Category Archives: Chapter 8 – Generation of Electricity

3- pin plug

Three pin plug.

The three pin plug contains;

The blue neutral wire which is held at around 0 volts.
The green/yellow earth wire.
The brown live wire which alternates between +240 volts and -240 volts.
The fuse (left hand side on diagram).
The cable grip, at the bottom of the plug to hold wires in securely.


The most common fuse sizes are 3A, 5A and 13A (an electric cooker can have a 30A fuse). If the amount of current flowing through the fuse gets bigger than the fuse rating the wire inside the fuse gets hot and melts which disconnects the live wire from the appliance. Fuses are cheap but operate more slowly than circuit breakers.

Calculate Fuse Ratings

There is a formula for working out the fuse rating, voltage or wattage for each appliance

P (Power in Watts) = V (voltage) x I (current in Amps)

The fuse rating can be calculated by dividing the power used by the appliance by the voltage going into the appliance.

I (Amps) = P (Watts) ÷ V (Voltage)

To calculate the fuse rating for a 1KW (1000Watts) , 230V appliance, divide the wattage (1000) by the voltage (230).

I (Amps) = P (Watts) 1000 ÷ V (Voltage) 230
I (Amps) = 4.35

A five amp ( 5 Amps) fuse would be required.

Earth Wires

Appliances with metal cases are earthed with an earth wire. Some appliances do not require an earth wire as they are double insulated. If a fault develops and the earth wire connects to the metal case of the appliance current begins to flow down the earth wire. The extra current being drawn in through the live wire causes the fuse wire in the fuse to heat up and melt. This disconnects the live wire and makes the appliance safe.


Electrical power supply for homes

Electrical power is supplied to our homes through the main supply cables from nearby substation. There are two types of current : direct current and alternating current. The type of current supplied to our home is alternating current with a voltage of 240 V.

Every house has an electrical wiring system that suplies electrical power to all electrical appliances in the house. There are two types of wiring systems; single-phase wiring system and three-phase wiring system. Single-phase wiring system is usually used in our homes because most consumers in residential areas only need a low voltage, for example, 240 V. The three-phase wiring system is usually used in commercial and industrial areas where there is a need for high voltage, for example, 450 V.


Electrical Power Transmission and Distribution System

The National Grid

In every country in the world, There is a standard system known as The National Grid system used to transmit electricity to all over the country. The national grid system is the network of cables which transport electricity from the power stations to homes, factories and other places that require it.

Power stations produce electricity at high currents. Electricity transmission at high currents would encounter a large resistance in the transmission wire and therefore lose a lot of its energy as heat. To prevent this, the current generated is passed through a step up transformer. Here the voltage is increased to as much as 400,000V and the current decreased (remember the relationship P = VxI an increase in voltage and decrease in current by the same ratio will give the same power).

The high voltage (400,000 Volts) electricity is carried along overhead lines and underground cables referred to as the supergrid. The voltage is the reduced in several stages making before reaching the end user. The voltage reduction is made in step down transformers. Here is a sample of the national grid system in Britain.

The national grid network in Malaysia


Step-up and step-down transformers

Step Up Transformer
This type of transformer used for increase the output voltage. Number of turns in secondary coil is larger than the number of turns in primary coil.

Step Down Transformer
This type of transformer used for decrease output voltage. Number of turns in primary coil is larger than the number of turns in secondary coil

Transformer Equations
Vp is the potential, Ip is the current, Np is the turn on the primary coil and Vs is the potential, Is is the current, Ns is the turn on the secondary coil. We use following equations to find potential, current or number of turns of any coil;



A transformer is a device for changing the voltage of an alternating current. An alternating current is one that flow back and forth, reversing its direction. A transformer consists of at least two sets of insulated coil windings on a soft iron core. The core is not a solid bar, but is constructed of many layers of thin iron called laminations. The coil connected to input voltage is called primary coil and the coil connected to to the output voltage is called the secondary coil.

When an alternating current is applied to the primary coil, electrical energy from primary coil is transferred to the secondary coil through the changing magnetic field.In other words when an alternating current is applied to the primary coil, it generates a magnetic field in the iron core, the changing magnetic field in the primary coil then induces an EMF in secondary coil which then produce electrical energy in the secondary coil.

The outout voltage depends on the input voltage and the number of turns of the primary coil and secondary coil on the iron core.


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