L12 - Living with Electricity

objectives

Living With Electricity

• When you plug a kettle into a mains socket, you are connecting it into a circuit, as shown below. The power comes from a generator in a power station.
• The supply voltage depends on the country. For household circuits, some countries use a voltage in the range 220-240 V, others in the range 110-130 V.
• Mains current is alternating current (a.c.).
• It flows backwards and forwards, backwards and forwards...
• 50 times per second, in some countries.
• The mains frequency is 50 hertz (Hz). In other countries, the mains frequency is 60 Hz

• A.c. is easier to generate than one-way direct current (d.c.) like that from a battery.

• Fuse This is a thin piece of wire which overheats and melts if the current is too high.
• Like the switch, it is placed in the live wire, often as a cartridge.
• If a fault develops, and the current gets too high, the fuse ‘blows’ and breaks the circuit before the cable can overheat and catch fire.
• Many circuits use a circuit breaker instead of a fuse

• Earth (grounded) wire* This is a safety wire. It connects the metal body of the kettle to earth and stops it becoming live. For example, if the live wire comes loose and touches the metal body, a current immediately flows to earth and blows the fuse. This means that the kettle is then safe to touch.
• Double insulation Some appliances - radios for example - do not have a body and does not need an earth earth wire. This is because their outer case is made of plastic rather than metal. The plastic acts as an extra layer of insulation around the wires.

• For extra safety, circuits may be fitted with a type of breaker called a residual current device (RCD). This compares the currents in the live and neutral wires. If they are not the same, then current must be flowing to earth - perhaps through someone touching an exposed wire.
• The RCD senses the difference and switches off the current before any harm can be done.

Plugs

• Plugs are a safe and simple way of connecting appliances to the mains. Over a dozen different types of plug are in use around the world. You can see an example on the right.
• A few countries use a three-pin plug with a fuse inside. The fuse value is typically either 3 A or 13 A. This tells you the current needed to blow the fuse. It must be greater than the normal current in the appliance, but as close to it as possible, so that the fuse will blow as soon as the current gets too high.
• For example:
  • If a kettle takes a current of 10 A, then a 13 A fuse is needed.
  • If a TV takes a current of 0.2 A, then a 3 A fuse is needed. The TV would still work with a 13 A fuse. But if a fault developed, its circuits might overheat and catch fire without the fuse blowing.

Electrical Hazards

• Water in sockets or plugs. Water will conduct a current, so if electrical equipment gets wet, there is a risk that someone might be electrocuted.
• Accidentally cutting cables. With lawnmowers and hedge trimmers, a plug-in RCD can be used to avoid the risk of electrocution.