Home circuits
What do you do when you go home? Switch on a light? Turn on the television? Put the kettle on? You may even do all three. As you move around your home switching electrical devices on and off, the supply of electricity may not look to be very organised but nothing could be further from the truth. Behind the walls and above the ceilings, wires are arranged in a very organised way to supply you with electricity safely. Here is the path that electricity can take when it enters your home.

Electricity enters your home by a service cable. This is also known as the main cable or the mains. It connects the street cable, which brings electricity from the power station to the circuits in the home. Inside the service cable are two wires called the live wire and the neutral wire. The current flows along these wires. The first place where these wires leave the service cable is called the sealed unit. Here, the live wire is connected to a fuse and the neutral wire is connected to a metal terminal. The fuse is designed to let a current of sixty or a hundred amps pass through it but if, owing to a fault, a larger current develops the fuse blows and the supply to the home is cut off. This protects the home from large currents which could cause a fire.

The electricity is then conducted from the sealed unit to the meter by another pair of live and neutral wires, and then a second pair of live and neutral wires take the current to a box called the consumer unit. Inside the consumer unit is a main switch. When this is switched on, as is usually the case, electricity can flow to all the circuits in the home. The current enters each circuit in the consumer unit. As the current enters the circuit it passes through a fuse. This is a second line of defence for the home. If a rise in current occurs due to a fault in the circuit, the fuse blows and stops the current flowing. This in turn reduces the risk of a fire developing.

There are several circuits which lead away from the consumer unit. For example, there is a lighting circuit to each floor in the home. The live and neutral wires run above the ceilings of the rooms. At each light socket a pair of wires run across the ceiling and down the wall to a light switch and a second pair of wires move on to another light fitting in the room or further on to a light fitting in another room on the same floor.

Just as there is a circuit supplying electricity to the light on each floor of a home so there is a circuit supplying power for the plug sockets on each floor. A circuit supplying power to the plug sockets is called a ring circuit. The live and neutral wires run from the consumer unit to each ring circuit but they are joined by a third wire called the earth wire. In normal circumstances no current flows along this wire. It is present as an extra safety device. If a fault develops in any electrical equipment plugged into the ring circuit the current may be able to reach the outside of the equipment and cause a shock. The earth prevents this happening in the following way. When the current reaches the outside of the equipment it passes directly along an earth wire in the cable connecting it to the plug. The earth wire offers little resistance to the flow of the current so a large current develops. This melts the fuse in the plug and stops the flow of the current.

Some pieces of electrical equipment such as ovens and shower units use so much power that they need a circuit to themselves. Each item has its own circuit, which starts in the consumer unit and passes to wherever the item is located in the home. These circuits which supply large amounts of power also have an earth wire for added safety.

Next time you switch on an electrical item in the home, think of the path the electricity is taking. It is flowing from a consumer unit in a cupboard or a garage through wires inside the walls or above the ceilings to make your life more comfortable.

How are the electrical currents prevented from flowing all over the home?
Electricity can only flow through materials called conductors. Metals are conductors and the cables and wires are made out of metal wires. Electricity does not flow through most other materials. These other materials are called insulators. Each cable and wire is coated in an insulator which keeps the current flowing along the wire and prevents it from touching other materials in the home.

Are the circuits in the home series circuits or parallel circuits?
The lighting circuits and ring circuits are parallel circuits. This means that if one item in the circuit, such as a light bulb, develops a fault and fails to work, all the other items in the circuit are not affected. If the lighting circuit, for example, was a series circuit, when one light failed all the lights would go out. Some items, such as a cooker or a shower, which need a great deal of power, have a circuit to themselves. These circuits are neither series nor parallel circuits.

How does a fuse prevent a fire?
When a fault develops in a circuit it can cause a larger current than normal to flow. As the current flows it creates more heat in the wires which carry it. The hot wires may melt their insulation and come into contact with carpets or curtains. When this happens, these fabrics can catch fire. The large current can only flow because the wire is strong enough to let it. In a fuse is a piece of wire, which is only strong enough to let a current of a certain size flow through it. If a larger current, caused by a fault, flows through the fuse, the wire simply melts and breaks the circuit. This stops the current flowing and prevents the wires becoming hot enough to start a fire.

What is an earth wire connected to?
It is connected to the earth. The way that this is done is for one end of the wire to be connected to a metal rod in the foundations of the home. When a fault develops the current flows along the earth wire to the metal rod until the fuse blows to stop it.

What would happen if a person touched something with an electrical fault that was not earthed?
The current would flow through them to the ground. When a circuit in the home is broken by a fault the electric current seeks a way of finding the earth. If there is not an earth wire it will go through anything that touches the item with the fault. If the current is large it can deliver a fatal shock.

How does an electricity meter work?
There is a horizontal disc and an electromagnet in an electricity meter. When you use any electricity in the home - such as for a lamp, a computer or a kettle, the current also flows through the electricity meter. The current makes the electromagnet turn the disc. There is a mechanism in the meter, which records how many times the disc turns. This mechanism passes on the information to a display. The display shows how much electricity has been used. The information on the display is used to calculate electricity bills.

Can experiments be done using mains electricity?
NO! You must NEVER do any experiments with mains electricity. The electricity is under such a high voltage (in England the voltage is 240V) that you could receive a fatal shock. The voltages used in batteries is very low - often only between 1.5V and 4.5V. These voltages are safe to use.