Have you noticed some appliances have an earth and some don’t? Maybe you bought a light fitting from the hardware store and it had a sticker “Double Insulated, Do Not Earth“?
What is an earthed appliance and what does the earth do?
To provide a level of protection against electric shock, a system known as the protective earth incorporated in the wiring circuit of buildings and some appliances.
Early forms of electrical wiring used only two wires; an active (live) and a neutral. This is still common in many countries.
To provide additional protection against electric shock, a third wire commonly called the earth or ground was included in the wiring of buildings and some appliances.
An appliance with a protective earth incorporated into it’s design has the usual an active (live) and a neutral conductors plus an extra conductor commonly known as the “earth conductor”. One end of the earth conductor is connected to the metal chassis of the appliance. The other end of the earth lead is connected to the earth pin of the flexible supply lead plug. Appliances that have a protective earth are called Class 1 Appliances.
The purpose of the earth is to protect the user of a defective electrical appliance from electric shock.
Here is a short explanation of how it works.
Let’s suppose you have a flood light with a metal chassis and the live wire has come lose inside the flood light and is touching the chassis. When you plug the flood light into the power and turn it on, the chassis will become live and if you touch it you have a high chance of getting an electric shock.
The good news is this flood light has a protective earth, so the current will flow “to earth” instead of through you. The earth conductor provides an alternative pathway that has less resistance than you do. The average adult has around 3,000 Ω – 4,000 Ω of resistance, whereas the protective earth should have less than 1Ω .
Here is a diagram that shows a typical protective earth circuit of a domestic home.
Looking at the diagram, follow the green earth wire from the metal chassis of the appliance to the earth pin on the supply lead plug. This earth pin makes a connection with the earth terminal in the power point, which in turn, is connected to the earth bar in the switchboard and then the earth stake.
In this example the current can flow along the earth lead, back to the switch board and then safely dissipate through the earth stake into the ground.
Although the protective earth is of a fairly simple design, it is very effective in protecting people from electric shock.
Part of the testing and tagging process is to check the integrity of the protective earth of the appliance so the user is protected. For more information about having your appliances checked, see out testing and tagging service or contact us for more information. Or to learn how to test and tag appliances, visit our test and tag training site.
Founder ACME Test and Tagging