Edexcel IGCSE Physics: Double Science 复习笔记： 2.3.3 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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