Electrical Safety

The History of Electricity 

Since ancient times, humans have been interested in Electricity. For thousands of years, humans have observed lightning, and thought this was the Gods warring with each other. Zeus of Greek mythology and Thor of Norse mythology are just two examples of where a God of Thunder was worshipped across the World. 

The word 'Electricity' is based upon the ancient Latin word 'Electrum' - meaning Amber. The ancient Greeks and Romans had discovered that by rubbing rods of the semi-precious stone Amber, it was possible to pick up small feathers with the rods. This is one of the earliest examples of Static Electricity. 

It wasn't until the 1600's when scientists began to truly explain the fundamentals of Electricity, which facilitated the progress that has given rise to the modern World. 

The video below shows an in-depth journey through the history of Electricity:-

Moving Charges and Current Flow

In the BGE course, the concept of Conductivity was covered. All materials can be defined within two groups:-

1. Conductors - Materials which allow Charge to flow easily through them.

2. Insulators - Materials in which it is difficult for Charge to flow through them.

Almost all of the conductors looked at were metals (Graphite is the exception). This is due to the structure of the atoms within metals. The diagram below shows the structure of a metal to the atomic scale:-

In a metal, Electrons are free to move from atom to atom within the metal. When there is no Electric field present, these Electrons move at random through the material. This means there is no overall flow in any direction. 

If an Electric field is present across the metal, the negatively charged Electrons are repelled away from the negative end of the metal, and attracted towards the positive end. This causes a flow of charge towards the positive end of the metal. The image below shows this process:-

Voltage & Current

The size of the Electric field is shown by the difference in Voltage between each end of the material. Voltage is the Energy given to the charges to move them around a circuit. 

The flow of Electrons through a conductor is called an Electric Current. The faster the flow, the larger the Current, just like in a river. 

All Current flow can be divided into two groups:-

1. Direct Current - occurs in a circuit using a battery.

2. Alternating Current - occurs in a circuit using Mains Electricity.

Direct Current

When a circuit is attached to a battery, Electrons are repelled by the negative terminal and attracted by the positive terminal. This causes a flow of Electrons from negative to positive. As all Electrons are moving in the same continuous direction, this is a direct Current.  On an oscilloscope, direct Current is shown by a horizontal, flat line as shown below:-

Alternating Current

When a circuit is attached to Mains Electricity, the direction of Electron flow changes back and forth many times a second, giving an alternating Current. On an oscilloscope, alternating Current is shown by a sinusoidal waveform:-

As an Electrical Engineer, you would design, build and maintain the electrical systems and equipment that are vital to industry, the railways and manufacturing. 

You would use computer-assisted engineering and design software to draw up new project plans and circuit diagrams for high and low voltage electrical equipment.

You’d lead the installation of a system and make sure it meets the safety regulations.

You’d also have to write reports, attend meetings and give presentations about projects. You’d need a good understanding of electrical health and safety regulations.

You would work on projects with other professionals, such as civil engineers, architects, engineering technicians and IT staff.

Mains Electricity 

The most common source of electrical Energy in the home is mains electricity.  Mains electricity provides an alternating Current with a stated Voltage of 230 V, alternating at 50 Hz. This is accessed by using the appropriate plug type for that region. In the UK, the standard plug is a 3-Pin plug (type G) as shown below:-

Note - There are 15 different types of plug in use across the World, each with different Voltage and Current limits. Just because an adaptor allows a plug to connect to a foreign socket does not mean it is safe to use - always check that the device is compatible with the country's mains values. 

The UK plug is designed with several important safety features, which can be seen in the diagram below :-

Safety features of a 3-Pin plug:-

1. Plastic coated - Electrical insulator to prevent electrical shocks.

2. Earth wire - Connected to any exposed metal, conducts Current safely to Earth if there is a fault preventing Electrical Shocks (see below). 

Fuse - Breaks flow of Current if the Current becomes too high, prevent risk of overheating or fire (see below).

3. Cable grip - Holds flex (cable containing the three wires) in place to prevent wires being exposed.

Fuses

A fuse in a 3-Pin plug is a device used to prevent too high Current flow within a device. A fuse consists of a thin wire within a ceramic cartridge, as shown in the image below:-

The fuse acts as simply a piece of wire, as long as the Current is at the operating value. If the Current is too high, the wire within the fuse melts, breaking the circuit. This prevents the device from overheating and possibly from starting a fire. 

Note - In everyday language, we say the fuse has 'blown'. Do not use the word 'blown' as this implies an explosion. The correct description is the fuse wire 'melts'. 

Fuses come in a range of values and every mains appliance will have a correct fuse to use. The correct fuse can be found by using information on the rating plate. The image below shows the rating plate of an electric drill:-

The correct value of the fuse is not given, but can be found using the .Power Rating'. Power is the 'energy per second' flowing in a circuit, measured in Watts (W). 

Once the Power is found, the correct fuse can be selected :- 

• Under 720 W - use a 3 A Fuse.

• Over 720 W - use a 13 A Fuse.

The Earth Wire

The Earth wire is an additional wire within the 3-Pin plug that, if the system is working correctly, does not carry any Current. The Earth wire's function is to provide a short, low Resistance path to 'Earth'. 

If a fault occurs by the Live wire coming into contact with the metal casing of a device, anyone touching that device will receive a dangerous electrical shock. This is because the person touching the casing will give the Current a path to 'Earth' through. 

The Earth wire works by providing a lower Resistance path which allows the current to flow easily to 'Earth'. This high Current flow causes the fuse to melt, breaking the circuit. This renders the device safe