Electrical safety：練習題

Why is it particularly dangerous to use an electrical appliance, such as a hairdryer, with wet hands or in a damp environment?

An appliance has a metal outer case and is connected to a mains supply via a three-core cable. Why is the earth wire connected directly to the metal case?

A washing machine is rated at \(2.3\text{ kW}\) and operates on a \(230\text{ V}\) mains supply. The available fuse ratings are \(3\text{ A}\), \(5\text{ A}\), \(10\text{ A}\), and \(13\text{ A}\).

Which fuse is the most appropriate to use for this appliance?

Which statement correctly describes the purpose of a fuse in an electrical circuit?

The diagram shows a symbol found on some electrical appliances.

What does this symbol indicate about the appliance's safety features?

Explain why the metal outer casing of an electric toaster must be connected to the earth wire of the mains supply.

A household electric kettle has a metal outer casing and is connected to the mains supply via a standard three-pin plug.

(a) Explain why it is essential for the switch to be connected to the live wire rather than the neutral wire.
(b) A fault occurs in the kettle, causing the live wire to come into direct contact with the metal casing. Describe how the earth wire and the fuse work together to prevent the user from receiving a fatal electric shock.

An electric heater is designed to operate on a \(230\text{ V}\) mains supply. The heater has a power rating of \(2.8\text{ kW}\).

(a) Calculate the current flowing through the heater when it is operating normally. (2)

(b) Suggest a suitable fuse rating for this heater from the following list: \(3\text{ A}\), \(5\text{ A}\), \(10\text{ A}\), \(13\text{ A}\), \(30\text{ A}\). Justify your choice. (2)

(c) The heater is used for \(45\text{ minutes}\). Calculate the total energy transferred by the heater in kilowatt-hours (kWh). (2)

(d) The heater has a metal case. Explain why a fault that connects the live wire to the metal case would be dangerous if the earth wire were broken. (2)