1

Mains Electricity
Sources of Mains Electricity
Water in high dams
Geothermal energy
Coal or diesel
Winds
Solar
Tidal waves in the seas
Nuclear energy
• The type of power generation chosen for a given location
depends on the most abundant source of energy available in
that area.
2
 The National Grid System
• It is a system of power cables connecting all the stations in a
country to each other and to consumers.
Advantage of national grid system
• Ensures that power is available to consumers even when one
of the power stations fails.
• Most power stations generate electricity in form of
alternating current (a.c) at voltage between 11kV and 25kV.
The voltage is then stepped up between 132kV to 400kV for
transmission so as to minimize power loss.
• The power can be stepped down to appropriate value for
domestic and other users. In Kenya domestic applicant
operate at 240V.
• An 𝒂. 𝒄 source voltage is represented by the symbol shown
below:

3
The National Grid System

4
 Electrical Power
𝑬𝒍𝒆𝒄𝒕𝒓𝒊𝒄𝒂𝒍 𝒑𝒐𝒕𝒆𝒏𝒕𝒊𝒂𝒍 𝒆𝒏𝒆𝒓𝒈𝒚 = 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 × 𝒄𝒉𝒂𝒓𝒈𝒆
𝒘𝒐𝒓𝒌 𝒅𝒐𝒏𝒆 = 𝑽𝑸
𝒘𝒐𝒓𝒌 𝒅𝒐𝒏𝒆
• 𝑬𝒍𝒆𝒄𝒕𝒓𝒊𝒄𝒂𝒍 𝒑𝒐𝒘𝒆𝒓 =
𝒕𝒊𝒎𝒆
𝑽𝑸
𝑷=
𝒕
𝒃𝒖𝒕 𝑸 = 𝑰𝒕
𝑽×𝑰×𝒕
𝑻𝒉𝒖𝒔, 𝑷 =
𝒕
⇒ 𝑷 = 𝑽𝑰
• Power loss during transmission
• Power dissipated in a circuit is given by P = VI.
But V = IR (Ohm’s law)
Thus P = I2 R
• The above equation shows that when current is high power loss is
also high
• Power loss is therefore low when transmitted at high voltage and
low current.
5
 During transmission power loss can be minimized by:
 Stepping up output voltage from power station so as
to be transmitted at low current hence low power loss
(H= 𝑰𝟐 R)
 Use of thick and good conductor transmission cable
to minimize resistance.

Aluminium is preferred than copper because:

Good conductor of electricity
It is light
It is cheaper and available

6
 Dangers of high voltage transmission.
• Harmful effects of strong electric fields.
• The risk of fire on nearby structures and
vegetation when cables get too low.
• The risk of electric shock in case the poles collapse
or hang too low.

7
 Domestic connection to power grid
• Electrical power is usually supplied at
240V from a step-down transformer.
• This power is connected to the house
using two wires;
• Neutral cable which earthed at zero
potential.
• Live cable which is at 240V potential Meter Box
• The life cable is connected to a higher
fuse value. The cables are then
connected to a meter where energy
consumed is registered. From meter
cable passes on to consumers fuse box.

8
 Domestic Wiring
Consumer fuse box consist of: Fuse box
Main switch
• It disconnects both live and
neutral wires simultaneously.
Live bus bar
• It is connected to live wire
and the fuse.
Neutral bus bar
• It is connected to all neutral
wires Circuit breakers
Earth Terminal
• It is earthed through a thick
copper bar buried deep in the
earth or through water
piping.

9
 Fuse
• Fuse is a thin wire (made alloy of copper
thin wire
and tins) which melts when current
exceeds its rating.
• Its function is to safeguard components
against excess current in the circuit.
• It has low melting point.
• It is connected on a live wire.
The fuse can blow due to the following:
a) Overloading the circuit
b) Short circuiting
Symbol of a fuse
c) Use of wrong fuse rating
The value of the chosen fuse should be
slightly above the value of the operating
current of the appliance. or

10
 Circuit Breakers
• Is an electronic device which
disconnecting the circuit when current
exceeds a certain value by
electromagnetism.
• It is more efficient than a fuse in that it can
be reset when power goes off unlike a fuse
which must be replaced with a new one.
• It breaks the circuit instantaneously unlike the
fuse with takes some time to melt.

