Answers

This ensures that even though current flows

through this cable, it remains at zero

potential and in this position, it cannot give

an electric shock.

Earth cable (E)is the safety wire used to

protect the appliances from excessive

current. The Earth cable (yellow/ green) is

connected to the earth pin (the upper hole)

of the three-pin plug.

Example 01

This wire is dipped in the ground (earthed)

near the house which protects the appliances

from damage in case of a defect or short

circuit.

The earth pin is longer than the Live and

Neutral.

Explain

Answers

The earth pin is usually longer than the other

two cables because

(i) The earth pin is the first to enter and open the

socket.

(i)

(ii)

It helps to open the socket and

(ii) It ensures that the device is earthed before

being connected to power

It ensures that the device is earthed

before being connected to power

Note the switch socket for the three-pin plug is

THE THREE PIN PLUG

shown below

This is a plug with three pins (Earth, Neutral

and Live) which fit into the socket.

The three cables are connected to the three

pins of three pin plug and fuse connected to

the neutral cable.

Example 02

The figure below shows a flex to the 13A –

3pin plug which has been incorrectly fitted

(a)

List the mistakes and suggest the

corresponding remedies

Answers

(i). Bulbs were connected in series yet they

have to be connected in parallel.

(ii). Fuse was connected to the neutral wire

yet it is supposed to be connected on the live

wire.

(iii) The switch was connected to the

neutral wire yet it is supposed to be

connected on the

live wire

(i)

Brown (Live wire) is connected

to the earth pin. It should be

connected to the fuse pin.

SHORT CIRCUIT

A short circuit: - is unwanted connection

which allows current to flow along the path

which is not part of the circuit designed.

(ii)

(iii)

Blue (Neutral wire) is connected

to the live pin/ fuse. It should be

connected to the neutral pin.

A short circuit: - is the path of electricity

that bypasses the load or part of the load in

an appliance and so results to higher current.

Causes of short circuit

Green / yellow (Earth wire) is

connected to the neutral pin. It

should be connected to the Earth

The following are some causes of short

circuit in a house

(i) Poor insulated wires

(b) What would happen if a plug is

connected to the main socket?

(ii) Worn out appliances

(iii)Incorrect wiring

Answer

How short circuit occurs in a house

Short circuit occurs when a live wire meets a

neutral wire without an appliance in the

circuit. This can happen when wires touch

each other.

The fuse will blow out (it will burn)

Example 03

The diagram below is a circuit connection.

Identify what are wrong connections in the

circuit.

Checking electric fault

Devices for checking and rectifying electric

faults are as follows

(i) A tester: -for testing the flow of

electric current

Answers

(ii) A multimeter: -for testing continuity

of a circuit. It tests for broken

connections and faulty connection of

the wires

Fuse are made of very thin wires so that they

can easily and timely melt down and break a

circuit in an event of an increase in electric

current.

(iii)Appropriate wires and fuses for

replacements

Electric heaters are made of thick wires so

that they do not melt as they convert electric

energy to heat energy.

Other components are

(iv)Leave mains lead indicator

Note that: Two devices are useful when

checking electrical appliances faults. These

are multi-meter and live main lead indicator.

FUSE

Fuse: - is a special wire which melts and

breaks a circuit whenever the current

through it exceeds its rated value.

Note: A fuse is a three-pin plug is connected

to the live cable

FUSE RATING

Fuse rating: -is the maximum current that a

fuse can tolerate without melting.

The following are common fuse rating 2A,

3A, 5A, 10A 13A, 14A and 30A.

Mode of action

Fuse blowing (melting) occurs mainly when

the circuit is over loaded or when a short

circuit occurs. The wire melts and this

prevents the current from the appliances and

hence is protected.

Example 01

Fuse wires are labeled 3A, 5A, 10A 12A,

18A and 20A. select a best fuse for a 240V,

2.856kW electric kettle.

Solution

Importance of a fuse

Power = 2.856kW = 2856WVoltage = 240V

A fuse protects the circuit from overloading

and short circuits by breaking the circuit

when a specific current exceeds the fuse

rating

P = IV

푃

I =

푉

Example 01

Why fuses are made of very thin wires while

electric heaters are made of thick wires?

2856

I =

240

Answers

I = 11.9퐴

(iii)

The electric iron rated 240V,

2kW

I ≈ 12퐴

solution

Power = 2kW,

The best fuse is the 12A fuse

Voltage =

240V,

Current =?

Example 02

P = IV

푃

I =

푉

Select the best fuse for the following

(i) A refrigerator rated 250V, 400W

2000

I =

solution

Power = 400W , Voltage = 240V

P = IV

240

I = 8.3퐴

I ≈ 10퐴

푃

I =

푉

The best fuse will be 10퐴

400

I =

Example 03

250

(a) With the aid of diagram, explain the

function of a fuse in electric appliance

I = 1.6퐴

Solution

I ≈ 2.0퐴

The best fuse will be 2.0퐴

The function of a fuse in an electrical

appliance is to disconnect the circuit

by melting when high current is passed

(ii)

The electric cooker rated 240V,

7.2kW

through

it

before

entering

the

appliance.

solution

Power = 7.2kW ,

The melting of the fuse protects the

appliance from high current electricity

which in turn could cause damage and

even fire outbreak.

Voltage =

270V

P = IV

푃

I =

푉

7200

I =

240

I = 30 퐴

(b) A circuit in a house is protected by a

10A fuse. The circuit is connected to

I ≈ 30퐴

the 240V mains.

The following

The best fuse will be 30퐴

appliances are connected to the circuit.