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physics Physics For High Schools (KS4)

Solved problems for the chapter:

Fuses, Earthing and Circuit Breakers

View the theory »

Electrical resistance

1. Definitions

2. Ohm's law

3. Kirchhoff's law

3.1. Kirchhoff's first law

3.2. Kirchhoff's second law

4. Total resistance

4.1. Parallel connection of resistors

4.2. Series connection of resistors

5. Resistivity of an electrical conductor

6. Current and voltage measurement

7. Forum

↓ Show theory [1] ↓

Detailed solution steps are available at:

https://en.openprof.com/wb/chapter:fuses%2C_earthing_and_circuit_breakers/3645/

Example 1.

A double-insulated electrical appliance must be connected safely to the electricity supply. Which statement is correct?

It can be connected with an earth wire only.
It must be connected with a fuse and an earth wire.
It can be connected with a fuse only.
It does not need a fuse or an earth wire.

Example 2.

Which statement about electrical safety is correct?

A device that has a normal operating current of 3.0 A must be protected by a 3.0 A fuse.
The metal casing of an electrical device must be connected to the neutral wire.
If a device is double insulated, it does not need a fuse.
Switches must always be connected into the live supply wire.

Either a fuse or a circuit breaker can be used to protect electrical cables from large currents that could cause overheating. When a fuse is used, where should it be connected, and when a circuit-breaker is used, where should it be connected in the figure below?

	position of circuit breaker	position of fuse
A	X	X
B	Y	X
C	X	Y
D	Y	Y

Where must a fuse be connected in a mains electric circuit?

the neutral wire only
the live wire and the earth wire
the earth wire only
the live wire only

An electrical appliance is powered from a mains supply. The appliance normally uses a current of , but the current briefly rises to at the instant the appliance is switched on. The cable to the appliance is designed for currents up to , and a fuse is used to protect the circuit. What should be the rating of the fuse?

The current in a lamp connected on its own to the mains supply is . A table decoration has three of these lamps connected in parallel. Which rating of fuse is suitable to protect this circuit?

Why is a fuse used in an electrical circuit?

to provide a path to earth if a fault occurs
to prevent the current becoming too large
for the current to have only one value
to save electrical energy

The diagram below shows a transformer used on a building site. The mains plug for the transformer contains a fuse.

Give a reason why the plug includes a fuse.
Explain how a fuse works.

Transformer

Which diagram shows the circuit symbol for a fuse?

10.

If the insulation within a mains cable becomes damaged, two of the wires in it may touch and cause a short circuit. Which row is correct?

	the danger when this happens	safety device which avoids this danger
A	a large current will overheat the wiring and lead to a fire	a relay
B	a large current will overheat the wiring and lead to a fire	a fuse
C	the appliance at the end of the cable will be damaged	a fuse
D	the appliance at the end of the cable will be damaged	a relay

11.

The circuit connecting a room heater to the mains electricity supply has a fuse in it. The fuse melts and switches off the circuit. Why does the wire in the fuse melt?

The air in the room reaches its required temperature.
The air in the room becomes too damp.
The current in the circuit becomes too large.
The insulation around the circuit wire becomes damaged.

12.

A different hair dryer has a fuse and two heat settings. When the hair dryer is used on the low heat setting, the current in the hair dryer is . When the hair dryer is used on the high heat setting, the current in the hair dryer is . What is the correct fuse rating for this hair dryer?

13.

Which of the components below are designed to improve the safe working of a mains electrical supply?

circuit breaker
earth wire
fuse

I and II only
I and III only
II and III only
I, II and III

14.

Why is a fuse used in an electric circuit?

to prevent short circuits
to increase the circuit resistance
to stop the cables from overheating
to reduce the power loss

15.

A teacher uses a power supply in a metal case. The circuit for the power supply includes a fuse. The metal case of the power supply is earthed. A fault occurs and a live wire touches the metal case. Explain how earthing the metal case protects the teacher.

16.

A kitchen tap supplies instant boiling water. Cold water passes over an electric immersion heater inside the tap. The metal tap is earthed and there is a fuse in the cable that connects the heater to the mains. Explain how the fuse protects the circuit.

17.

A microwave oven has a metal case and is connected to a electricity supply. The microwave oven is fitted with a fuse and an earth wire is connected to the metal case of the microwave oven. A fault occurs and the live wire of the microwave oven touches the metal case. Explain how the fuse and an earthed metal case protect the appliance and the user.

18.

What is an advantage of using a circuit breaker instead of a fuse in an electrical circuit?

19.

An electric heater is plugged into the mains supply using a fused plug. The current in the heater is . The cable attached to the heater is rated at . The fuses available are rated at , , and . Which fuse should be used?

20.

There is a desktop computer and it is connected to a mains supply by a plug containing a fuse. The computer has a metal case. A fault occurs and a live wire touches the metal case. Explain how an earth wire and the fuse in the plug protect the user.