Electrical Safety (Edexcel IGCSE Physics)

• Mains electricity is potentially lethal
  • Potential differences as small as 50 V can pose a serious hazard to individuals

An important symbol for electrical safety

Signs, like the above, warn of the risk of electrocution

• Common electrical safety 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 insulation, exposing live wires
  • Damp conditions – if moisture comes into contact with live wires, it could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk

• 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

• To improve electrical safety wires are covered with an insulating material, such as rubber

Insulating electrical wires to improve electrical safety

The conducting part of a wire is covered in an insulating material for safety

• Some appliances do not have metal cases, 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 electrical 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

The earth wire is an electrical safety feature

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 electrical 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)
  • As explained in the Selecting fuses revision note a fuse consists of a glass cylinder containing a metal wire
  • A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value

A circuit breaker is the most important electrical safety feature in houses

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

• A circuit breaker has a major advantage over a fuse as an electrical safety device 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 as the most important electrical safety device
  • Sometimes they are misleadingly named "Fuse boxes"