A metal-cased heater is rated 1380 W at 230 V. Ignore start-up surges. Available fuse ratings are 3 A, 5 A, 10 A and 13 A; choose the smallest rating strictly above its normal current. (a) Calculate the current and choose the fuse. (b) Explain why the fuse belongs in the live wire. (c) Explain the protective role of an earth connection if the live wire touches the case. This is a paper exercise, not an instruction to work on mains wiring.

Question

Accepted Answer

6 A; choose 10 A; the live fuse disconnects the source, and earthing provides a low-resistance fault-current path. Use electrical power P = VI for normal operation, so I = P/V = 1380/230 = 6 A. The selected fuse must carry this operating current under the rule given in the question. From the stated options, 10 A is the smallest rating strictly above 6 A. A greatly oversized fuse could allow excessive current in the cable without disconnecting promptly. The fuse is placed in the live wire so melting it disconnects the appliance from the live supply. A fuse only in the neutral could stop normal current while leaving internal parts connected to live potential. If live touches the metal case, a sound earth connection provides a low-resistance route for a large fault current. This operates the protective device and limits the case voltage. The earth wire normally carries no operating current; it is not a substitute for insulation.

Protecting a metal-cased heater 9

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