in the cable is P = I²R. With I = 13 A instead of 10 A, the heating is (13/10)² = about 1.7 times greater. The 1.0 mm² conductors are too thin for this current, so the insulation softens and there is a real risk of a short circuit or fire. The correct action is to use a lead rated at 13 A or more with thicker conductors, or plug the heater directly into the wall socket.
2. Explain why leaving the lead coiled makes the problem worse, what the discoloured plug suggests, and what should be done immediately.
Read the worked answer
A coiled cable has its heat-producing conductors packed together. Heat cannot escape by convection and is trapped between the layers of cable, so the temperature rises much more than it would if the lead were uncoiled and spread out. The plastic reel also insulates it. The same current therefore produces a much higher temperature.
The discoloured plug suggests overheating at the contacts, caused by resistance at a poor connection or by the plug pins and fuse holder carrying more current than they are rated for. Discolouration and a hot smell are warning signs of a developing fault; the plastic may char and the contacts may eventually arc.
Immediate action: switch off and unplug the heater, do not use the lead again, uncoil it and let it cool, and have the plug and lead checked or replaced by a competent person. The heater should be run from a suitable socket, not this lead.
3. Explain the different roles of the 13 A fuse, the 32 A MCB and the 30 mA RCD in this situation, and why none of them prevented the overheating.
Read the worked answer
The 13 A fuse and the 32 A MCB are overcurrent devices. They protect the wiring from currents far above the design value. The MCB protects the fixed house circuit; the fuse protects the appliance and its lead. But a 13 A fuse is designed to carry 13 A continuously, and the heater draws only 13.0 A, so the fuse does not blow - a fuse is not a precise current limiter and it only reacts to faults or large overloads.
The 30 mA RCD compares the current in the live and neutral conductors. It trips when about 30 mA leaks to earth, for example through a person touching a live part or through damp insulation. It protects people from electric shock; it does not detect an overloaded cable or a hot plug, because no current is leaking to earth.
So the overheating was caused by a correctly operating appliance on an incorrectly rated lead - a design and selection error rather than an electrical fault, which is why none of the protective devices operated. The real protection is choosing cable of adequate current rating.