MAINS ELECTRICITY.

Specific objectives.

1. State the source of mains electricity.
2. Describe the transmission of electric power from generating station.
3. Explain the domestic wiring system.
4. Define the kilowatt- hour (KwH).
5. Determine the electrical energy consumption and lost.
6. Solve numerical problems involving mains electricity.

Sources of mains electricity.

• Electricity is produced in a generator by rotating coils placed between powerful magnets in a generator.
• To produce electricity therefore, a coil must rotate or made to cut the magnetic flux or cause a change in the magnetic flux.
• The source of main electricity are the agents that make a coil rotate in a magnetic field. These are;

Running or falling water.

• The energy conversion is the potential energy of high level water is converted to kinetic energy of falling water then to mechanical energy in rotating the turbines then to electrical energy. This is called hydroelectric power.

Steam

• The coil or turbines can be rotated by using high pressure steam.
• The steam is obtained from heated water using;

1. The hot core of the earth. This is called geothermal electricity.
2. Coal.
3. Nuclear reactors (nuclear fission and fusion).

Wind.

Tides.

Transmission of mains electricity.

Electricity is distributed and transmitted from the generating power station to the consumers by a system of power stations interconnected by cables and other necessary appliances. This is called national grid system. The generating station is called the main station. Others are called sub- stations. The current from the main station is carried through thick cables (copper) to a step- up transformer. Current to the transmission line is made of aluminum wires since aluminum is cheaper and lighter. The aluminum wires are twisted to make it strong and prevent Eddy current. The power is transmitted at high voltage called high tension transmission between 132- 400 kV. To the consumer, the voltage is stepped down to 11 kV then to 240 V by a system of step- down transformers. The transmission lines are supported by a metal iron or post.

                                    Dangers of high tension transmission.

1. Risk of electric shock in case of poles collapses or cables hang too low.
2. The risk of fire on nearby structures and vegetation when the cables get too close.
3. The harmful effect of strong electric field which ionizes the air around and these charges may cause electric shock or interfere with electric signals.

Power lost during transmission.

• The power lost during transmission is in form of heat. This is caused by;

1. Resistance in the transmission line due to the distance involved.
2. The high current.

How to reduce the power losses.

1. Use thick wires made of materials of good conductor of electricity e.g. aluminum.
2. Transmit power at very high voltage to reduce the current.

Domestic wiring or house circuit.

Electric power is supplied at 240 V from the national grid of 11 kV by a step- down transformer. It is brought to the house by a two wire cable. One wire is live and marked red or brown and at 240 V. The other wire is earthed at the transformerand is neutral. It is marked blue or black and has a p.d of zero. A third cable is introduced in the house is called earthed wire. It is marked green. It is for safety and protect the user against electric shock. It is earthed at the main meter box. The cable goes through the supply or company’s fuse box with a fuse rated 60 A or 80 A.

                                    Power socket.

• These are the power points where power is tapped from and has three holes for l, N and E.
• The live and neutral hole are covered by a shutter which is only opened when the plug is fixed.
• Is used to protect against small children.

                                    Fused plug.

• Has 3 pins where the lead are connected. These are L, N and E pin.
• The plug connects the appliance to a power source through the socket. The earth pin is longer than the other two. Is used to open the live and neutral hole in the socket and earth appliances immediately.
• The plug has a fuse rated 13 A or 15 A to safeguard the appliance from damage due to excessive current in the circuit.
• The rating of the fuse depends on the operating current of the appliance and the value chosen. Should be slightly above the value of the operating current of the appliance.

                                    Fuse.

• It is a safety device to safeguard an appliance from electric fire.
• Is a short length of a wire of material of low melting point often tin or copper. It melts and breaks the circuit when the current though it exceeds a certain value.
• The reasons for excessive current are;

1. Overloading the power point.
2. Short-circuiting due to worn out insulation on connection wires or the wire has been weakened or by oxidation or by straining it too tightly in the fuse.

• Without a fuse, the wiring would become hot and could cause a fire.
• A fuse should never be replaced by another conductor.

                                                Earthing.

• Is a third wire in the ring main circuit and goes to the top point in the socket on all power points.
• Is a safety precaution to prevent electrical shock should an appliance develop a fault.
• Is connected to the metal case of an appliance which is thus joined to the earth by a path of almost zero resistance.
• Is earthed by being connected either to metal water pipe or to the house or to an earth connection- copper rod on the supply cable of the metal box.
• If the element of an appliance develop an electrical fire or sags and touches the case, a large current flow to the earth and blows the fuse. Otherwise, the case becomes live and anyone touching it receives shock which might be fatal.

                                                Precautions.

1. Fuse and switches.

• They should be connected in the live wire of the main. If they were in the neutral, the appliance would be permanently connected to live side and touching a faulty appliance would complete the circuit through the person’s body causing an electric shock.

1. Insulation materials.

• All household wires must be covered by insulation materials usually vulcanized rubber or plastic.

1. All circuits should be connected in parallel.
2. Sockets should have shutters over live and neutral holes to safeguard against children and should be in above the ground.

Circuit breakers.

• Is commonly used in the consumer control unit and helps to distribute power to different parts of the house e.g. lighting system, power points, cooker and heater.
• They are preferred to a fuse because they break the circuit instantaneously whereas the fuse wire take time to melt.
• It is reset for use again once the faulty has been corrected as opposed to the fused wire which has to be replaced.