11
 Fuse
Example
• House has a lighting circuit operated from 220V mains.
Nine bulbs rated at 120W 240V are switched on at the same
time. What is the most suitable fuse for this circuit?
• P = VI
𝑷
• I=
𝒗
𝟗𝑷
• Total current 𝑰 =
𝑽
𝟗 𝑿 𝟏𝟐𝟎
=
𝟐𝟐𝟎
= 4.91A
• The suitable fuse is a 5A fuse

12
Domestic wiring

13
 Domestic Wiring

LIGHTING CIRCUIT RING MAIN

CIRCUIT

14
 The Lighting Circuit.

For lighting circuit the lamps are connected in parallel so that:

a) They are operated with a switch independently.
b) To reduce the effective resistance.
c) They can be operated at the same potential of the mains
d) If one bulb/lamp is faulty the rest still work.
Note :The cables in lighting circuit are relatively thinner than cables
in cooker and heater circuits because lamps consume smaller amount
of current

15
 The Rings Mains Circuit
Used to connect all the sockets in the house.

16
 A Three – Pin Plug
A three pin plug is used to connect electrical devices to the
sockets in a ring main circuit

The earth pin is longer so as

to open the protective cover
or shutter of the live and
Colour coding neutral pins.
Live wire – red/brown Three pin-plugs have the
Neutral wire – Black/ blue earth pin which provides the
Earth wire – Green/green with yellow stripes path for excess current.
17
The cost of electrical energy
Commercial companies charge for electrical energy supplied to
consumers. The amount of energy used by consumers depends
on:
• Power rating of appliances
• Time for which they have been in use.
Energy = Power x time
The unit is used for costing 1 unit = 1kWh
Kilowatt-hour (KWh) is amount of electrical energy spent in one
hour at rate of 1000 J/S (watts).
The total cost of electrical energy consumed by a household is
given by the product of the number of units consumed and the
charges per unit;
Total cost= number of units used x cost per unit.
18
Example 1
An immersion heater rated 3000 W is used continuously
for 45 min per hour per day. Calculate the cost per week at
sh 12.00 per unit.

Solution
𝟑𝟎𝟎𝟎
Number of kW =
𝟏𝟎𝟎𝟎
= 3.0 kW
Number of kWh per day
𝟑
= 3.0 kW x x 1 x 24
𝟒
Number of kWh per week
𝟑
= 3.0 x x 24 x 7
𝟒
= 378 kWh
Cost per week = 378 x 12.00
= sh 4536.00
19
Example 2
A house has five rooms with 240 V, 60 W bulbs. If the bulbs are switched
on from 7.00 p.m. to 10.30 p.m.
(a) Calculate the power consumed per day in kilowatt-hours
(b) Find the cost per week for lighting these rooms at Sh 12.00 per unit.
Solution
(a) Energy spent per second
= (60 W x 5)
= 300 W
= 300 𝐉𝐬 −𝟏
Energy spent per evening
= 300 x 3.5 x 60 x 60
Energy spent in 7 days
= 300 x 3.5 x 60 x 60 x 7
Number of kilowatt-hours per week
𝟑𝟎𝟎 𝐱 𝟔𝟎 𝐱 𝟔𝟎 𝐱 𝟑.𝟓 𝐱 𝟕
=
𝟑.𝟔 𝐱 𝟏𝟎𝟔
= 7.35 kWh
(b) Cost per week = No. of kilowatt-hours x cost per kWh
= 7.35 x 12.00
= Sh. 88.20 20
1. Explain why mains electricity is transmitted through alternating current and
not direct Current
2. State the energy transformation that takes place in a hydroelectricpower
station.
3. Electrical energy is distributed as alternating current (a.c) at high voltage.
Explain why:
(i) A very high voltage is necessary.
(ii) Thick aluminium transmission lines are recommended to carry the
current.
(iii) The power is transmitted as Alternating voltage (a.c)
4. State two dangers of high voltage transmission.
5. What is the purpose of having a national grid in power transmission.
6. State two factors which affect heating in electric circuits.
7. A man driving a car in rain discovers that the moment he alights from it,
while touching its body he gets an electrical shock. Why was he not getting the
shock while inside even if he touches metallic parts?
8. Name a device used to change light energy directly into electrical energy.

