Mains Electricity

Electricity in the home Electrical dangers: Heating effect Biological effect

too much current in the wrong place ⇒ fires! electrical current causes muscles to contract out of control and can cause the heart to stop

The three pin system: The mains electricity supply to our homes uses a "three pin system" The mains supply is A.C. (alternating current). The electrons in the wires are pushed back and forth 50 times each second (frequency of 50Hz). The live (brown) and neutral (blue) wires carry the electric current back and forth. The diagram below shows a lamp circuit.

If a wire becomes loose the body of the appliance may become live – electrocution risk. It is the live wire that changes potential. It varies from +240V (or possibly even 300V) and -240V (-300V?) and is the most dangerous wire. If you touch the live wire there will be a circuit connected between you and the earth and a large current will flow. This can easily kill. The neutral wire is at a zero potential. If you touch it, you will be at a zero potential and the wire will also be at zero potential. There is no potential difference and current should not flow! However, this is not advisable. The third wire, the Earth wire (yellow/green), usually carries no current. It is connected to the casing and then to the Earth (hence its name). If the live wire becomes free, the Earth wire will cause a circuit from the live wire to the Earth. A current will begin to flow and build up to a point where the fuse will melt and render the appliance harmless. Replacing the fuse and switching the appliance back on will just blow the fuse again.

Fuses A fuse is connected between the live wire and the live pin. The fuse is made from a metal which has a low melting point and is designed to protect an appliance. All appliances have a "working current" which is safe for their operation. However, if the current exceeds this value the appliance could be damaged. The fuse (opposite) is designed to melt if the current in an appliance exceeds the working current. All appliances will have their fuse rating written on their back and now usually come with a correctly fused plug. Typical values would be 3A, 5A and 13A fuses where appliances below 700W - 3A fuse, above 700W - 5A fuse, above 1200W - 13A fuse)

The Electric shock demonstration

In normal operation, the switch is connected to the __________ wire. The fuse is connected to the ____________ wire. The ___________ wire is connected to the casing of the appliance. When the heater is on, current flows in the ___________wire and the _____________ wire. If the live wire breaks loose and touches the casing a big enough current flows in the__________ wire and ___________ wire to melt the ________. This disconnects the __________ wire and the appliance is made safe. If the earth wire is not connected, no current flows and the ________ does not melt and the _________ wire is not disconnected. If a person touches the casing they become _________ and current flows through them to ___________. The person will be ________________________.

Other safety devices: Appliances with "double insulation” don’t require an Earth connection. The cable has two layers of insulation around each wire to increase safety (often food mixers, electric drills and vacuum cleaners have this). The symbol for double insulation is opposite. There are also two types of “circuit breakers” known as "magnetic trips" and "thermal trips". A circuit breaker is used when devices uses large currents and dangers such as cutting through wires (lawn mower, power drill/cutter etc) are used and bare cables could be left live.

Wring a plug

Obviously we need to be very careful with mains electricity so check everything twice and don’t cut corners!

Never connect a wired plug without a device attached to the mains!