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Safe Electricals – Revision Pack (P4) Resistance: Resistance slows the flow of electrical current or flow of electrons. A variable resistor, or rheostat, changes the resistance. Longer lengths of wire will have more resistance and shorter lengths will have less resistance. Thinner wires have higher resistance and thicker wire have lower resistance (see left). Voltage (potential difference) is measured in volts on a voltmeter which must be connected in parallel to the resistor. For a fixed resistor, as the voltage increases so does the current. For a fixed power supply, as the resistance increases, the current decreases. The formula for resistance is: RESISTANCE = Voltage / Current Resistance is measured in Ohms (Ω). Voltage is measured in Volts (V). Current is measured in Amps (A). Live, neutral and earth wires: The live wire carries a high voltage into and around the house. This wire is brown. The neutral wire completes the circuit; it provides a return path for the current which enters through the live wire and exits via the neutral wire. This wire is blue. The earth wire is there as a safety measure. It is connected to the case of the appliance to prevent it from going ‘live’. This is yellow and green. A fuse contains a wire which breaks the circuit if the current gets too high. It is a safety feature. The fuse is in the live wire. If there was a fault, like the live wire touches the case, then the earth wire allows a huge current to surge through the live wire which melts the fuse and breaks the circuit. The earth wire and fuses work together to prevent people from experiencing an electric shock. If the live wire was to touch the casing of the appliance, like an electric cooker, then you would get a shock when you touched the appliance. However, the earth wire is connected to the metal casing so takes the current away from it, stopping it from becoming ‘live’. Safe Electricals – Revision Pack (P4) The image to the left highlights what was discussed before; the earth wire will take the current away from the casing if it comes into contact with the live wire. NOTE – A re-settable fuse (circuit-breaker) doesn’t need to be replaced to restore power, it can be reset. Electrical Power: The rate at which an appliance transfers energy is its power rating: (Electrical) Power = Voltage x Current NOTE – the mains voltage of a home in the UK is always 230V The formula for electrical power can be used to calculate the correct fuse to use in an electrical device; for example: A microwave oven has a power rating of 900W. Which of these fuses is the most appropriate for this microwave which will be used at home? 7A fuse 2A fuse 4A fuse 13A fuse ANSWER Power = voltage x current Current (in Amps) = Power / Voltage Voltage at home = 230V Power = 900W 900 / 230 = 3.913 A -SO, the 4A fuse is the most appropriate fuse to use. Double insulation: An appliance with a outer plastic case doesn’t need to be earthed, this is because their outer case is an electrical insulator NOT (like metal) a conductor – so it CANNOT become live. The symbol for a double insulated appliance is shown the left. Safe Electricals – Revision Pack (P4) Past Papers: PPQ(1): PPQ(2): Continued on next page... Safe Electricals – Revision Pack (P4) Safe Electricals – Revision Pack (P4) PPQ(3): Safe Electricals – Revision Pack (P4) Mark Schemes: PPQ(1): PPQ(2): PPQ(3):