Electrical Safety (Oxford AQA IGCSE Physics)

Revision Note

Earthing

  • Many electrical appliances have metal cases

  • This poses a potential safety hazard:

    • If a live wire with a high voltage (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted

    • This is because metal is a good conductor of electricity

  • The earth wire is an additional safety wire that can reduce this risk

  • If the live wire does touch the metal casing:

    • The earth wire provides a low-resistance path to the earth

    • The current is harmlessly conducted away

    • This is because the route to Earth via the earth wire has a much lower resistance than through a person's body

Earth wire connected to metal casing

An earth wire connects to the metal casing of an appliance and leads back to the plug, for GCSE & IGCSE revision notes
The earth wire in a plug connects to the metal casing of an appliance in case the live wire breaks

Worked Example

Some plugs are 'double insulated', meaning an additional layer of thick insulation surrounds the live wires.

Explain why this means these plugs do not need an earth wire.

Answer:

Step 1: Recall the purpose of an earth wire

  • The earth wire provides a low resistance route to earth if the live wire breaks and touches the metal casing

Step 2: Explain how double insulation makes this unnecessary

  • The additional layer of insulation prevents the live wire from touching the casing if the live wire breaks

Fuses & Circuit Breakers

Fuses

  • Fuses are used to protect individual appliances

  • Fuses are located in the plug of an appliance

  • Fuses usually consist of a glass cylinder containing a thin metal wire

  • If the current in the wire becomes too large (e.g. in a surge from the national grid):

    • The wire heats up and melts

    • This breaks the circuit and stops the current

Circuit symbol of a fuse

The symbol of a fuse is a straight line (the wire) passing through a rectangle, for IGCSE & GCSE Physics revision notes
The circuit symbol for a fuse has a wire running through it (not to be confused with a resistor)
  • Fuses have ratings, which signify the maximum current that can flow through the fuse wire without it melting

    • If the current exceeds that value, the fuse wire melts and the individual appliance is disconnected from the mains supply

  • Fuses come in values of 3 A, 5 A and 13 A

    • The correct fuse to use is the value just above the current required for the appliance

Circuit breakers

  • Similar to fuses, circuit breakers are safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

    • This is sometimes referred to as the circuit breaker tripping when the current is too high

  • Domestic electricity is supplied to homes with a set potential difference and set maximum current

  • The current enters the house at the consumer unit (sometimes referred to as a fuse box)

  • The consumer unit consists of a series of circuit breakers which control the amount of current supplied to each circuit within the house

Example of a domestic circuit

The consumer unit connects to circuits controlling appliances like mains sockets, lights and kitchen appliances, for GCSE & IGCSE Revision Notes
The consumer unit distributes current to all the circuits in the house
  • Circuit breakers stop excess current flowing faster than fuses

  • Circuit breakers can be reset, unlike fuses which must be replaced

Worked Example

A 2 kW kettle is purchased to be used on the mains power supply. 

Determine the value of the fuse required for the kettle. The fuses available are rated at 3 A, 5 A and 13 A.

Answer:

Step 1: List the known quantities and convert to SI units

  • Power rating, P = 2 kW = 2000 W

  • Voltage of mains supply, V = 230 V

Step 2: Write out the equation

  • The current is needed to determine what fuse to use

  • Therefore, the power, voltage, and current equation should be used

 P space equals space V I

Step 3: Rearrange the equation to make current the subject

  • Divide both sides by V

P over V space equals fraction numerator space up diagonal strike V I over denominator up diagonal strike V end fraction

 I space equals fraction numerator space P over denominator V end fraction

Step 4: Substitute in the known values to calculate

I space equals fraction numerator space 2000 over denominator 230 end fraction

 I space equals space 8.7 space straight A

Step 5: State the fuse required for the kettle

  • The correct fuse is slightly higher in value than the current required by the appliance

  • Fuses come in 3 A, 5 A, or 13 A

  • Therefore, the correct fuse is 13 A

Safety Features of Appliances

  • Earth wires are connected to appliances with metal casings to conduct current to the Earth when the live wire breaks and touches the casing

  • However, when this happens, a large current flows

    • This is because the resistance of this path to Earth is low

  • A fuse is connected along the live wire in this circuit such that, when this large current flows, the fuse melts and stops any current flowing

Examiner Tip

To be considered safe, the plug of an appliance must have the correct fuse and either have an earth wire or be double insulated. The latter could be either two layers of insulation around the live wire or a plastic casing which acts as an insulator.

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