21
1. Explain why three pin plugs are preferred to two pin plugs in the use with
iron box
2. Explain the letters L, N and E usually marked on the three pin plug
3. State two reasons why the earth pin is normally longer than the other two
pins.
4. Give two advantages of a circuit breaker instead of a fuse in domestic wiring
5. Distinguish between a fuse and a circuit breaker
6. State the importance of a fuse in a circuit and explain how it works.
7. What property does a fuse wire have that makes it suitable for controlling
excessive current in circuit?
8. In domestic ring there are various circuits which may be used. Name three
9. What is normally contained in a consumers control unit in a domestic
wiring system
10. Explain why bulbs in a long corridor are not wired in series.
11. An electric iron rated 240V, 750w is to be connected to a 240v mains
supply through a 3A fuse. Determine whether the fuse is suitable or not.
12. An 8000w heater is used for a total of 1.5 hours a day. Calculate the cost of
using this shower for a month of 30days if the cost of an electrical unit is ksh.
5.40.
22
13. How many 100W electric irons could be safely be connected to a 240V
mains circuit fitted with a 13A fuse?
14. Determine the largest number of 75W bulbs which can be safely used to
run on a 240V source with a 5A fuse.
15. Find the maximum number of 75W bulbs that can be connected to a 13A
fuse on a mains supply of 240V.
16. Calculate the power of a devise which has a p.d of 250V applied across it
when a current of 0.5A passes through it.
17. An electric bulb rated 12V, 36W allows current to pass through for 1hour.
How much electrical energy is dissipated by the bulb?
18. An immersion heater rated 1.5KW is used continuously for 30min per hour
per day. Calculate the cost per week per ksh 6.70 per unit.
19. An immersion heater rated 300W is used continuously for 45 minutes per
day. Calculate the cost per week at 60cts per unit.
20. A 50w bulb is used continuously for 36 hours. Determine the cost of
energy consumed at a cost of Kshs. 2 per unit.
21. A 60W bulb is used continuously for 36 hours. Determine the energy
consumed, giving your answer in kilowatt hour (kWh)
23
22. Electrical energy costs Kshs. 1 per kWh unit. Find the cost of using an
electric heater of power 7.5kW for a day.
23. An electric iron box is rated 2500W and uses a voltage of 240V. Given that
electricity costs Kshs. 1.10 per Kwh, what is the cost of using it for 6 hours?
What is the cost of operating all the parts at once for 30 minutes if electricity
costs Sh.10.00 per unit?
24. An electric cooker has an oven rated 2500w, a grill rated 1500w and two
plates each of 1000w. This cooker operates directly from 240v supply. What is
the cost of operating all the parts at once for 30 minutes if electricity costs
Sh.10.00 per unit?
25. Four 40W bulbs and six 100W bulbs were switched on for 5 hours a day for
domestic use in a certain institution. Find the monthly bill for the consumer
given that the cost of electricity in the country is at Sh. 5.50 per unit (Take 1
month = 30 days and standing charge of Sh. 150)
26. A 2kW electric heater is used for 10hrs. Calculate the cost of electricity if it
costs 30ksh/ unit
27. An immersion heater is rated 5 KW 250V. It is used for 2 hours daily. If
electricity cost sh12.00 per unit, calculate the weekly cost of running the
heater.
24