Mains electricity：練習問題

Which safety feature in a domestic appliance is specifically designed to protect the circuit by melting when the current exceeds a certain value?

An electric oven with a power rating of \(3.45 \text{ kW}\) is connected to a \(230 \text{ V}\) mains supply. The oven is switched on for a total of \(2.0 \text{ hours}\) to cook a meal. Calculate the current \(I\) flowing through the oven and the total energy \(E\) transferred during this time.

An appliance has a metal case and is not double insulated. Which wire is essential to prevent the casing from becoming live if a fault occurs?

Explain why a double insulated domestic appliance does not require an earth wire connection for safety.

Electrical safety is essential when dealing with mains electricity.
(a) Explain how the plastic insulation on a mains cable protects the user.
(b) Some appliances are marked with a 'double square' symbol, indicating they are double insulated. Explain why these appliances do not require an earth wire.

A high-power electric heater is rated at \(2.3 \text{ kW}\) and is connected to a standard \(230 \text{ V}\) mains supply.
(a) Calculate the current flowing through the heater when it is operating at its rated power.
(b) Standard fuses are available with ratings of \(3 \text{ A}\), \(5 \text{ A}\), and \(13 \text{ A}\). Select the most appropriate fuse for this heater and explain why the other options are unsuitable.