Electricity

Mains Electricity
1. Electric current is the movement of charge carriers
2. Mains supply is AC, Battery supply is DC
  1. Uk mains supply is approximately 230 volts
    1. It is an AC supply (alternating current) which means the current is constantly changing direction
    2. The frequency of the AC mains supply is 50 cycles per second or 50Hz
  2. Cells and batteries supply direct current (Dc). This means the current always keeps flowing in the same direction
3. Electricity supply can be shown on an Oscilloscope
  1. A cathode ray oscilloscope (CRO) is basically a voltmeter
  2. If you plug an AC supply into an oscilloscope you get a trace on the screen that shows how the voltage of the supply changes with time. This trace goes up and down in a regular pattern some of the time its positive and some of the time its negative
  3. If you plug in a DC supply the trace you get is just a straight line
  4. The vertical height of the AC trace at any point shows the input voltage at that point. By measuring the height of the trace you can find the potential difference of the AC supply
  5. For DC its a lot simpler the voltage is just the distance from the straight line trace to the center line
4. How to read an oscilloscope
  1. A DC source is always at the same voltage so you get a straight line
  2. An AC source gives a regularly repeating wave. You can work out the period and the frequency of the supply
  3. Frequency = 1 / Time period (s)
Electricity in the Home
1. Hazards in the home
  1. Long cables
  2. Frayed cables
  3. Cables in contact with something hot or wet
  4. Water near sockets
  5. Shoving things into sockets
  6. Damaged plugs
  7. Too many plugs into one socket
  8. Lighting sockets without bulbs in
  9. Appliances without their covers on
2. Most Cables have three separate wires
  1. Most electrical appliances are connected to the mains supply by three core cables. This means that they have three wires inside them each with a core of copper and a coloured plastic coating
  2. The brown live wire in a mains supply alternates between a high positive and a negative voltage
  3. The blue neutral wire is always at OV. Electricity normally flows in and out through the live and neutral wires only
  4. The green and yellow earth wire is for protecting the wiring and for safety it works together with a fuse to prevent fire and shocks. It is attached to the metal casing of the appliance and carries the electricity to earth should something go wrong and the live or neutral wires touch the metal case
3. Three pin plugs and cables
  1. Get the wiring right
    1. The right coloured wire is connected to each pin and firmly screwed in
    2. No bare wires showing
    3. Cable grip tightly fastened over the cable outer layer
    4. Different appliances need different amounts of electrical energy. Thicker cables have less resistance so they can carry more current
  2. Plug features
    1. The metal parts are made of copper or brass because these are very good conducters
    2. The case, cable grip and cable insulation are made of rubber or plastic. Because they are good insulators and flexible
    3. This keeps all the electricity flowing where it should
Fuses and Earthing
1. Earthing and Fuses prevent electrical overloads
  1. The earth wire and fuse (or circuit breaker) are included in in electrical appliances for safety and work together like:
    1. If a fault develops in which the live wire somehow touches the metal case then because the case is earthed, too great a current flows in through the live wire through the case and out down the earth wire
      1. This surge in current melts the fuse (or trips the circuit breaker in the live wire) when the amount of current is greater than the fuse rating. This cute off the live supply and breaks the circuit
        This isolates the whole appliance making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by the heating effect of a large current
        As well as people fuses and earthing are there to protect the circuits and wiring in appliances from getting fried if there is a current surge
        Fuses should be rated as near as possible but just higher than the normal operating current
        The larger the current the thicker the cable you need to carry it. Thats why the fuse rating needed for cables usually increases with cable thickness
2. Insulating materials make appliances Double insulated
  1. All appliances with metal cases are usually earthed to reduce the danger of electric shock
    1. Earthing just means the case must be attached to an earth wire
      1. An earthed conductor can never become live. If the appliance has a plastic casing and no metal parts showing then its said to be double insulated
        Anything with double insulation like that doesnt need an earth wire. These are known as two core cables
3. Circuit breakers have some advantages over fuses
  1. Circuit breakers are an electrical safety device used in some circuits. Like fuses they protect the circuit from damage if too much current flows
  2. When circuit breakers detect a surge in current in a circuit they break the circuit by opening a switch
    1. A circuit breaker can easily be reset by flicking a switch on the device. This makes them more convinient than fuses which have to be replaced
  3. However they are more expensive
  4. One type of circuit breaker used instead of a fuse and an earth wire is a Residual current circuit breakers (RCCBs)
    1. Normally the exact same current flows through the live and neutral wires. If somebody touches the live wire a small but deadly current will flow through them to the earth. This means the neutral wire carries less current then the live wire. The RCCB detects this difference in current and quickly cuts off the power by opening a switch
    2. They also operate much faster than fuses they break the circuit as soon as there is a current surge no time is wasted waiting for the current to melt a fuse. This makes them safer
    3. RCCBs even work for small current changes that might not be large enough to melt a fuse. Since even small current changes could be fatal this means RCCBs are more effective at protecting against electrocution

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	Created by sian.allison almost 12 years ago