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16 Cards in this Set
- Front
- Back
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Explain the structure of a mains electric cable used in the home. |
A mains electricity cable contains two or three inner wires.
Each has a core of copper, because copper is a good conductor of electricity. The outer layers are flexible plastic, because plastic is a good electrical insulator. The inner wires are colour coded: |
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How are the inner wires are colour coded? |
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Explain the features of a mains plug. |
The features of a plug are:
The case is made from tough plastic or rubber, because these materials are good electrical insulators. The three pins are made from brass, which is a good conductor of electricity. There is a fuse between the live terminal and the live pin. The fuse breaks the circuit if too much current flows. The cable is secured in the plug by a cable grip. This should grip the cable itself, and not the individual wires inside it. |
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Describe how to correctly wire a mains plug. |
Brown is the live wire and is connected to the fuse, which in turn is connected to the live pin. Blue is the neutral wire and is connected to the neutral pin. Green and yellow is the earth wire and is connected to the earth pin. |
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What rules must be followed when wiring a plug? |
Always check that the correct colour is attached to the correct pin. Make sure that the wires are not stretched. Make sure that the screws on the pins grip the copper wire firmly. No bare copper wire should be visible inside the plug. The cable grip should hold the outer insulation firmly. The correct fuse should be used for the appliance power rating. |
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(a) What thickness of cable would you need to use for a 3 kW kettle? Explain your answer. (b) If a cooker needs to draw a current of 22 A, what thickness of cable is needed? |
(a) Cable thickness of 1.5 mm: this has maximum power rating of 3 600 W, which is more than the kettle. A thicker cable would be a waste of copper and would be less safe (would allow a larger current to pass before burning out). (b) Cable thickness of 4.0 mm needed. |
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In the figure, each plug has a fault. Explain what is wrong in each case. |
A has the brown and the blue wires going to the wrong pins. B has the fuse replaced by a piece of metal foil. C has a piece broken off the plug casing. D has the earth wire disconnected. |
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Earth wires have green and yellow stripes. Explain why this might help people who have difficulty seeing differences between colours. |
Even if the person cannot tell which wire is which from the colour, they can tell which is the earth wire. |
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Explain why modern plugs have insulating plastic shrouds around part of the live and neutral pins. |
So that children are unable to touch the pins while the pins are carrying electric current. |
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Explain why the cable for a television needs to be stranded but a cable buried in the wall does not. |
The cable for the television needs to be stranded because the cable needs to bend. A cable buried in the wall does not bend backwards and forwards so it does not need to be stranded. |
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The inner insulation in a cable is soft and flexible. Explain why it would be dangerous to grip the inner insulation with the cable grip rather than the outer insulation. |
The inner insulation could be easily damaged and has little strength so if the cable was pulled, the strain would be on the copper. |
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Power cables carried by electricity pylons are made from aluminium with a steel core down the centre. Suggest why aluminium is used rather than copper and why there is a steel core to the wire. |
Aluminium is less flexible than copper but is cheaper and conducts electricity better for its weight so although it would not be a good choice for wires in a house it is an excellent metal for conducting large amounts of current in wires that don’t move much. The steel core gives the wires extra strength. |
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From the table, plot a graph of maximum power against cross-sectional area. Use the graph to work out what would be the minimum cross-sectional area to use with a 4 kW heater. |
Graph of maximum power against cross-sectional area plotted correctly from data. Approx 1.8 mm^2. |
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Many electrical products are now supplied with moulded plugs that cannot be taken apart, although the fuse can still be replaced. Describe the advantages and disadvantages there might be for the consumer. |
The moulded plug is safer because there is no way for the consumer to get inside and incorrectly wire the plug. However, if the plug is damaged the whole cable and plug need to be replaced, which is more expensive. |
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An electric iron has a special type of cable which often has a woven covering on it.
What do you think this is for and why would ordinary electrical cable be unsafe to use? |
The woven fabric coating is heat resistant so if the hot foot of the iron touched it, the cable would be protected. With a normal plastic-coated cable the outer casing would melt and the wires could become exposed. |
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What are the key points to remember? |
Plugs and cables are used to connect electrical devices to the mains electricity supply. Wires are colour-coded to make identification easier. Most electrical devices use a three-pin plug. Three-pin plugs must be wired correctly. Appliances of different power need different thicknesses of cable. |
