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
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
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
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
Step 3: Rearrange the equation to make current the subject
Divide both sides by V
Step 4: Substitute in the known values to calculate
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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