An electric circuit in a domestic house is

protected by a 6A fuse. The circuit is

connected to the 240V mains. The following

appliances are connected to the circuit.

Appliance

Bulb 1

Bulb 2

TV

Power rating

100 W

75 W

Appliance

Bulb 1

Bulb 2

TV

Power rating

100W

300 W

75W

300W

Heater

1500 W

Heater

Iron

1500W

900W

Determine whether the fuse will blow

on or off if all appliances are turned

on.

What will happen to a fuse if all appliances

are turned ON? Justify your answer.

Solution

Solutions

Given appliances and their paower ratings.

Step I: To find the total power

Total power = 100W + 75W + 300W

+1500W

Step 1: to calculate the toatl power

consumption

Power = 1975W

푃_푇= 푃_퐵1+ 푃_퐵2+ 푃_푇ꢀ+ 푃_퐻푇+ 푃

ꢁ푅

[

]

푃_푇= 100 + 75 + 300 + 1500 + 900 푊

Step II : To find the current

푃_푇= 2 875 푊

From the Joules law equation

STEP 2: to calculate the total current drawn

푃 = 퐼푉

Where P = 1975W

V = 240V

푡표푡푎푙 푝표푤푒푟

퐶푢푟푟푒푛푡, 퐼 =

푉표푙푡푎푔푒

Then

2 875 푊

퐼 =

푃

푉

I =

240 푉

퐼 = 11.98퐴

1975푊

240푉

I =

The total current drawn is 11.98A

The 6A fuse will blow off (will fail) if all

appliances are turned ON because the total

current drawn ( 11.98A) exceeds the fuse’s

current rating of 6A.

I = 8.23퐴

∴ The fuse will blow on because the

appliances will use low current of 8.23A

than the limit of the fuse which is 10A.

Example 05

(a) Explain why electrical appliances must

be earthed?

Example 04

Circuit Breakers: - is a type of a switch

that cuts off the flow of current when current

in a circuit exceeds a specific value.

Solution

Earthing of metallic appliances is to prevent

severe electric shock to the users. The

metallic body of electric appliances is

connected to earth wire so that any leakage

of electric current is transferred to the

ground.

(b) What happens if an appliance is not

earthed?

Solution

The circuit breaker operates by opening the

circuit in the event of excess current.

Without the earth wire, if a fault occurs

and the live wire becomes loose, there is a

danger that it will touch the metal case.

The next person who uses the appliance

could get electrocuted.

(a) State and explain the precautions to

take during electrical wiring of a

house.

Answers

Points to note.

(i) Power off. Make sure the power is

(i) Unlike a fuse, a circuit breaker can be

off at the breaker before you start

and use voltage tester to verify that.

(ii) Be careful with what you touch,

because if you touch a live wire can

result to short circuit.

reset once current in a circuit has returned to

normal

(ii) However just like the fuse, the circuit

breaker is connected in series with the

circuit it controls.

(iii)Replace old wiring that shows signs

of deterioration or fraying.

(iii) Common circuit breakers are either

thermal or magnetic device the incorporate

a bimetallic strip or electromagnetic to

open and close the switch respectively.

(iv)Make sure hands are dry and wear

chappals

(v) Wiring of electrical appliance should

be done by a trained person.

Example 01

Distinguish between a fuse and a circuit

breaker

CIRCUIT – BREAKERS

In the lighting circuit, the lamps are

connected in parallel and not series

Reasons

Answers

A fuse is a wire of low melting point which

melts when a certain amount of current is

exceeded.

(iv) This ensures that if one of the bulbs

fails, the rest will continue working

(v) This ensures that all the bulbs

operate at the same main voltage

A circuit breaker is a magnetic device that

breaks the circuit through an electromagnet

when a certain amount of current exceeds.

The switches are on the live wire and not the

neutral wire

Reason: This is so that when the switches

are off, no appliance is at full potential

(voltage).

CONSUMER UNIT

Consumer unit: - is the type of distribution

board that electricity distributes from the

main supply to separate circuits.

The lightning circuit does not require the

earth connection unless florescent tubes

are used for lighting purpose.

From the consumer unit (distribution board),

the cables branch into several parallel

circuits which carry current to various parts

of the house

The wires used in the lighting circuit are

relatively thinner than for other circuits.

Reason: The lighting circuit carries smaller

There are two methods of wiring a house

from the main switch; these are the ring

main system and tree system.

current

TYPES OF LIGHTING CIRCUITS

There are two lighting circuits namely loop

– in lightning circuit and junction box

lighting circuit.

RING MAIN SYSTEM

In this circuit, a cable containing three wires

(Live, Earth and Neutral) form a loop.

Power sockets in the various rooms are

connected at convenient points.

Loop – in lighting circuit:

In loop – in lighting circuit, the live, neutral

and earth wires run from the consumer unit

to each other of the ceiling designs, one after

another. From each rose, another set of

wires run to the switch which operates the

light.

LIGHTNING CIRCUIT

circuit breaker for each circuit in a common

enclosure. Also is known as Panel board,

breaker panel, electric panel, fuse box or DB

box.

Example 02

How does an electrical distribution box

work?

Junction box lighting circuit

In junction box lightning circuit, neutral and

earth wires run to one junction box after

another. From the junction box, one wire

runs

Answer:

One or more supply lines are connected to

the distribution box, and outgoing cables are

connected from it. A power distribution box

is a part of an electrical system whose task is

to distribute electrical energy. It includes

to the light and other runs to the switch for

that light.

distribution,

protection,

measurement,

control and signaling instruments

Example 03

What is fuse box?

Answer

COOKER CIRCUIT

Fuse boxes are metal boxes that store fuses,

which is an electrical safety measure that

will shut-off power when the power is

beyond that fuses ability to contain the

power.

Cooker and water heater circuits are earthed

and their wires are relatively thicker than

those for lighting circuits.

Reason: Cooker and water heater circuits

carry large currents.

Example 04

What is the function of fuse box?

Example 01

Answers

What is distribution fuse box?

The fuse melts under excess current to

protect the home electrical appliances

against damage due to overloading. When

too much electricity, flows through a circuit

the fuse will blow or the circuit breaker will

trip

Answer

A distribution board is a component of an

electricity supply system that divides

an electrical power feed into subsidiary

circuits while providing a protective fuse or