Electrical Safety (Edexcel IGCSE Physics)
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Katie MExpertise
Physics
Electrical Safety
- Mains electricity is potentially lethal
- Potential differences as small as 50 V can pose a serious hazard to individuals
Signs, like the above, warn of the risk of electrocution
- Common hazards include:
- Damaged Insulation – if someone touches an exposed piece of wire, they could be subjected to a lethal shock
- Overheating of cables – passing too much current through too small a wire (or leaving a long length of wire tightly coiled) can lead to the wire overheating. This could cause a fire or melt the insulations, exposing live wires
- Damp conditions – if moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk
- In order to protect the user or the device, there are several safety features built into domestic appliances, including:
- Double insulation
- Earthing
- Fuses
- Circuit breakers
Insulation & Double Insulation
- The conducting part of a wire is usually made of copper or some other metal
- If this comes into contact with a person, this poses a risk of electrocution
- For this reason, wires are covered with an insulating material, such as rubber
The conducting part of a wire is covered in an insulating material for safety
- Some appliances do not have metal cases and so there is no risk of them becoming electrified
- Such appliances are said to be double insulated, as they have two layers of insulation:
- Insulation around the wires themselves
- A non-metallic case that acts as a second layer of insulation
- Double insulated appliances do not require an earth wire or have been designed so that the earth wire cannot touch the metal casing
Earthing
- Many electrical appliances have metal cases
- This poses a potential safety hazard:
- If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted
- The earth wire is an additional safety wire that can reduce this risk
A diagram showing the three wires going to a mains powered appliance: live, neutral and earth
- If this happens:
- The earth wire provides a low resistance path to the earth
- It causes a surge of current in the earth wire and hence also in the live wire
- The high current through the fuse causes it to melt and break
- This cuts off the supply of electricity to the appliance, making it safe
Fuses & Circuit Breakers
- Fuses and 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)
The circuit symbol for a fuse (not to be confused with a resistor)
- Fuses usually consist of a glass cylinder containing a thin metal wire
- If the current in the wire becomes too large:
- The wire heats up and melts
- This causes the wire to break, breaking the circuit and stopping the current
- A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value
The main circuit breaker can quickly shut off electricity to the whole house. The branch circuit breakers can shut off electricity to specific areas of the house
- This has a major advantage over a fuse because:
- It doesn't melt and break, hence it can be reset and used again
- It works much faster
- For these reasons, circuit breakers are used in mains electricity in homes
- Sometimes they are misleadingly named "Fuse boxes"
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